hydroxy compd

7/28/2019 Hydroxy Compd

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L.S.T. Leung Chik Wai Memorial School

F.6 Chemistry

Chapter 34: Hydroxy-Compounds

HYDROXYCOMPOUNDS (ALCOHOLS AND PHENOLS)

I. Introduction

(A) Alcohols

Alcohols are compounds containing one or more hydroxyl groups (OH) attached to a

saturated carbon atom.The saturated carbon may be a carbon or a simple alkyl group.

Monohydric aliphatic alcohols alcohols containing one hydroxyl groups.

Examples:

CH3CH2OH

CHCH3

OH

CH3CH2 CCH3

OH

CH3

CH3

ethanol ________________________ ______________________

Polyhydric aliphatic alcohols alcohols containing two or more hydroxyl groups

Examples:

CH2CH

2

OH OH

CH2CH

2

OH

CH2

OH

CH2CH

OH

CH2

OHOH

__________________ ___________________ _____________________

Note The saturated carbon atom which is attached to the hydroxyl group can be of an alkenyl

or alkynyl group.

Examples

CH2=CHCH

2CH

2OH OHCH

2C CH ClCH

2CH=CHOH

___________________ _____________________ ____________________

The carbon atom may be attached to the side chain of a benzene ring.

CH2OH

CH3

CH3

OH

CH2CH

2OH

_____________________ ________________________ _____________________

(B)Phenols (Aromatic alcohols or alcohols)

Phenols are compounds in which the hydroxyl group is directly attached to the benzene ring.

Examples:

OH OH

Br

OHO2N

chapt. 34: p.1


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L.S.T. Leung Chik Wai Memorial School

F.6 Chemistry

Chapter 34: Hydroxy-Compounds

II. Preparation of Alcohols

(A) Hydrolysis of halogenoalkanes through (SN) reaction

to prepare primary and secondary alcohols from a primary and secondary halogenoalkanes.

Mechanism:

OH- +CH

2Br

CH2CH

3

Slow CH2

CH2CH

3

BrOH

CH2

CH2CH

3

OH

Note: Tertiary alkvl halides undergo ELIMINATION too easily to be of use for synthesizing

tertiary lcohols.

(B) Reduction of aldehydes or ketones

to prepare primary and secondary alcohols from an aldehyde and a ketone (both contain thecarbonyl group >C=O

1. Reduction by using hydrogen gas under high pressure with Pt or Ni as catalyst

R

C

R

O+ H2

high pressure R

CH

R

OH

2. Reduction by using Lithium tetrahydridoaluminate (LiAlH4) which can release hydride ion (H-)

R

C

R

O+H

from LiAlH4

R

CH

R

O

H+

from dil. acid

R

CH

R

OH

Note : Since LiAlH4 reacts violently with water, it is necessary to use an inert solvent such as

ether (ethoxyethane). Hydrolysis of the intermediate b dilute acid gives thedesired alcohol.

A less powerful reducing agent, sodium tetrahydridoborate (NaBH4) can also be used. It

is more convenient because it can be used in aqueous or methanolic solution.

R

C

R

O

NaBH4

methanol / water

R

CH

R

OH

Example:

CH3CH

2CHO

1. LiAlH

4/ dry ether

2. H

3O+

chapt. 34: p.2


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1. LiAlH

4/ dry ether

2. H

3O+

O

1. LiAlH

4/ dry ether

2. H

3O+

HOOCCH2 C

O

CH3

1. LiAlH

4/ dry ether

2. H

3O+

C

O

CH3

CCH2

O

CH3O

Note : LiAlH4 can also reduce carboxylic acid, acid chloride, acid anhydride and esters groups

to alcohols.

(C) Hydration of Alkenes

to prepare secondary and tertiary alcohols from alkenes.

In the presence of acid catalyst, water can be added onto an alkene to form a secondary or a

tertiary alcohol (except for ethene) . The reaction reversible and the mechanism is just thereverse of that for dehydration of an alcohol.

