alcohols phenols and ether by aarkumar

8/13/2019 alcohols phenols and ether by aarkumar

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ALCOHOLS AND PHENOLSALCOHOLS AND PHENOLS

CLASS XIICLASS XII


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Introduction

Alcohols are substituted hydrocarbon molecules containing one or more -OH

group bound to a saturated sp3hybridized carbon atom. The more common

alcohols are derived from saturated hydrocarbons or alkanes that contain

hydroxyl groups linked to alkyl radicals. They may be regarded as being

derived from them by replacing one or more hydrogen atoms by hydroxyl

groups, even though they are not prepared in this way. The hydroxyl group

is the functional group that characterizes the alcohols.

However, alcohols differ from bases such as NaOH or KOH in that they do not

furnish hydroxide ion (OH-

) in water nor do they have other properties of

bases. This is because the alcohols are covalent compounds while the

inorganic bases are ionic compounds.

Phenols are an important class of aromatic compounds that are hydroxyl

derivatives of aromatic hydrocarbons. The hydroxyl group is directly linked

(and not as a part of a side chain) to the carbon atoms of the benzene rings.

Interestingly, it was the first compound that was to be specifically employed

as an antiseptic (Lister, 1867) under the name of carbolic acid. In solution,

phenol acts as an antiseptic (0.2% solution) or a disinfectant (1% solution).

Ethers are hydrocarbon molecules with -C-O-C- as the functional group.

Here, oxygen is bound to two organic (alkyl) groups i.e., R-O-R' where R and

R' may be same or different alkyl or aryl groups or one of them may be an

alkyl group and the other aryl group. They can be straight or branched

chains, cyclic rings, saturated and unsaturated and aromatic compounds.

All of these molecules are widely represented in nature and are important in

industry and as pharmaceuticals. For instance, ethanol is used as a solvent,

as a fuel (can be mixed with petrol), used to make 'ethyl esters' and is the

'potent' chemical present in alcoholic drinks. Ordinary spirit used for polishing


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wood contains ethanol the hydroxyl compound. Most of the biomolecules of

nature like cellulose (used to make paper and cloth), lactose in milk, glucose,

sucrose and fructose found in vegetables and fruits contain large amounts of

-OH groups. Their use is so widespread that many have common names. For

example:

Some of the large variety of plastic materials that we see around is derived

from phenolic resins and plastics. Ethers are widely used as solvents both in

the laboratory and industry because of their almost inert nature and good

dissolving power. Life would have been different without these compounds.

Classification

Alcohols are compounds of the general formula ROH, where R is any alkyl or

substituted alkyl group. The group may be open-chain or cyclic; it may

contain a double bond, a halogen, an aromatic ring, or additional hydroxyl

groups. Based on these possibilities all alcohols are divided into two broad

categories called aliphatic alcohols and aromatic alcohols.

Aliphatic Alcohols

The aliphatic alcohols are a homologous series of compounds containing oneor more hydroxyl groups [-OH] attached to an alkyl radical.

The aliphatic alcohols can be regarded as derivatives of alkanes in which one

or more hydrogen atoms have been replaced by hydroxyl groups [-OH]. The

general formula of saturated aliphatic alcohols is CnH2n+1OH, where n=1,2,3,


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etc. The saturated carbon chain is often designated by the symbol R, so that

ROH can represent any alcohol in the homologous series.

Methanol and ethanol are the first two members of the series.

Methanol: CH3-OH

Ethanol:

The next two members of the series are propanol and butanol, whose names

also end in ...ol, which means the molecule is an alcohol.

Propanol: CH3-CH2-CH2-OH

Butanol:

Other examples of aliphatic alcohols with a closed ring cyclclic structure are,

Compounds of this type with one hydroxyl group per molecule are known as

monohydric alcohols.

Aromatic Alcohols

Any of the compounds containing the hydroxyl group in a side chain to a

benzene ring are aromatic alcohols. An aromatic alcohol where a methanol is

bonded to a benzene is benzyl alcohol.


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Mono, Di, Tri or Polyhydric Compounds

Aliphatic and aromatic alcohols and phenols are classified as monohydric,

dihydric, trihydric and polyhydric compounds according to the number of

hydroxyl groups in their molecules. Monohydric alcohols have one hydroxyl

group per molecule, dihydric alcohols have two hydroxyl groups and

polyhydric alcohols contain three or many hydroxyl groups in their structures

as shown below:

Monohydric alcohols are classified according to the hybridization of the carbon

atom i.e., sp3or sp2to which the hydroxyl group is attached.

Compounds Containing sp3C-OH BondThe sp3hybridized monohydric alcohols are classified into three categories as

primary (1), secondary (2) and tertiary (3) in the same manner as alkyl

halides.


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This classification is important because the type of reaction an alcohol

undergoes depends on what is attached to its carbon. If the carbon to which

the -OH group attached also binds to one carbon atom, the alcohol is

designated primary (1). If two carbons are attached to that carbon, the

alcohol is secondary (2) and if three carbons are attached the alcohol is

tertiary (3). This terminology refers to alkyl substitution of the carbon atom

bearing the hydroxyl group (colored blue).

Allylic Alcohols

When the hydroxyl group is attached next to the double bonded carbon atom

(C=C) then such compounds are called allylic alcohols. The carbon atom to

which the hydroxyl group is attached is sp3 hybridized in such compounds.

Allylic alcohols can also be primary, secondary and tertiary as follows:


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Benzylic Alcohols

In aromatic compounds when the hydroxyl group is attached to a carbon

atom bonded to a benzene ring benzylic alcohols result. Similar to the allylic

alcohols, the carbon atom to which the hydroxyl group is attached is sp3

hybridized in such compounds.

Compounds Containing sp2C-OH BondVinylic Alcohols

Sometimes the hydroxyl group is directly attached to double bonded carbon

atoms (C=C). In such compounds the carbon atom to which the -OH group is

attached is sp2 hybridized and they have special names like vinylic alcohols.

The replacement of hydroxyl group for these types of hydrogen atoms results

in alkyl alcohols having the corresponding common names.


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Aryl Alcohols or Phenols

Aryl alcohols or phenols are hydoxyl derivatives of aromatic hydrocarbons,

which are derived by replacing hydrogen atom attached to sp2hybridizedcarbon atom(s) of the benzene ring by a hydroxyl group. For example,

Ethers

Like alcohols ethers are divided into two broad categories called aliphatic

ethers and aromatic ethers.

Aliphatic Ethers

Aliphatic ethers are those in which R and R' are both alkyl groups. For

example,

Ethers in which the two alkyl groups R and R' are identical they are simple

ethers or symmetrical ethers e.g., CH3.O.CH3.

Aromatic Ethers


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Aromatic ethers are those in which either one or both R and R' are aryl

groups. For example,

Aromatic ethers are further subdivided into phenolic ethers and diaryl ethers.

Phenolic Ethers

Ethers in which one of the groups is aryl while the other group is alkyl are

called phenolic ethers or alkyl aryl ethers. Examples are:

Diaryl Ethers

In diaryl ethers both the groups are aryl groups. For example:

C6H5-O-C6H5

Diphenyl ether

Both aliphatic and aromatic ethers are also classified as symmetrical and

unsymmetrical ethers.

Symmetrical Ethers


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Many simple ethers are symmetrical, in that the two alkyl substituents are the

same CH3.O.CH3. These are named as 'dialkyl ethers'. Examples are: CH3-

CH2-O-CH2-CH3, diethyl ether (sometimes referred to as ether), and CH3-O-

CH2-CH2-O-CH3, ethylene glycol dimethyl ether (glyme).

Unsymmetrical Ethers

If the ethers are different they are mixed ethers e.g., CH3.O.CH2H5and are

called unsymmetrical ethers.

Nomenclature

Alcohols

Since the vast majority of alcohols have their -OH groups attached to

alkanes, their names are most easily generated by using those alkanes as

their base. Alcohols have the highest priority of the groups so if an -OH is

present it's an alcohol.

Common System

The common name for alcohols is generated simply by taking the name of

the alkane chain and adding alcohol to the end. This is how common names

are usually generated. The isopropyl alcohol molecule provides an example.

In CH3-CHCl-CH3 if one replaces the Cl in isopropyl chloride with -OH

isopropyl alcohol is obtained.

For monofunctional alcohols, this common system consists of naming the

alkyl group followed by the word alcohol.


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One should also know the common names of a few popular alcohols:

IUPAC System

Alcohols are given IUPAC names based on the alcohol being derivatives of

the parent alkane. This is common for the carbon-carbon double and triple

bonds which have the respective suffixes ene and yne. In the IUPAC system

of nomenclature, functional groups are normally designated in one of two

ways: the presence of the function may be indicated by a characteristic suffix

and a location number.

Alcohols are usually named by the first procedure and are designated by an

ol suffix, as in ethanol, CH3CH2OH (note that a locator number is not needed

on a two-carbon chain). On longer chains the location of the hydroxyl group

determines chain numbering.


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A stepwise process for naming alcohols is as follows:

Find the longest carbon chain which includes the carbon to which the -

OH group(s) is attached. Select this as parent chain containing the hydoxyl

group. Name the molecule as if it were an alkane, replacing -e with -ol.

With complex alcohols containing multiple functional groups, the longest

chain will be the hydroxy-containing chain with the largest number of

principle functional groups such as alcohol, alkene etc., but not halide,

sulfide, ether etc.

Number the longest chain from the hydroxyl group end. Start counting

from the end closest to the -OH group.

Place the -OH location prior to that chain name using numbers (if

required to remove ambiguity).

When more than one -OH functionality is present i.e., polyhydric

alcohols retain the 'e' of alkane and use the endings '-diol', '-triol,' etc., for

two or three hydroxy groups in the molecule.


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Number all the substituents by position on the chain. The positions of

the substituent alkyl (or other) groups are denoted by using the lowest

possible numbers for the associated carbon atoms in the main chain. If

there is more than one 'type' of substituent e.g., methyl.... and ethyl..

etc., they are written out in alphabetical order in composing the final name

(di, tri are ignored in using this rule).

Many functional groups have a characteristic suffix designator, and only one

such suffix (other than "ene" and "yne") may be used in a name. When the

hydroxyl functional group is present together with a function of highernomenclature priority, it must be cited and located by the prefix hydroxy

along with an appropriate number. For example, lactic acid has the IUPAC

name 2-hydroxypropanoic acid.

Cycloalcohols (Cycloalkanols) are named on the basis of the number of

carbon atoms in the ring (minimum 3) and the prefix 'cyclo' and the suffix

'ol'. The OH attached to the carbon is numbered 1. The prefix alkane name

e.g. 'prop' has an 'a' added but leaves out the end 'ne' if more than one OH

group e.g., in mono-hydroxy alcohols it is propan... and in diols/triols etc., it

is propane.

Other examples of IUPAC nomenclature are shown below, together with the

common names often used for some of the simpler compounds.

Common Name IUPAC Name Structure

methanol methanol

ethanol ethanol

n-propanol 1-propanol

isopropanol 2-propanol


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n-butanol 1-butanol

sec-butanol 2-butanol

isobutanol 2-methyl-1-propanol

tert-butanol 2-methyl-2-propanol

cyclohexanol cyclohexanol

octanol or caprylic

alcohol

2-octanol

ethylene glycol ethane-1,2-diol

glycerol propane-1,2,3-triol

menthol 5-methyl-2-isopropyl

cyclohexanol

D-glucose 2,3,4,5,6-

pentahydroxyhexanal

Phenols

The general formula for monohydric phenols is ArOH, Ar = phenyl,

substituted phenyl, or any other aryl group.

When the -OH is attached directly to a benzene ring the molecule is called a

phenol.


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Common System

All substituted phenols are named as derivatives of phenol. The substituted

compounds are named in terms of ortho, meta and para in the common

system.

IUPAC System In the IUPAC system all substituted phenols are named as

derivatives of phenol. The position of the substituent with respect to hydroxylgroup is indicated by Arabic numerals with the carbon carrying the OH group

being numbered 1 e.g., ortho (1,2- disubstituted), meta (1,3-disubstituted)

and para (1,4-disubstituted).

If there is a 'higher ranking' functional group in the molecule, the substituent

OH is called by the prefix 'hydroxy'. Example:


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In the IUPAC name the di, tri and poly hydroxy phenols are named as

hydroxyl derivatives of benzene.

Ethers

Ethers are compounds having two alkyl or aryl groups bonded to an oxygen

atom, as in the formula R1-O-R2.

Common System

In simple ethers containing no other functional groups, name the two alky or

aryl groups linked to the oxygen atom in alphabetical order and add the word

ether, e.g., CH3CH2-O-CH2CH2CH3(Ethyl propyl ether)

In case of symmetrical ether the prefix 'di' is used before the name of the

alkyl or aryl group.


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IUPAC System

The ether functional group does not have a characteristic IUPAC

nomenclature suffix, so it is necessary to designate it as a substituent. The

ether functional group is named as an alkoxysubstituent on the parentcompound. Ethers are named on the basis of the longest carbon chain with

the O-R or alkoxy group. The common alkoxy substituents are given names

derived from their alkyl component, e.g. methoxy CH3O- or ethoxy CH3CH2O-

etc., treated as a substituent group.

Alkyl Group Name Alkoxy Group Name

CH3- Methyl CH3O- Methoxy

CH3CH2- Ethyl CH3CH2O- Ethoxy

(CH3)2CH- Isopropyl (CH3)2CHO- Isopropoxy

(CH3)3C- tert-Butyl (CH3)3CO- tert-Butoxy

C6H5- Phenyl C6H5O- Phenoxy

The parent compound is the longest carbon chain if both are alkyl groups;

but with any primary functional group (alkene, alcohol, aldehyde, acid etc.)

ethers become substituent groups on that primary chain, for example:


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CH3CH=CHCH2OCH2CH3is 1-Ethoxy-2-butene.

Structure of Functional Groups

In alcohols, phenols and ethers the carbon-oxygen bond is the most powerful

bond that makes their chemistry different. A carbon-oxygen bond is a

covalent bond () between sp3hybridized carbon and oxygen atoms. Here,

oxygen with its 6 valence electrons prefers to share two electrons by bonding

with carbon, leaving the remaining 4 non-bonding electrons as 2 lone pairs.

Alcohols

In alcohols the simplest representatives of the C-O bond can be thought of

organic derivatives of water. They also contain hydroxyl (OH) group as the

functional group attached to an alkyl group.

Bond Length

The bond lengths for C-O bonds and O-H bonds are in the range of 143 and

96 picometer respectively, which is shorter than that of C-C or C-H bonds.

The shortening of C-O and O-H is due to increasing electronegativity of O

(Electronegativity C vs O = 2.55 : 3.44).


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Bond Poarity

An alcohol molecule is dipolar in nature with the oxygen carrying a partial

negative charge (-) and carbon and hydrogen each carrying a partial positive

charge (+). For example in the case of methanol, the charge separation is as

follows:

This is supported by the face that the methanol has a dipole moment of 1.71

D.

Bond Strength

The C-O bond strength is larger compared to C-N or C-C, for example, it is

91 kcal/mol (298 K) in methanol (87 in methyl amine and 88 in ethane).

Bond Angle

All the C-C-H, H-C-H, C-O-H, C-C-O or H-C-O bond angles are approximately

109o in all the non-cyclic alcohols or alkanols. This is slightly less than the

tetrahedral angle (109 28) due to the repulsion between the two lone

electron pairs of oxygen. As the CO bond is strongly polarized towards

oxygen many alcohols are water soluble.


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Phenols

In phenolsthe hydroxyl (OH) functional group attached to an aryl group.

Phenol had a lower dipole moment of 1.54 D than methnaol. Shortened

single bonds are found in phenols (136 pm). This is due to partial double

bond character of the sp2 hybridized carbon of the benzene ring, and

conjugation of unshared electron pair of oxygen with the aromatic ring.

Ethers

In ethers, the oxygen atom forms a bridge between two alkyl groups by

forming two single bonds each with the alkyl C atoms just like the one in

water where the oxygen forms a bridge between the two hydrogen atoms.

The structure of diethyl ether (Ethoxy ethane) is shown below.


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Two of the four sp3orbitals of the oxygen atom overlap with sp3 orbitals of

the alkyl groups to form two sp3-sp3() bonds with the C-O bond length of

141 pm almost equal to that in alcohols. The remaining two sp3 orbitals of

oxygen contain a lone pair of electrons each.

The four electron pairs, i.e., the two bond pairs and two lone pairs of

electrons on oxygen are arranged approximately in a tetrahedral

arrangement. The bond angle of 110ois slightly greater than the tetrahedral

angle due to the repulsive interaction between the two bulky (-R) groups.

Alcohols and Phenols

As mentioned earlier, alcohols refer to a class of chemical compounds

consisting of atoms of carbon, hydrogen and oxygen. A variety of alcohols

exist and they contain at least one hydroxyl group (OH-) as the functional

group in them. The oxygen atom of the hydroxyl group is bonded to the

carbon atom.

Although alcohols are mostly soluble in water they do not get ionized by

water i.e., no hydroxide ion (OH-) is formed. Hence, alcohols have the

characteristic property of the alcoholic group.

For example, in alcohols like ethanol and phenol, -OH is the functional group.


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Preparation of Alcohols

Hydration of Alkenes:

In an aqueous acid solution, alkenes add water across the double () bond

thereby saturating the molecule producing an alcohol. The reaction is region

selective since only the more highly substituted alcohol is produced

(Markovnikov's rule).

In accordance with Markovnikov's rule reaction with water and sulphuric acid

as catalyst gives the predicted product. In these reactions molecular

rearrangement often occurs which indicates that the reaction involves a

carbocation intermediate. The reason why Markovnikov's Rule always results

in the more substituted alcohol can be understood if one looks at the reaction

mechanism proposed for this reaction.