Mechanism:

H+C C

+

H

H

H

H

H

OH2

C C

H

H

H

H

H

O+

H H

C C

H

H

H

H

H

OH

In practice. the alkene is bubbled into conc. sulphuric acid to form an alkylhydrogensulphonate. When this is diluted with water and distilled, an alcohol is formed:

CH2=CH2 + H2SO4 CH3CH2HSO4

CH3CH2HSO4 + H2O CH3CH2OH + H2SO4

(D) Hydrolysis of Esters with alkali to prepare prinary, secondary and tertiary alcohols

Example:

CH3COOC2H5 + NaOH

(E) Oxidation of alkenes by alkaline KMnO4 to prepare a diol from alkenes.

Example :ethane-1,2-diol can be prepared by bubbling ethene into alkaline potassium


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manganate(VII) solution.

CH2

CH2

KMnO4

/ OH-

CH2

CH2

OH OH

III. Preparation of phenol

(A) By fusion of sodium hydroxide with the sodium salt of benzenesulphonic acid :SO

2ONa

+NaOH

O

+ Na2SO3

Na+

Phenol is then released from sodium phenoxide with dilute acid:

O Na+

+ H+

OH

(B) By warming of an aqueous solution of benzene-diazonium chloride(a common laboratory method)

+ OH2

OHN+

N Cl

+ N2 + ClH

(C) By the hydrolysis of chlorobenzene with sodium. hydroxide, under he drastic conditions of 150

atm. and at 400 0C

(an industrial method)

ClNaOH

O Na+

OH2 H+

IV. Reactions of Alcohols and Phenols : Basic consideration

Both alcohols and phenols contain the hydroxyl group (OH) However, as this group is

attached to the benzene ring for phenols and to the saturated Carbon for alcohols. the reactions

of such compounds are quite different.


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(A) Reactions of Alcohols

There are two main types of reactions for alcohols:

1. Reactions involving fission of ROH bond (or just CO bond)

C O Halkyl hydroxy fission

2. Reactions involving fission of ROH bond (or just OH bond)

C O Halkoxy hydrogen fission

(B) Reactions of phenols

There are two main types of reactions for phenols:

1. Reactions involving the OH group.

2. Reactions involving the benzene ring.

Note : For phenols. the direct attachment of a hydroxyl group to the benzene ring has mutualeffects on the reactivity of both the OH group and the benzene ring.

The electronrich benzene ring in phenol can make it undergo electrophilic aromatic

substitution.The reactivity of the OH group can also be modified by the benzene ring through

delocalization effects.

V. Reactions of Alcohols

(A) Reaction involving fission of ROH bond (cleavage of C0 bond)1. Dehydration

(a) intramolecular dehydration (forming alkene)

The conditions for dehydrating alcohols depend closely on the structure of individual alcohols. For primary alcohols, the conditions required are conc. sulphuric acid a temperature of

1700C

CH3CH

2OH

CONC. H2SO

4CH

2=CH

2+ OH

2

1700C

Secondary alcohols dehydrate under milder conditions than primary alcohols.

OH85 % H3PO4

+ OH2

165 1700C

Tertiary alcohols dehydrate under even milder conditions.


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CH3

C

CH3

OH

CH3

20 % H2SO4CH

3C CH

2

CH3

Mechanism:

For secondary and tertiary alcohols, the following mechanism is generally accepted.

C

H

C

OH

+ H+

Note : The main function of the acid is to transform the poor leaving group OH into the verygood leaving group. OH2

The ease of dehydration of alcohols istertiary > secondary > primary

Reason : Tertiary carbocation is the most stable one.i.e. The order of stability of the carbocations follows the number of electron

releasing groups

CH3

C+

CH3

CH3

CH3

C+

H

CH3

H C+

H

CH3

H C+

H

H

most stable least stable


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Dehydration of secondary and tertiary alcohols containing more than three carbon atoms

will give a mixture of alkenes whose amounts will be determined by the following ruleAlcohol dehydrations generally produce the more highly substituted alkenes.

i.e. the major product is that contains the higher number of alkyl groups attached to theC=C bond.

e.g.