In the first step the hydrogen ion from the acid catalysts attaches itself to

the alkene at the double bond to the carbon which will result in the most

stable carbocation.


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In the next step the carbocation attaches itself to an oxygen atom of a water

molecule with the aid of a lone pair making the oxygen atom positively

charged oxonium ion.

In the final step, a water molecule pulls a hydrogen ion off the oxygen atom

to produce the final alcohol.

The obvious synthetic problem with this reaction is that it may undergo

molecular rearrangement thereby producing more than one alcohol. In

addition it always produces the more substituted alcohol.

Hydroboration-oxidation

Alkenes react with diborane (B2H6) to form trialkylboranes, which upon

subsequent treatment with alkaline H2O2gives alcohol.

The principal adding reagent in this reaction is borane (BH3), but borane is

much too reactive in air to be stored in a bottle. Therefore diborane is used

in a tetrahydrofuran solvent (THF) where it cleaves and the resulting borane

molecules are stabilized by complexing with the oxygen atom in the cyclic

ether, THF. Borane adds across the double bond of the alkene, with one of

the hydrogens attaching to the more substituted sp2

carbon and the boron

attaching to the least substituted sp2carbon.


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This is because boron being a larger atom needs more room and is less

sterically hindered on the least substituted carbon. As long as boron has

hydrogen attached to it, this reaction will occur again with another alkene

molecule until the boron is attached to three alkene molecules producing the

organoborane an addition product. Even the organoboranes will react with

air, so they must be kept in a suitable solvent like THF.

Organoboranes are useful in themselves. They can be used to produce

alkanes from alkenes by reduction of the organoborane with acetic acid

under reflux conditions or alcohol by oxidation with hydrogen peroxide in

presence of aqueous NaOH.

This replaces boron with a hydrogen atom on each of the three organic

groups attached to the boron thus producing three molecules of alcohol for

every one molecule of the organoborane.

Oxymercuration-reduction

Alkenes react with mercuric acetate (CH3COO)2Hg to form adducts which

upon reduction with NaBH4in basic medium gives alcohol.


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This reaction occurs in 3 stages. In the first part mercury (II) acetate,

Hg(oAc)2 forms a bridged species with the mercury atom, bridging the two

sp2carbons.

Then in the second step a water molecule attaches itself to the least

substituted of the sp2carbons in the bridged ring intermediate with the aid of

a lone pair on the oxygen atom of the water molecule. This produces the

oxonium intermediate and breaks the bridged structure.

In a third step an acetate ion pulls a hydrogen ion off the oxygen to produce

the hydroxy organo mercury product. Now a strong reducing agent sodium

borohydride in basic solution is added to the mercury compound when the

mercury group is replaced by a hydrogen atom to produce the final alcohol

product.


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Like the hydration reaction, the oxymercuration-reduction reaction is region

selective and follows Markovnikov's rule, producing only the more substituted

alcohol. However, no molecular rearrangement is possible in this reaction

indicating that no carbocation is produced. This is a distinct advantage over

acid catalyzed hydration giving less product mixing.

Hydrolysis of Haloalkanes

Haloalkanes on treatment with aqueous solution of KOH or moist silver oxide

(Ag2O/H2O) give alcohols.

Reduction of Acyl (Carbonyl) CompoundsAn acyl compound is one that has a carbon doubled bonded to an oxygen

(C=O) in its molecule. This group is also referred to as a carbonyl group.

There are several categories of acyl compounds:

Aldehydes

Ketones

Carboxylic Acids

Esters

Acyl compounds may undergo reduction with a suitable reducing agent. The

reducing agent undergoes oxidation in the process.


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Organic Reducing AgentsThere are four reducing agents that will reduce

one or more of the acyl compounds.

Lithium Aluminum Hydride, LiAlH4(LAH)

Sodium Borohydride, NaBH4(NBH)

H2/Cu + CuCr2O4at 5000psi, 175oC

H2/Pt

Lithium aluminum Hydride is stronger than sodium borohydride and will

reduce any of the acyl compounds to products shown above. Indeed, lithium

aluminum hydride is so reactive that it must be prepared in a totally

anhydrous environment as it reacts explosively with water.

Sodium Borohydride is a weaker reducing agent that is less effective and is

capable of reducing only the aldehydes and ketones; it will not reduce

carboxylic acid or ester functions.

The use of hydrogen gas in the presence of Cu and copper (II) chromate is

called 'hydrogenolysis' and is a common way of reducing esters in small scale

laboratory reductions. This is also called high pressure hydrogenation and

copper and copper (II) chromate serves as catalysts.

The fourth method uses hydrogen gas in the presence of a noble metal such

as platinum to reduce aldehydes, ketones, and carboxylic acids.

Aldehydes


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These can be reduced to primary alcohols in presence of platinum or

palladium metals or sodium borohydride.

Ketones

These can be reduced to secondary alcohols in presence of sodium

borohydride.

Carboxylic Acids

These acids are reduced to primary alcohols via aldehydes by lithium

aluminum hydride.


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Esters

Esters are reduced to two alcohols, one from the acid fragment of the ester

and one from the alcohol fragment by hydrogenolysis.

Reaction with Organometallic Reagents

Organometallic reagents are organic compounds that are bonded to a metal.

There are a number of important organometallic reagents. Of these, two are

important in the synthesis of alcohols i.e., organomagnesium (Grignard's

reagent), R-+MgX and organolithium, R-Li+.

Grignard's reagent react with aldehydes, ketones and esters to form additionproducts (adducts).

Upon decomposition with water in presence of dilute HCl or H2SO4, the

adducts give alcohol.


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When a Grignard's reagent reacts with formaldehyde a primary alcohol is

produced. Indeed, this is the only way to prepare a primary alcohol via an

organometallic reagent.

Grignard's reagent reacts with all other aldehydes to produce secondary

alcohols.

Grignard's reagent reacts with ketones to produce tertiary alcohols.

Hydrolysis of Esters

Alcohols are prepared from naturally occurring esters by hydrolysis with

aqueous alkalis. This reaction is called saponification reaction.

Reaction of Primary Amines

Primary aromatic amine reacts with nitrous acid to form a diazonium salt,

which liberates nitrogen to form an unstable carbonium ion. These carbonium


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ions react in aqueous medium to form an alcohol.

Preparation of Phenols

Phenols are prepared by the reaction of benzene derivatives with different

reagents.

Alkali Treatment of Haloarenes

Chlorobenzene is converted into phenol when it is heated with 6-8% aqueous

NaOH at temperatures above 623 K under 300 atm. pressure. This reactionis also called as Dow's process.

Hydrolysis of Aromatic Diazonium Salts

Aromatic diazonium salts on treatment with water or preferably with dilute

acid form phenols. The reaction is usually carried out by adding the solution

of a diazonium salt to a large volume of boiling dilute sulphuric acid. The

acidic conditions minimize the side reaction of the coupling of the phenol with

diazonium salt.


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This is one of the most convenient methods of preparing phenols.

Reaction with Grignard's Reagent

When a solution of an aryl halide (bromo benzene) in dry ether is treated

with magnesium, an aryl magnesium halide (phenylmagnesium bromide) is

formed.

When oxygen is bubbled through an ethereal solution of phenylmagnesium

bromide, it forms an addition product.

The treatment of the adduct with dilute mineral acid gives phenol.


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Alkali Fusion of Benzene Sulfonic Acid

This is one of the oldest methods useful for the industrial synthesis of

phenol. When benzene sulphonic acid is fused with NaOH at 300-350oC, it

forms sodium benzene sulphonate as an intermediate. The reaction is

followed by an acidification reaction to neutralize the phenoxide and give

phenol. This reaction occurs via the addition-elimination mechanism with

SO32-functioning as the leaving group.

Acidic Oxidation of Cumene

Oxidation of cumene (2-phenylpropane or isopropylbnezene) at the benzylic

position gives hydroperoxide which upon subsequent hydrolysis with aqueous

acid cleaves to give phenol and acetone.


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Low-cost reagents and important products make this process attractive.

Decarboxylation of Salicyclic Acid

Phenol is obtained by the decarboxylation of sodium salt of salicyclic acid

with soda lye (CaO + NaOH) followed by acidification with dilute HCl.

Physical Properties

The physical properties of an alcohol are best understood if one thinks of it

as a composite of an alkane and water. As such it contains a lipophilic,

alkane-like group and a hydrophilic, water-like hydroxyl group. The hydroxyl

-OH group gives the alcohol its characteristic physical properties, while the

alkyl group which modifies these properties.

Boiling Point:

Alcohols have a very high boiling point when compared to the related

alkanes.

Boiling point increases with increasing molecular weight.

The boiling point of the diols is quite high, hence their use as an

additive to car radiators.


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Melting Point:

Melting point increases with increasing molecular weight, expectedly,

because of the of van der Waal attraction.

Symmetric branching increases melting point.

Specific Gravity:

Specific gravity or density increases with increasing molecular weight.

Alkyl alcohols are less dense than water, cyclic alcohols are

approximately the same as water, and aromatic alcohols and diols are

greater than water.

Water Solubility:

Lower weight alcohols, up to C4, are infinitely soluble in water.

Solubility decreases with increasing molecular weight as

hydrophobicity of alkyl groups dominates the nature of the molecule.

Most diols are infinitely soluble in water.

Name M.P. oC B.P. oC S.G. 20oC Solub g/100

Methyl -97 0.793 I

Ethyl -115 0.789 I

n-Propyl -126 0.804 I

n-Pentyl -78.5 0.817 2.3

n-Octyl -15 0.825 0.05

n-Decyl 6 0.829 ----


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Isopropyl -86 0.789 I

Isobutyl -108 0.802 10.0

tert-Butyl 25.5 0.789 I

Cyclohexanol 24 0.962 ---

Allyl -129 0.855 I

Benzyl -15 1.046 4

Ethylene Glycol -16 1.113 I

Propylene Glycol 1.040 I

Glycerol 18 1.261 I

Acidity of Alcohols:

Alcohols are weakly acidic, as might be expected due to their similarity to

water. They can also function as weak Lewis bases, becoming protonated to

give oxonium ions:

pKavalues: CH3CH2OH = 16.0; H2O = 15.7; CH3OH = 15.5; HCl = -7.0.

Hydrogen Bonding

Alcohols and phenols differ significantly from hydrocarbons (alkanes,

alkenes, alkynes, and aromatics) and alkyl halides in their melting points and

their boiling points. Hence, for short chain alcohols and for phenols one finds

much higher boiling points than one would expect on the basis of their sizes.

The most dramatic differences occur with the smallest molecules: methane

(MW = 16), chloromethane (MW = 50), and methanol (MW = 32) with

boiling points of -161.4 C, -23.7 C, and 64.5 C, respectively. Thus in alcohols


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boiling points are not related to size. They obviously require a different

explanation.

Compound M

W

Boiling Pt

(C)

Sol. in Water

(g/100mL)

CH3CH2OH 46 78 100% Soluble

CH3CH2CH3 44 -42 Insoluble

CH3CH2CH3OH 60 97 100% Soluble

CH3CH2CH2CH3 58 0 Insoluble

CH3CH2CH2CH2CH2

OH88 138 2.3 g/100mL

CH3CH2CH2CH2CH3C

H286 69 insoluble

The high boiling points of alcohols can be explained by the existence of

hydrogen bonds (H-bonds). This is the same explanation for the incredibly

high boiling point of water (water has about 4 H-bonds per molecule, vs.

about 2 for alcohols). Hydrogen bonds involve a very strong polar attraction

combined with very small atomic radii, resulting in a partial covalent bond

being formed. Although this bond is significantly weaker than the covalent

bonds common in organic molecules (i.e. 400 kJ/mol for O-H bonds vs. about

20 kJ/mol for an H-bond), but is much stronger than the weak van der Waals

forces holding alkanes together (about 0.4 - 0.8 kJ/mol).


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The -OH group is highly polar and is capable of hydrogen bonding with other

alcohol molecules and also to other neutral molecules and to anions. This

property is particularly important in defining boiling point and aqueous

solubility. The increased intermolecular bonding increases the energy needed

to boil by decreasing the vapor pressure. As expected the polar nature

increases with the diols.

Chemical Reactions

The functional group of the alcohols is the hydroxyl group, -OH. Unlike the

alkyl halides, this group has two reactive covalent bonds, the C-O bond and

the O-H bond. Accordingly, alcohols and phenols will undergo two distinct

types of reactions.

Reactions involving the cleavage of the O-H bond with substitution or

removal of hydrogen as a proton.

Reactions involving the cleavage of the C-O bond with substitution or

removal of OH group.

Nature of the -OH Group

The electronegativity of oxygen is substantially greater than that of carbon

and hydrogen. Consequently, the covalent bonds of this functional group are

polarized so that oxygen is electron rich and both carbon and hydrogen are

electrophilic. As a result, there is a low electron density on H atom of -OH

group alcohol, and with H+ character being more, alcohols are acidic (pKa ~

16).

Acidity of Alcohols

Nevertheless, alcohols are weaker acids than water (pKa~ 14) because the

electron releasing inductive effect of the alkyl groups increases the electron

density around the oxygen atom. As a result the electrons of the O-H bond


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are not sufficiently attracted towards the oxygen atom and so the release of

proton becomes difficult.

On comparing the acidity of primary, secondary and tertiary alcohols the

electron releasing inductive effect of the alkyl groups is maximum in tertiary

alcohol and minimum in primary alcohols. Thus, primary alcohols are the

strongest acids while tertiary alcohols are the weakest. The acidic strength of

alcohols follows the order:

Primary > Secondary > Tertiary

In addition, the presence of lone pairs in the alcohol O atom makes it a

region of high electron density. Alcohol oxygen atoms are Lewis bases and

allow the ability to alcohols to react as either bases or nucleophiles at the

oxygen atom.

Indeed, the dipolar nature of the O-H bond is such that alcohols are much

stronger acids than alkanes - by roughly 1030 times, and nearly that much

stronger than ethers which are oxygen substituted alkanes that do not have

an O-H group. The most reactive site in an alcohol molecule is the hydroxyl

group, despite the fact that the O-H bond strength is significantly greater

than that of the C-C, C-H and C-O bonds, demonstrating again the difference

between thermodynamic and chemical stability.

Removal of the proton generates the anion - alkoxide ion. Alkoxides are

important bases in organic chemistry.

Acidity of Phenols

In some respects, the chemical behavior of phenols is different from that of

the alcohols, so it is sensible to treat them as a similar but characteristically

distinct group.


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In contrast, with phenols the substitution of the hydroxyl hydrogen atom is

even more facile, and makes them roughly a million times more acidic than

equivalent alcohols.

This phenolic acidity is further enhanced by electron-withdrawing

substituents ortho and para to the hydroxyl group, as displayed.

The alcohol cyclohexanol is shown for reference at the top left. It is

noteworthy that the influence of a nitro substituent is over ten times stronger

in the para-location than it is in meta, despite the fact that the latter position

is closer to the hydroxyl group. Furthermore additional nitro groups have an

additive influence if they are positioned in ortho or para locations. The

trinitro compound shown at the lower right is a very strong acid called picric

acid.

Why is phenol a much stronger acid than cyclohexanol? To answer this


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question one must evaluate the manner in which an oxygen substituent

interacts with the benzene ring. In electrophilic aromatic substitution

reactions, an oxygen substituent enhances the reactivity of the ring and

favors electrophile attack at ortho and para sites. It is proposed that

resonance delocalization of an oxygen non-bonded electron pair into the pi-

electron system of the aromatic ring was responsible for this substituent

effect.

The resonance stabilization in these two cases is very different. The

contributing structures to the phenol hybrid all suffer charge separation,

resulting in very modest stabilization of this compound. On the other hand,

the phenolate anion is already charged, and the canonical contributors act to

disperse the charge, resulting in a substantial stabilization of this species.

The conjugate bases of simple alcohols are not stabilized by charge

delocalization as in phenols, so the acidity of these compounds is similar to

that of water.

An energy diagram showing the effect of resonance on cyclohexanol and

phenol acidities is shown below. Since the resonance stabilization of the


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phenolate conjugate base is much greater than the stabilization of phenol

itself, the acidity of phenol relative to cyclohexanol is increased. However,

phenols are weaker acids than carboxylic acids (pKa= 5) or even carbonic

acids (pKa= 7).

The phenolate negative charge is delocalized on the ortho and para carbons

of the benzene ring and comes from the influence of electron-withdrawing

substituents at those sites. Electron withdrawing substituents (e.g., NO2) can

make phenols as acidic as many carboxylic acids.

Electron donating substituents like -NH2, -OR, -R etc., which destabilize the

phenoxide ion tend to decrease the acid strength of phenols. The table given

below summarizes some of these facts.

Compound pKa Compound pKa

Phenol 10.0


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o-Methoxyphenol 10.0 p-Methoxyphenol 10.2

o-Methylphenol 10.3 p-Methylphenol 10.3

o-Chlorophenol 8.6 p-Chlorophenol 9.4

o-Nitrophenol 7.2 p-Nitrophenol 7.2

m-Nitrophenol 8.4

The -OH group is a poor leaving group and needs to be converted to a better

leaving group before substitution can occur.