CH3 CH

OH

CH2CH3 H

+

CH3 CH

O+

CH2CH3

H H

-H2OCH

3C

+CH

2CH

3

H

(a) Intermolecular dehydration (forming ether)

When the dehydration is carried out at a temperature of 1400C with an excess of alcohol. ether willbe formed.

CH3CH

2OH

CONC. H2SO

4+ OH2CH3CH2OCH2CH32

2. HalogenationAlcohols react with hydrogen halide and phosphorus halides give halogenoalkanes.

(a) Reaction with hydrogen halides

Mechanism:

Step1: Protonation of the alcohols (same process for 1, 2 and 3 alcohols)

The alcohol acts as a weak base and accept the proton donated by the hydrogen halide.The equilibrium lies well

ROH + H+

R O+

H

H

Step 2 : Displacement the halide on for a water molecule.

For primary and secondary alcohols, it is a SN2 reaction.

RCH2

O+

H

H

XR-CH

2-X

For tertiary alcohols, it is a SN1 reaction.

R3C O

+H

H

R

C+

R R

X

R

C+

R R

R

C

RR

X


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Note Secondary alcohols also proceed through a mechanism involving the formation

of carbocation.

Reactions of primary and secondary alcohols with hydrogen halide are catalysedby zinc chloride in the Lucas reagent. (a solution of ZnCl2 in conc. HCI).

C2H5OH + ClHReflux

ZnCl2 Catalyst

C2H5Cl + OH2

Example Chlorination of 2-methylpropan-2-ol in a solution of ZnCl2 and conc. HCl

(i) Mechanism for this reaction : ( a ____ reaction )

(ii) Energy profile for this reaction:

(iii) Rate of the reaction for 10 , 20 and 30 alcohols:

The order of rates of reaction:

30 alcohol > 20 alcohol > 10 alcohol

The rate can be shown by the turbidity in the aqueous layer since thechloroalkane formed is immiscible with water.

(iv) Dependence of the chloride anion concentration.For tertiary alcohols, the rate is independent of the concentration of chloride

ion because it is a SN1 reaction.

Example: Bromination of ethanol in a mixture of conc. H 2SO4 and solid NaBr

C2H

5OH C

2H

5Br + OH2

conc. H2SO

4/ NaBr

heating


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(b) Reaction with phosphorus halides

Alcohols react with phosphorus tribromide and phosphorus tri iodide to form bromo and

iodoalkanes respectively.

+PBr

3 3 R-Br3 R-OH+ H

3PO

3

+ PI3 3 R-I3 R-OH

+ H3PO

3

Note :The phosphorus trihalides are prepared by the reaction of red phosphorus and the halogen.

Phosphorus pentahalide or thionlychloride are used to prepare chloroalkanes at roomtemperature

+ PCl5 R-ClR-OH + ClH + POCl3

+ SOCl2 R-ClR-OH + SO2

+ ClH

Exercise : -

The following apparatus is used to prepare bromoethane (b.p. 38C) from ethanol; using red

phosphorus and bromine.

(a) What advantage is there in using red phosphorus instead of white phosphorus?(b) What is the purpose of the soda lime tube?

(c) Why is a water condenser included in the set-up.

(d) Explain why a cold water bath is used while bromine is added?(e) The mixture has to be refluxed for 30 minutes after addition of bromine. Why?

After the heating process the apparatus is converted for distillation and he product is collected in

a receiver immersed in cold water. The distillate is then washed with water before drying.

(f) How can you tell when the distillation is complete?

(g) Suggest one impurity that can be removed by washing with water.(h) Give the name of a drying agent for bromoethane.

(I) What further treatment is required in order to obtain pure bromoethane?

(j) What would you do when setting up the apparatus and reagents, bromine liquid is

accidentally split on our hand?