Reactions Involving the Cleavage of the O-H Bond

Both alcohols and phenols readily undergo cleavage of the O-H Bond. The

ease of cleavage follows the sequence:

Phenols > primary alcohols > secondary alcohols > tertiary alcohols

Electrophilic Substitution at Oxygen

Substitution of the Hydroxyl Hydrogen

Because of its enhanced acidity, the hydrogen atom on the hydroxyl group is

rather easily replaced by other substituents. A simple example is the facile

reaction of simple alcohols with alkali metals like sodium or potassium similar

to water to give the alkoxide:

The mechanism by which many substitution reactions of this kind take place

is straightforward. The oxygen atom of an alcohol is nucleophilic and is

therefore prone to attack by electrophiles. The resulting 'onium' intermediate

then loses a proton to a base, giving the substitution product. If a strong

electrophile is not present, the nucleophilicity of the oxygen may be


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enhanced by conversion to its conjugate base (an alkoxide). This powerful

nucleophile then attacks the weak electrophile. These two variations of the

substitution mechanism are illustrated in the following diagram.

Like the alcohols, the phenolic hydroxyl hydrogen is rather easily replaced by

alkali metals.

However, phenol reacts easily with alkalis itself to form phenoxides or

phenates.

Alcohols do not react with NaOH solution and so this test distinguishes

phenols from alcohols.


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Reaction with Alkyl Halides

Alkyl substitution of the hydroxyl group leads to ethers. Alkoxide salts react

with alkyl halides to give ethers by the Williamson ether synthesis reaction.

This reaction provides examples of both strong electrophilic substitution, and

weak electrophilic substitution. The weak electrophilic substitution is a SN2

reaction and is generally used only with primary alkyl halide reactants

because the strong alkoxide base leads to E2 elimination of secondary and

tertiary alkyl halides.

Primary alkyl halides are most reactive (E2 competes with more hindered

substrates). Thus unsymmetrical ethers are best prepared with the less

hindered member being the alkyl halide, and the more hindered member

being the alkoxide. For example, if we use a tertiary alkoxide ion and a

methyl halide the SN2reaction will predominate:

However, if a primary (or any other) alkoxide ion attacks a tertiary halide the


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reaction is blocked by steric hindrance and thus the E2 elimination is

preferred:

The reaction of phenols with alkyl halides is carried out by first dissolving

phenols in NaOH solution to form the phenoxide and then heating it with the

alkyl halide to from the ether.

The phenate ion is an excellent nucleophile.

Reaction with Carboxylic Acids Alcohols react with carboxylic acids in the

presence of a few drops of concentrated H2SO4or dry HCl acid as catalyst, to

form esters.

This reaction is called Fischer esterification reaction and is usually slow and

exothermic. Phenols do not easily undergo the esterification reaction with

carboxylic acids because the reaction is endothermic.


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Reaction with Acid Chlorides and Anhydrides

When alcohols or phenols are treated with acid chlorides or anhydrides in the

presence of a base such as pyridine or dimethylaniline, the H-atom of the -

OH group is replaced by the acyl (RCO-) group to form esters.

Phenols react with acid chlorides or acetic anhydride to give phenyl acetate.

Similar to acylation, the introduction of acetyl (CH3CO) group in alcohols or

phenols is known as acetylation.

The reaction of alcohols and phenols with benzoyl chloride in presence of


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aqueous NaOH is called Schotten Baumann reaction.

Reaction with Grignard's reagent

Both alcohols and phenols react with Grignard's reagent to form

hydrocarbons.

Reactions Involving the Cleavage of the C-O Bond

Astep toward improving the reactivity of alcohols in breaking the C-O bond

would be to modify the -OH functional group in a way that improves its

stability as a leaving anion.

One such modification is to conduct the substitution reaction in strong acid

so that -OH is converted to -OH2(+). Since the hydronium ion (H3O(+)) is a

much stronger acid than water, its conjugate base (H2O) is a better leaving

group than hydroxide ion. The only problem with this strategy is that many

nucleophiles, including cyanide, are deactivated by protonation in strong

acid, effectively removing the nucleophilic co-reactant needed for the

substitution.

The strong acids HCl, HBr and HI are not subject to this difficulty because

their conjugate bases are good nucleophiles and are even weaker bases than

alcohols. Nucleophilic substitution of 1 -alcohols proceeds by a SN2

mechanism, whereas 3 -alcohols react by a SN1mechanism. Reactions of 2 -

alcohols may occur by both mechanisms and often produce some rearranged

products.


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Action of Halogen Acids

Alcohols when treated with halogen acids (HX) get converted into

haloalkanes. This reaction does not occur with phenols. The nature of alcohol

and the halogen acid both influence the rate of the reaction.

Primary and secondary alcohols form chloroalkanes when hydrochloric acid

gas is passed through alcohol in the presence of anhydrous zinc chloride

(Groove's process).

ZnCl2is a Lewis acid and it readily coordinates with the oxygen atom of the

alcohols. As a result, the C-O bond weakens and breaks to form carbocation,

which reacts with chloride ion to form chloroalkanes. Thus, anhydrous ZnCl2

helps in the cleavage of the C-O bond.

Tertiary alcohols are very reactive and therefore, they react readily with

concentrated HCl even in the absence of zinc chloride.

The three classes of alcohols show difference in reactivity with HCl. In fact

this is one of the methods called Lucas test that is used to distinguish them

from one another. For example, primary alcohols on reacting with Lucas

reagent (conc. HCl and ZnCl2) does not produce turbidity at room

temperature while in case of tertiary alcohols, turbidity is produced


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immediately since they form the halides easily.

Similarly, when an alcohol is heated with hydrobromic acid (48%) bromo

alkanes are obtained. HBr can also be generated in situ (during the reaction)

by the action of concentrated H2SO4on KBr or NaBr.

On heating alcohols with constant boiling hydroiodic acid (57%) generated in

situ by the action of phosphoric acid on potassium iodide, iodoalkanes are

obtained.

Secondary and tertiary alcohols cannot be used to prepare respective

bromides and iodides unlike alkyl chlorides. This is because secondary and

tertiary alcohols undergo dehydration on heating with concentrated H2SO4, to

form alkenes.

The order of reactivity of halogen acids on alcohols is in accordance with the

bond dissociation energies of H-X bonds:

HI > HBr > HCl

Reactivity of alcohols towards this reaction is:

Tertiary > Secondary > Primary

The cleavage of C-O bond becomes easy and reactivity increases when the

polarity of C-OH increases with the number of electron releasing groups on

the -carbon atom of the alcohol.


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Action of Phosphorus Halides

Phosphorus halides react with alcohols to form haloalkanes. The action of

phosphorus pentachloride (PCl5) or phosphorus trichloride (PCl3) on alcohols

brings about the cleavage of C-O bond.

Likewise, bromoalkanes and iodoalkanes are generated by the action ofphosphorus tribromide (PBr3) and phosphorus tri-iodide (PI3) respectively on

alcohols. As PBr3and PI3, are not very stable compounds, they are prepared

in situ by the action of red phosphorus on Br2, or I2, as follows:

Treatment of phenols with PCl5gives only a small amount of chlorobenzene,

the major product being tiphenyl phosphate.

Action of Thionyl Chloride

By refluxing alcohols with thionyl chloride in the presence of pyridine, chloro

alkanes can also be prepared from alcohols.


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Phenols do not react with thionyl chloride.

Action of Ammonia

When a mixture of the vapours of an alcohol and ammonia are passed over

heated alumina (Al2O3) at 633 K, a mixture of primary, secondary and

tertiary amines is produced.

The reaction of ammonia on phenol give aniline under severe conditions only,

as phenol is less reactive towards nucleophilic displacement reactions.

Reaction with Zinc Dust

Phenols but not alcohols on distillation with zinc dust give the corresponding

aromatic hydrocarbon. For example,


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Reactions Involving the Alcohol Molecule as a Whole

There are two types of reactions that alcohols are prone to and engage in;

one is essentially the reverse of addition of water to an alkene. This is called

dehydration (the loss of water from an alcohol). The other is called oxidation.

This reaction results in loss of hydrogen and the conversion of the C-O single

bond to a double bond.

Dehydration of Alcohols

In the case of dehydration either water is removed as the reaction

progresses or the equilibrium already favors alkene production. The

dehydration of alcohols as a method of synthesizing alkenes with the

products usually predicted by Zaitsev's rule - the more highly substituted

alkene will be the major product. This reaction is acid catalyzed, commonlybeing carried out in sulphuric acid or heated alumina requiring higher

temperatures.

Tertiary and secondary alcohols react under mild conditions, such as with

phosphoric acid at lower temperatures.


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Different products are obtained under different reaction conditions and

temperature e.g., ethers are obtained when excess alcohol is used.

The mechanism of the dehydration reaction is an E1elimination reaction:

Alcohols act as bases due o the presence of lone pair of electrons on the

oxygen atom. Therefore they react with strong mineral acid to form the

oxonium salts in the first step.

In the 2ndslow step the C-O bond weakens due to the presence of positive

charge on the highly electronegative oxygen. As a result, the protonated

alcohol readily eliminates a molecule of water to form ethyl carbocation.


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In step 3 the ethyl carbocation being a reactive chemical species looses a

proton to form the ethene molecule.

The relative ease of dehydration is as follows:


This path is essentially the reverse of the water addition path. For interior -

OH groups, the final location of the C=C bond depends on the branching of

the alkane. For example:

In general, the double bond will locate on the carbons with the most

branching, although frequently there will be some of the other product

present.

Oxidation of Alcohols

Simple 1 and 2-alcohols in the gaseous state lose hydrogen when exposed to

a hot copper surface. This catalytic dehydrogenation reaction produces

aldehydes and ketones. In the case of tertiary alcohols alkenes are produced.


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Since the carbon atom bonded to the oxygen is oxidized, such alcohol to

carbonyl conversions are generally referred to as oxidation reactions. Gas

phase dehydrogenations of this kind are important in chemical

manufacturing.

However, when alcohol oxidations are carried out in solution, the hydroxyl

hydrogen is replaced by an atom or group that is readily eliminated together

with the alpha-hydrogen. The most important reactions of alcohols are their

oxidation to aldehydes, ketones, and carboxylic acids (carbonyl compounds)and may be viewed as follows:

The outcome of oxidation reactions of alcohols depends on the substituents

on the carbinol carbon. In order for each oxidation step to occur, there must

be H on the carbinol carbon.

Primary alcohols are oxidized to aldehydes, which in turn may be oxidized to

acids, both containing the same number of carbon atoms as the original

alcohol. Secondary alcohols are oxidized to ketones with the same number of

carbon atoms as the original alcohol. Tertiary alcohols are not normally

oxidized since it would be necessary to break a C-C bond (C-C bonds arevery strong and stable, thus resistant to reaction without destabilizing

structural relations or high energy).


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While secondary alcohols can be oxidized to ketones, only under drastic

conditions further can oxidation occur, when for instance, carboxylic acids

with lesser number of carbon atoms than the original alcohol is formed.

Tertiary alcohols cannot be oxidized easily (no carbinol bond C-H) and only

under strong acid oxidizing agents they convert to a mixture of ketone and

an acid each containing lesser number of carbon atoms than the original

alcohol.

Common oxidizing agents consisting of chromate and manganate species are

summarized below.


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Primary alcohols can be oxidized to aldehydes or further to carboxylic acids.

In aqueous media, the carboxylic acid is usually the major product. However,

they can be oxidized to aldehydes by the careful selection of an oxidizing

agent like PCC or PDC in dichloromethane. The best reagent for laboratory

scale reactions is pyridinium chlorochromate, C5H6NCrO3Cl (PCC), in

dichloromethane solvent:

Other oxidizing agents, such as sodium dichromate (Na2Cr2O7) in aqueous

acid or chromium trioxide (CrO3), continue the oxidation process to the

carboxylic acid product. The intermediate aldehydes are not generally

isolated because they are oxidized to acids too rapidly:


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Secondary alcohols are readily oxidized to ketones:

In each case a chromate ester of the alcohol substrate is believed to be an

intermediate, which undergoes an E2like elimination to the carbonyl product.

The oxidation state of carbon increases by 2; while the chromium is reduced

and decreases by 3. Since chromate reagents are a dark orange-red color (VI

oxidation state) and chromium III compounds are normally green, the

progress of these oxidations is easily observed. Indeed, this is the chemical

transformation on which the Breathalizer test is based.

Jones' reagent: a solution of sodium dichromate in aqueous sulphuric

acid

Oxidation of Phenols

Generally phenols are more easily oxidized than simple alcohols. Oxidation

can be achieved by reaction with silver oxide (Ag2O) or chromic acid

(Na2Cr2O7), or other oxidizing agents.

Particularly important are the oxidation of 1,2- and 1,4-benzenediol

(pyrocatechol and hydroquinone, respectively) and their derivatives.


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These types of systems are important in biological redox-systems such as

coenzyme Q and vitamin K.

Here's a closer look at the two one electron transfers that are believed to

take place when hydroquinone is oxidized to benzoquinone.

Loss of a proton and an electron generates a phenoxy radical.

Loss of a second proton and a second electron completes the oxidation.

Electrophilic Reactions

Phenols are potentially very reactive towards electrophilic aromatic


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substitution. This is because the hydroxy group -OH, is a strongly activating,

ortho-/para- directing substituent. Substitution typically occurs para to the

hydroxyl group unless the para position is blocked, then ortho substitution

occurs.

The strong activation often means that milder reaction conditions than those

used for benzene itself can be used. Phenols are so activated that

polysubstitution and oxidation can be a problem at times.

Halogenation

Phenols react with halogens in the presence of less polar solvents like CS2,

CHCl3and CCl4to give ortho and para isomers of monohalophenols.

Phenols when treated with chlorine or bromine water give polyhalogen

derivatives in which all the H-atoms present at the ortho- and para- positions

with respect to OH group are replaced by chlorine and bromine atoms.


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NitrationWith dilute HNO3at 293 K phenol give mixtures of ortho and para

isomers of mononitrophenols.

The yield of mononitrophenols is quite low due to the partial oxidation of the

ring by HNO3. However, if phenol is first treated with nitrous acid,

nirosophenol is formed which on further oxidation with dilute nitric acid yields

good yeild of mononitrophenol.

The ortho and para isomers can be separated by steam distillation. o-nitrophenol undergoes intramolecular hydrogen bonding while p-nitrophenol

undergoes intermolecular hydrogen bonding. While o-nitrophenol is steam

volatile and p-nitrophenol is less volatile due to the association of molecules

and so can be separated by distillation.


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With concentrated nitric acid and in the presence of sulphuric acid, phenol

gives 2,4,6-trinitrophenol (picric acid). The yield is low due to oxidation of

phenol.

Sulphonation

When phenol is treated with concentrated H2SO4 sulphonation occurs. A

mixture of ortho- and para- isomers are obtained. If the sulphonation is

carried at low temperatures (293 K), the ortho isomer predominates and at

high temperatures (2373 K) the para isomer is the chief product.

Alkylationand Acylation


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The alkylation and acylation reaction, known as 'Friedel-Craft reaction', is

carried by treating phenol with alkyl chloride or acyl chloride in the presence

of a catalyst like anhydrous aluminium chloride. For example,

Carboxylation of Phenols (Kolbe-Schmitt reaction) Heating the

nucleophilic phenolate salt with carbon dioxide under high

pressure/temperature results in regioselective ortho-substitution.

On further acidification the o-and p-isomers of o-hydroxybenzoic acid are

formed. This process is also known as the Kolbe-Schmitt synthesis.


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However, o-hydroxybenzoic acid is the main product and is commonly known

as salicyclic acid. This is the starting material for preparing 2-acetoxybenzoic

acid (aspirin) which is widely used as analgesic and antipyretic. The

acetylation of salicylic acid produces aspirin.

Reimer-Tiemann Reaction

On treating phenol with chloroform in the presence of aqueous sodium or

potassium hydroxide at 340 K followed by hydrolysis of the resulting product,

gives 2-hydroxybenzaldehyde (salicylaldehyde) as the major product and 4-

hydroxybenzaldehyde as a minor product.

If instead of chloroform, CCl4 is used salicyclic acid is formed as the major

product.


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The Reimer-Tiemann reaction occurs by the generation of dichlorocarbene

(:CCl2) electrophile which attacks the ortho- para- electron rich centres to

substitute the hydrogen atom at these positions.

Some Commercially Important Alcohols

Methanol (Wood Alcohol)

Methanol or Methyl Alcohol can be obtained by the destructive distillation of

wood, hence its popular name of wood alcohol. Commercially, methanol is

prepared by the hydrogenation of carbon monoxide in the presence of a

metal oxide catalyst such as zinc oxide and chromic oxide under high

temperature and pressure.

Methane is also used for producing methanol.

Properties

Methanol is a clear, colourless, organic liquid with a pleasant odour. It is

inflammable and has a high calorific value and hence it is used as a fuel.


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It burns with a good smokeless flame.

Methanol is used mostly for industrial purposes. It is an important solvent for

many substances used in dyes, drugs etc. When mixed with water, the

mixture does not freeze even at sub-zero temperatures. Hence, in cold

countries, it is used as antifreeze for automobile radiator.

It is used as a starting material for making other organic compounds e.g.,

when methanol is oxidized by acidified potassium permanganate or

potassium dichromate, formaldehyde is formed.

Formaldehyde on further oxidation gives formic acid.

Methanol is highly poisonous. If consumed it causes blindness due to

destruction of the cells of the optic nerve. If consumed in large amount, it

can cause death.

Spurious alcohol or illicit liquor is often made by improper distillation or by

using methylated spirit. It is cheap and is mostly made available to the lower

strata of our society. Spurious alcohol contains higher percentage of methyl

alcohol which is poisonous. Consumption of such liquor may cause blindness,

serious health problems and even death. Sometimes, to give the consumer

gets a pronounced feeling of 'intoxication', other chemicals are mixed with

ethyl alcohol. Such mixtures can prove to be exceedingly poisonous and can

cause severe damages to the body organs, nervous system resulting in


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death.