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(B) Reaction involving fission of ROH bond (cleavage of OH bond)

1. Reaction with active metals (e.g. sodium)Alcohols are very acids because the alkyl group pushes electrons towards the OH group, so

that the oxygen does not strongly attract the electrons in the OH bond.

Furthermore. once a RO- ion is formed. it cannot be stabilized by the delocalization of thecharge. Thus. alcohols react only to a very slight extent with alkalis, but will react wtth very

electropositive metals under anhydrous conditions to give alkoxide.

Example: reaction of ethanol with sodium

2CH3CH2OH + 2Na 2CH3CH2O- Na+ + H2

Addition of water will regenerate the alcohol readily.

CH3CH2O-Na+ + H2O CH3CH2OH + NaOH

Note The reaction is much slower then the reaction of water with sodium.

The reaction of alcohol with sodium can be used to depose the excess sodium in

the laboratory.

2. Esterification

Alcohols and carboxylic acids react to give esters.The functional groups of acids and esters are

(where R is an alkyl group)

C

O

OHC

O

O-R

Carboxylic acid Carboxylic ester

Esterification takes place much faster in the presence of a catalyst such as conc. H2SO4.

Example :

C

O

OHCH

3CH

2

+ CH3CH

2OH C

O

OCH2CH

3

CH3CH

2+ OH

2

conc. H2SO

4

Reflux

Alcohols can also react with acid chlorides and acid anhydrides to form esters.

Example:

C

O

ClCH

3

CH2

+ CH3CH

2OH C

O

OCH2CH3

CH3CH

2+ ClH

propanoyl chloride

C

O

OCH

3CH

2

C

O

CH3CH

2

+ CH3CH

2OH C

O

OCH2CH

3

CH3CH

2+ CH

3CH

2COOH

propanoic anhydride


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3. Oxidation

Alcohols can he oxidized by various oxidizing agents to aldehyde, ketones or carboxylic acids.

depending on the nature of the alcohol and the strength of the oxidizing agents being used.

Oxidizing agents used:

Acidic or alkaline potassium permanganate, acidified potassium dichromate,chromic acid or dilute nitric acid.

(a) Primary alcohols are readily oxidized through aldehydes to carboxylic acids.

R-CH2OH

RC

O

H RC

O

OH

Primary alcohol Aldehyde Carboxylic acid

Note: The alcohol , aldehyde and acid preserve the same number of carbon atoms.

(b) Secondary alcohols are oxidized to ketones under normal conditions

Secondary alcohol Ketone

Note The ketone formed has to be undergo prolonged drastic treatment before it can be

broken down into acids with smaller number of carbon atoms.

CH3CH

OH

CH3

O

CH3C

O

CH3

O

CH3C

O

OH CO2+

(c) Tertiary alcohols are normally resistant to oxidation in the neutral or alkaline medium.

because it would involve the breakage of the high energy CC bonds in the alcohol

molecule.

CH3

CH3

CH3

OH

NO reaction

In acidic solutions, tertiary alcohols can he oxidized to give a mixture of ketone and acid,both with fewer carbon atoms

than the alcohol.

CH3

CH3

CH2CH

3

OH

CH3

O

CH3

+ CH3COOH

Note: Characterization of the oxidation products of alcohols is a means of distinguishingbetween primary , secondary and tertiary alcohols.


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The aldehyde may be detected by its reaction with 2,4-dinitrophenylhydrazine. Tollens reagent

or Schiffs reagent.

Ketones may be detected by its reaction with 2,4-dinitrophenylhydrazine but not with Tollens

reagent or Schiffs reagent. Carboxylic acids can be detected by reaction with sodium hydrogencarbonate solutions or ester

formation.