Ethanol

Ethanol or ethyl alcohol has been used for thousands of years especially in

the form of wine. Usually the term 'alcohol' refers to ethanol.

The raw materials used for the preparation of alcohol are plant products that

contain some form of starch or sugar. Grapes, barley, rice, potatoes, apples

etc., are examples of such materials. The process of converting the starch to

sugar is called fermentation.

Today, most of the alcohol produced in the world is by the fermentation of

molasses, a brownish syrupy liquid obtained in the sugar industry. It contains

a large amount of sugar, which cannot be further crystallized. Yeast is added

to these molasses, and it is allowed to ferment for usually three weeks. The

enzyme maltase present in yeast converts the sucrose of molasses to

glucose.

Yeast also contains an enzyme zymase, which converts glucose to ethanol

and carbon dioxide.

The alcohol so formed is separated by fractional distillation to obtain rectified

spirit. The product-rectified spirit contains about 96% ethanol and 4% water.

Pure alcohol (100% alcohol) called 'absolute alcohol' is obtained from this

product by distillation with benzene (C6H6).


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Ethanol is also be obtainable from petroleum products. During the cracking of

petroleum, large quantities of ethene (ethylene) are produced. This can be

hydrated using sulphuric acid as a catalyst to produce ethanol.

Water molecule adds to an alkene molecule across the double bond in the

presence of dilute acids and a catalyst. For example, ethane gives ethanol

when a mixture of ethene and steam is passed over phosphoric acid and

silica under a pressure 65 atm, and at 300 C

Properties

Ethanol is colourless and has a pleasant odour. Its boiling point is 78 oC and

its freezing point is -114oC. It is soluble in water and almost all the organic

solvents. It is highly intoxicating in nature. It is combustible and burns with a

blue flame.

Ethanol is used for many industrial applications because of some

characteristic reactions. The products formed are starting materials for

carrying out many organic reactions both at industry and laboratory. For

example,

When a piece of sodium is dropped in ethyl alcohol, bubbles of hydrogen gas

are observed.


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Ethanol reacts with phosphorus trichloride to form ethyl chloride.

When treated with concentrated H2SO4 at 170 oC, ethyl alcohol undergoes

dehydration to form ethane.

At lower temperature of 140 oC, and when present in excess, ethyl alcohol

forms a pleasant smelling substance called diethyl ether.

Alcohols on oxidation give aldehydes. The aldehydes on further oxidation

give carboxylic acids.

Based on these reactions important uses of alcohol are as follows:

Ethyl Alcohol is used as a solvent for many organic solutes, especially

which are insoluble in water.

It is used in the preparation of perfumes.

It is used in the manufacturing of 'power alcohol', which is 80%

mixture of petrol (gasoline) and 20% absolute alcohol. It can be used to

generate power as a motor fuel and help to save gasoline. As absolute


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alcohol does not mix with petrol, benzene, ether or tetralin is added into

this mixture.

Ethyl Alcohol is used in making tinctures and medical syrups.

It is used as a solvent for paints, varnishes, dyes etc.

It is used in the production of many organic compounds.

It is used in alcoholic beverages.

The chemical usage and common usage of the term 'alcohol' differ.

Chemically, the term alcohol refers to a group of organic compounds having -OH group in their composition; the common man reference to alcohol relates

to ethyl alcohol or ethanol. Ethyl alcohol has a variety, of uses as seen

above, especially as a solvent. But by far the greatest use of ethyl alcohol is

in the form of alcoholic beverages, such as wine, beer, rum, brandy, whisky

etc.

Alcohol consumed in sizeable amounts is detrimental and affects the nervous

system In small quantities it may serve as a source of energy, but

consumption of alcohol is a habit forming activity.. If consumed over a period

of time, alcohol can ruin one's health especially the liver, which gets affected

by cirrhosis. . The person loses his or her sense of balance and mental ability

when consumed in excess and this type of consumption often ruins family life

and can have tragic consequences.

Alcoholic drinks are heavily taxed by the government, so as to discourage

people from over consumption. Certain state have banned alcoholic drinks

altogether, inspite of the taxes being a sizable revenue earning for state

government coffers. Alcohol used for industrial and surgical purposes is not

taxed heavily. But in order to prevent unscrupelous people fromm buying and


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consuming industrial and surgical alcohol, it is mandatory that ethyl alcohol

be mixed with a certain percentage of highly poisonous methyl alcohol or

methanol. This renders the ethyl alcohol unfit for human consumption. This

mixture is called 'Methylated Spirit'. If chemicals like copper sulphate or

pyridine are added to ethyl alcohol it is called 'denatured alcohol'. Both these

mixtures are prepared so as to prevent people from drinking alcohol heavily.

Ethers

Ethers can be prepared by the following general methods.

Preparation of Ethers

Williamson Ether Synthesis

Ethers are usually prepared from alcohols or their conjugate bases. One importan

procedure is the Williamson Ether Synthesis mentioned earlier. This reaction proceed

by an SN2 reaction of an alkoxide nucleophile with an alkyl halide. The substitutio

reaction permits the convenient synthesis of both symmetrical and non-symmetrica

ethers.

This type of reaction happens in two steps. The alcohol firstly reacts with sodium (o

some other substance that removes a proton from the alcohol (-OH) group. Then a

haloalkane is added to a solution of the resulting anion, when the alkyl nucleophile from

the haloalkane attaches to it. Na+ion is a spectator ion forming a halide salt.

For preparation of ethers with different R groups bound to oxygen (non-symmetrica

ethers), all one has to do is use an alcohol with one organic group and an organi


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chloride with a different group. Reactions of a secondary alkyl alkoxide and a primary

aryl halide and vice versa are shown as two examples of this procedure.

In this case unsymmetrical ether benzyl isopropyl ether is obtained. As there are tw

different combinations, two different types of reactants are possible, one usually bette

than the other. Since alkoxide anions are strong bases, the possibility of a competing E

elimination also occurs here. Bearing in mind the factors that favor substitution ove

elimination, a primary alkyl halide is a preferred reactant as it gives a better and

cleaner yield of benzyl isopropyl ether than does other reaction which generate

considerable elimination product. Best yields of unsymmetrical ethers are obtaine

when the alkyl halides are primary and the alkoxides are tertiary.

Aryl halides are much less reactive than alkyl halide and so are usually prepared b

treating sodium phenoxide with alkyl halides.

Alkoxy Mercuration

A second general ether synthesis involves the oxymercuration reaction. The alcoho

reactant is used as the solvent, and a trifluoroacetate mercury (II) salt is used a

nucleophile in preference to the acetate).

Alkenes readily react with mercury (II) trifluoroacetate acetate in the presence of an

alcohol to give alkoxy mercurial compounds, which on reduction with NaBH4 in basi


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medium gives ethers in high yield. Reactions of butene and cyclopentene as example

of this two-step procedure are given below.

Note that this reaction gives ether in accordance with Markovnikov's addition rule and

does not involve a carbocation and competing elimination reaction.

Dehydration of Alcohols

Symmetrical ethers (R-O-R) can be prepared by gently heating an alcohol in th

presence of an acid catalyst like concentrated sulphuric acid. This process is ver

similar to that used to dehydrate an alcohol to an alkene. The primary difference is tha

dehydration uses somewhat higher temperatures.

The yield of ether depends upon the nature of the alcohol whether it is 1, 2, or 3 . Th

formation of ethers from these alcohols is best achieved when alcohol is used in excess

However, the order of dehydration of alcohols leading to the formation of ethers follow

the sequence,

Primary > Secondary > Tertiary

Acid-catalyzed dehydration of small 1 alcohols constitutes a specialized method o

preparing symmetrical ethers. As shown in the following two equations, the success o


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this procedure depends on the temperature. At 110 C to 130 C an SN2reaction of th

alcohol conjugate acid leads to an ether product. At higher temperatures (> 150 C) a

E2elimination takes place.

The dehydration of an alcohol to either an alkene or ether shows the reactions to

proceed over a relatively narrow temperature range (20 for ethanol). Such a narrow

range of conditions suggests that when these reactions are carried out one may obtai

more than one product and such is frequently the case.

Catalytic Dehydration

Dehydration of alcohols can also be achieved by passing the vapours of an alcohol ove

alumina (Al2O3) at 623 K (350 C).

For example,

The order of the ease of dehydration of alcohols is,


Reaction of Alkyl Halides with Dry Silver Oxide

Haloalkanes can also be converted into ethers by heating with dry silver oxide.


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Reaction With Grignard's Reagent

Higher ethers can be produced by the action of -halo ethers with Grignard's reagent.

Reaction With Diazomethane

Ethers can be prepared by the reaction of diazomethane with alcohols in the presence

of tetrafluoroboric acid (HBF4) as catalyst.

Physical Properties

Physical State

Higher Ethers are gases at ordinary temperature while the other lower members ar

colourless liquid with a characteristic 'ether smell'.

Dipole Nature

The C-O bonds in ethers are slightly polar because of greater electronegativity o

oxygen than carbon atom. Since the two C-O bonds in ethers are inclined to each othe

at an angle of 110o the two dipoles do not cancel each other and thus they a dipole

moment of 1.15-1.3 D.

Boiling Points

Ethers are isomeric with monohydric alcohols but their boiling points are much lowe

than the isomeric alcohols. This is because unlike alcohols ethers do not form hydroge


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bonds and exit as associated molecules. Further the weak polarity of the C-O bonds d

not affect the boiling points of ethers which are comparable with those of alkanes o

same molecular masses.

Diethyl ether (MW 74): 308 K; butyl alcohol (MW 74): 351 K; pentane (MW 72) 309 K

Solubility

Ethers have water solubilities intermediate between alkanes and alcohols. Becaus

ethers have no O-H bonds, they cannot participate in hydrogen bonding to the sam

extent that alcohols do. Nevertheless, the oxygen in the ether can form a hydroge

bond to the hydrogen in water. The presence of only single site on the ether for a

limited kind of hydrogen bonding interaction means that ethers generally hav

significantly smaller solubilities in water than do alcohols. Still they have highe

solubilities than any hydrocarbon.

As their molecular mass increases, the solubility of ethers in water decreases due t

corresponding increase in the hydrocarbon portion of the molecule. Ethers are fairl

soluble in common organic solvents like alcohol, benzene, chloroform, acetone etc.

Chemical Reactions

Ethers are generally unreactive and make good as solvents. They do not react with

halogens, nucleophiles or mild acids or bases. They are widely used as solvents for a

variety of organic compounds and reactions, suggesting that they are relativel

unreactive themselves. With the exception of the alkanes, cycloalkanes an

fluorocarbons, ethers are probably the least reactive, common class of organi

compounds. The inert nature of the ethers relative to the alcohols is undoubtedly due to

the absence of the reactive O-H bond.

Reactions of Ethereal Oxygen


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The ethereal O atom is a region of high electron density due to the presence of lone

pairs. Hence, ether oxygen atoms are Lewis bases undergoing reactions involving the

formation of co-ordinate bonds.

Formation of Oxonium Salts

The oxygen atom of ether has two lone pairs of electrons. As a result they behave a

Lewis bases and thus dissolve in cold concentrated inorganic acids to form stabl

oxonium salts. For example,

Formation of Coordination Complexes

Ethers form coordinate bonds with Lewis acids such as BF3, AlCl3, FeCl3, Grignard'

reagent etc., to produce complexes called as etherates.

The etherate complexes from Grignard's reagent dissolve in ether and that is th

reason Grignard's reagent are usually prepared in ethers.

Reactions Involving the Cleavage of C-O bond

Like an alcohol -OH group, the -OR group is a poor leaving group and needs to b

converted to a better leaving group before substitution can occur.

The most important reaction of ethers is their cleavage by strong acids. Alkyl ethers ar

cleaved by the strong acids HI or HBr in a nucleophilic substitution reaction similar t

that of alcohols. Aqueous HI is the acid most commonly used, but HBr also work


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sometimes.

Protonation of the ethereal oxygen creates a good leaving group, a neutral alcoho

molecule. The halide ion, bromide or iodide are both good nucleophiles. Depending o

the structure of the alkyl groups, the reaction can be SN1or SN

2.

Reaction with Halogen Acids

The cleavage of the C-O bond by strong halo acids is the most common reaction o

ethers. This occurs by SN1or E1mechanisms for 3-alkyl groups or by an SN

2mechanism

for 1-alkyl groups. The conjugate acid of the ether is an intermediate in all thes

reactions as shown below.

Ethers are readily cleaved by hydroiodic acid at 373 K to form an alcohol and an alky

halide.

However, if excess of acid is used the alcohol first formed reacts further with th

halogen acid to form the corresponding alkyl halide.


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The reaction is a typical nucleophilic substitution, taking place via a SN1 or SN

mechanism.

Primary and secondary alkyl ethers are attacked by iodide ion to give an SN

mechanism with acid catalysis:

Firstly, ether being Lewis bases undergoes protonation to form oxonium salts as th

intermediate. Protonation of the alcoholic oxygen takes place to make a better leavin

group. This step is very fast and reversible.

In the second step the formed protonated ether undergoes nucleophilic attack by th

halide ion. The halide ion functions to displace the good leaving group, neutral alcoho

molecule, by cleaving the C-O bond. This results in the formation of an alkyl bromide

and an alcohol.

Note that the iodide attacks the less hindered alkyl group, so in the unsymmetrica

ethers alkyl halide is always formed from the smaller alkyl group.

Protonation of tertiary ethers in acid lead to spontaneous cleavage to give a carbocatio

intermediate, resulting in either an SN1or E1mechanism for cleavage:


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There is a similarity to the analogous alcohol reactions when 'water' is lost, except an

alkyl group has been substituted for the H of water to give an alcohol leaving group.

The order of halogen acids follow the sequence:

HI > HBr > HCl

In case of alky aryl ethers the products are always phenol and alkyl halide and never a

aryl halide and alcohol.

.

This occurs because the phenol formed in this reaction does not react further, since SN2

SN1and E1reactions do not take place on aromatic rings.

Reaction with Sulphuric Acid

Ethers undergo hydrolysis reaction to from alcohols when heated with dilute sulphuri

acid under pressure.

When heated with concentrated sulphuric acid alcohol and alkyl hydrogen sulphates are

formed.


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Reaction with Acid Chlorides and Anhydrides

When ethers are treated with acid chlorides or anhydrides in the presence of anhydrou

ZnCl2or AlCl3esters and alkyl halide are formed.

Reaction with Phosphorus Pentachloride

The action of phosphorus pentachloride (PCl5) on ethers brings about the cleavage of C

O bond leading to the formation of alkyl halide.

Reactions Involving the Alkyl Group

Action of Air and Light

Ethers in which oxygen is bonded to 1 - and 2 -alkyl groups are subject to peroxid

formation in the presence of air (gaseous oxygen) and light.

This reaction presents an additional hazard to the use of these flammable solvents

since peroxides decompose explosively when heated or struck. The mechanism o

peroxide formation is believed to be free radical in nature due to the presence of tw


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unpaired electrons molecular oxygen.

Action of Halogens

Ethers react with chlorine or bromine to give substitution products. The extent o

halogenation depends on the reaction condition. For example,

Electrophilic Substitution Reactions

Like phenols, the aromatic ethers having the alkoxy group undergo electrophili

substitution reactions of the benzene ring at ortho- and parap- positions.

Halogenation

Anisole reacts with halogens in the presence of less polar solvents like CS2, CHCl3an

CCl4to give ortho and para isomers of monohaloanisoles.


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Nitration

With concentrated HNO3/H2SO4the nitrating mixture at 293 K anisole give mixtures o

ortho and para isomers of mononitroanisoles.

Friedel-Craft's Alkylation and Acylation

The alkylation and acylation reaction, known as 'Friedel-Craft reaction', is carried by

treating anisole with alkyl chloride or acyl chloride in the presence of a catalyst lik

anhydrous aluminium chloride. For example,


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Using Ethers as Protective Groups

Due to their relative lack of chemical reactivity, ethers have proven to be usefu

protective groups for alcohols and phenols. By converting a hydroxyl function to etherits acidity and ease of oxidation (in the case of 1 and 2-alcohols) can be suppressed to

such a degree that normally incompatible reactions, such as those employing Grignard

reagents, may be carried out.

Summary

Alcohols contain -OH functional group attached to a saturated sp3hybridized

carbon atom like alkane (R-OH), while phenols have a -OH group attached to the

carbon atoms of an aromatic ring (Ph-OH).

Ethers are organic molecules with -C-O-C-as the functional group.

Alcohols are divided into two broad categories - aliphatic alcohols and aromatic

alcohols.

Aliphatic and aromatic alcohols and phenols are classified as monohydric,

dihydric, trihydric and polyhydric according to the number of hydroxyl groups in

their molecules.

Alcohols containing sp3C-OH bond are classified into three categories as primary(1), secondary (2) and tertiary (3). These can be allylic or benzylic alcohols


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(attached to next to double bonded carbon atom).

Alcoholscontaining sp2C-OH bond are named as vinylic alcohols (directlyattached to double bonded carbon atoms) or aryl alcohols or phenols that are

hydoxyl derivatives of aromatic hydrocarbons.

Ethers are of two broad categories - aliphatic or aromatic ethers which can be

symmetrical and unsymmetrical ethers.

Alcohols are named using the -ol ending. On longer chains the location of the

hydroxyl group determines chain numbering.

Phenols are named using the word 'pheno' at the end. All substituted phenols

are named as derivatives of phenol with the position of the substituent with respect

to-OH group indicated by Arabic numerals.

The ether functional group is named as an alkoxysubstituent on the parentcompound.

alcohols phenols and ether by aarkumar

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