Reactions of Ethanol, a Typical Aliphatic Alcohol

C2H

5OH

conc. H2SO

4

Ethoxyethane CH3CH

2OCH

2CH

3Ethene CH

2=CH

2

Na

Sodium ethoxide CH3CH

2ONa

K2Cr

2O

7/ H

+Ethanal CH

3CHO

KMnO4

H+

Ethanoic acid CH3COOH

CH3COCl

CH3COOH

OR

Ethyl ethanoate CH3COOCH

2CH

3

I2

+ red P

CH3CH

2I

Iodo ethane

KBr / conc. H2SO4CH

3CH

2Br

PCl5

/ SOCl2

CH3CH

2Cl

Methods of distinguishing between 10 , 20 and 30 alcohol

Reagents Primary alcohol Secondary alcohol Tertiary alcohol

Acidified

K2Cr2O7

Aldehyde,

RCHO formed

Solution change fromorange to green

Ketone,

R2CO formed

Solution change fromorange to green

No observable change

Conc. H2SO4 Alkene formed slowly Intermediate in speed Alkene formed fast

Conc. HCl

+

ZnCl2Add to alcohol and place

in boiling water bath

Cloudiness due to

formation of RCl is slow

to appear.

Cloudiness appear in

5 minutes

Cloudiness appears in

1 minute owing to

the formation of RCl,

which is insoluble inwater


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VI. Reactions of Phenols

1 Reactions of phenols can he classified into:

(A) those involving the -OH group(B) those involving the benzene ring attached to it

(A) Reactions of the OH group

1. Dissociation (Acidic properties salt formation)

Phenol can dissociate in waterOH

+ OH2

O

+ H3O+

It behaves as a weak acid because its dissociation occurs to only a light extent

(pKa = 10.0)

Thus, phenol can react with sodium metal

OH

+ Na

O

+ H2

Na+

2 2

Unlike alcohol, it can react with NaOH and is a stronger acid than alcohol.

Note : Explanation of the higher acidity of phenol than aliphatic alcohol.

In phenol. the OH bond breaks more readily and the resultant phenoxide anion is

stabilized by resonance.

In the phenoxide ion, a p orbital of the oxygen atom overlaps with the orbital of the ring

carbon atoms.

Therefore. the equilibrium favours dissociation of phenol into H3O+ and the phenoxide ion.

Thus it can be seen that owing to the direct attachment to the benzene ring. the acidity of phenolis greatly enhanced by resonance. and is very different from aliphatic alcohol.


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The phenoxide ion formed from the reaction of phenol with alkalis can act as a powerful

nucleophile and can be used in the synthesis of certain organic compounds.

OH

+ NaOH

ONa

+ OH2

When comparing the acidity with carbonic acid and carboxylic acid, phenol is the weakest.

Methods to distinguish between alcohols, phenols, and carboxylic acid:

(a) Carboxylic acid can liberate CO2 when treated with sodium hydrogen carbonate solution

whereas alcohols and phenols cannot.

(b) Phenols can react with NaOH to give a salt whereas alcohols cannot.

Example: A scheme outlining the chemical reactions of the following compounds A, B and C is

listed as below:OH

A

CH3CH

2OH

B

CH3COOH

C

Mixture of A, B and C in ethoxyethane is shaken with NaHCO3 solution.

Aqueous layer Ethereal layer

Evapourate ether

B

Add H+aq

, then

filter

C

Shaken with aq.

NaOH, then

separate

acidify with dil.HCl,

extract with etheragain,

evaporate ether

A

separate

Ethereal layer Aqueous layer


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2. Other reactions of the OH group

As stated before, the OH group of phenol behave differently from those of aliphatic alcohols,

as a result of the delocalization of electrons with the benzene ring directly attached to it.

OH

The resonance effect strengthens the C-O bond, the partial double bond character between thecarbon atom and the oxygen is confirmed by its bond length being shorter than that of normal C

O bond.

Thus, Reactions of the OH group of phenol are quite different from that of alcohol:

(a) Displacement of the OH group by halogen, pcurs only at more extreme conditions.

(b) Phenol is not oxidized to some breakdown products but form complex polymers.

(c) Phenols does not undergo elimination as primary and secondary alcohol do.

In alkaline medium, phenol generate the phenoxide ion C6H5O-. Such anion is a more powerful

nucleophile than /the neutral phenol molecule, and can take place in some nucleophilic reactions.

OH

NaOH

room temp.

O Na+

OCH2CH

3

CH3CH

2Br CH

3COCl

OOCCH3

CH3

O

O

O

CH3

OOCCH3

Note : The reaction between sodium phenoxide and bromoethane is a typical SN2reaction inwhich the rate depends on the availability of electron pairs at oxygen.

OCH

3CH

2Br

If there is an electron withdrawing group (e.g. NO group) attached to the benzene

ring, the rate will be reduced.

NO

O

O


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(B) Reactions of the benzene ring Substitution

Phenol is more reactive than benzene towards electrophilic reagents because there is an

interaction between the lone pairs on the oxygen atom in OH or O and the ring; whichincrease the availability of electrons in the aromatic ring.

Note : The greater activity of the ring in phenol than in the simple benzene molecule is

reflected in the milder

1. Nitration

Monosubstituted compound is obtained with dilute nitric acid at room temperature.

OHdil. HNO

3OH

NO2

+

OH

NO2

If conc. nitric acid is used. trisubstituted product is obtained readily.

OHdil. HNO

3

OH

NO2

O2N

NO2

2. Halogenation (Bromination)

(a) Chlorine, in the absence of solvent, gives 2 and 4chlorophenol.

(b) Bromine, in a nonpolar solvent (e.g. CS2 or CCl4) gives 2 ,4bromophenol.OH

+ Br2

CS2

OH

Br

+

OH

Br

(c) Bromine water gives a precipitate of 2,4,6tribromophenol.

The faster reaction in water is due to the presence of phenoxide ions.

OH

+ Br2

OH

BrBr

Br


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3. Sulphonation

When phenol is treated with concentrated sulphuric acid. different substituted products will

result, depending on the reaction temperature.

OH

OH

SO3H

OH

SO3H

Exercise

Arrange the following compound in order of increasing rate of reaction with concentratedsulphuric acid. Give reasons for your order.

OH

CH3

OH


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VII. Alkanediols

Alkanediols are examples of polyhydric alcohols which contain more than one OH group in

their structures.

Example 1 : Ethane-1,2-diol (ethylene glycol)

(A) Preparation

Ethene is bubbled into alkaline solution of potassium manganate(VII), the purple fades outas ethene ids oxidized to ethane1 . 2diol.

CH2=CH

2

MnO4- H

+

CH2CH

2

OHOH

A brown suspension of manganese (IV) oxide also appears.

(B) Properties

The properties are similar to those of monohydric alcohols.

(C) Uses 1

It as used antifreeze in car radiators and as a radiators and as a de-icing fluid on aeroplane

wings.

Note : Alkanetriols Propane1.2,3 triol (glycerol)

It is the byproduct in the manufacture of soap. Glycerol is used for the manufactureof ester which it forms from nitric acid, propane1,2.3triyl trinitrate.

CH2

CH OH

CH2OH

OH

+ HNO3

CH2

CH O-NO2

CH2O-NO

2

O-NO2

3 + 3 OH2

glycerol propane-1,2,3-triyl trinitrate

This ester is also called nitroglycerine. It is used as both explosivee and a drug inmedicine to treat heart diseases.


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OH

O Na+

Br2

O

BrBr

Br

(CH3CO)

2O

OCOCH3

CH3CH

2Br

OCH2CH

3

SO3Na

NaOH

OH- H+

Cl

NaOH

HNO3

OH

NO2

Conc. HNO3

OH

NO2

O2N

NO2

Conc. H2SO

4

OH

SO3H

Cl2 In FeCl3

OH

Cl

FeCl3

Violet colour (Test for a phenol)

N2Cl

Diazonium saltOH

2

Soluble in NaOH, but

it does not react with NaHCO3

to give CO2

Reaction of Phenol

hydroxy compd

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