l 10 alcohols-structure_nomenclature_classification_etc_pch217_2013_2014
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1
Organic Chemistry
Course Number: PCH 1120-217
Lecture # 10
Sunday September 29, 2013
Alcohols: Structure, Nomenclacture, Classification, Physical Properties, Basicity &
Acidity and Preparation of Alcohols
Prof. Oludotun Phillips
Room # 2-81, 2nd Floor Pharmacy Building
Email: dphillips@hsc.edu.kw
Tel: 4986070
2
Alcohols: Learning Objectives
At the end of the class students should be able to:
discuss the structures and properties of alcohols.
classification of alcohols.
discuss the nomenclature (common and IUPAC names) of alcohols.
describe the physical properties of alcohols, basicity and acidity of alcohols.
discuss the different methods for the preparation of alcohols.
3
• The functional group of an alcohol is an -OH group bonded to an sp3 hybridized carbon
– bond angles about the hydroxyl oxygen atom are approximately 109.5°
• Oxygen is also sp3 hybridized
– two sp3 hybrid orbitals form sigma bonds to carbon and hydrogen
– the remaining two sp3 hybrid orbitals each contain an unshared pair of electrons
Alcohols - Structure
4
• IUPAC names
– the parent chain is the longest chain that contains the -OH group
– number the parent chain in the direction that gives the -OH group the lower number
– change the suffix -e to -ol
• Common names
– name the alkyl group bonded to oxygen followed by the word alcohol
Alcohols - Nomenclature
5
• Examples:
Ethanol(Ethyl alcohol)
1-Propanol(Propyl alcohol)
2-Propanol(Isopropyl alcohol)
1-Butanol(Butyl alcohol)
OH
OH
OHOH
2-Butanol(sec-Butyl alcohol)
2-Methyl-1-propanol(Isobutyl alcohol)
2-Methyl-2-propanol(tert-Butyl alcohol)
OH
Cyclohexanol(Cyclohexyl alcohol)
OHOH
OH
Nomenclature of Alcohols
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• Problem: Write the IUPAC name of each alcohol
OH
CH3(CH2)6CH2OH
OH
(a)
(b)
(c)
Nomenclature of Alcohols
7
Polyhydric alcohols: compounds containing…. – two -OH groups are named as diols,– three -OH groups are named as triols, etc.
CH3CHCH2
HO OH
CH2CH2
OHOH
CH2CHCH2
OHHOHO
1,2-Ethanediol(Ethylene glycol)
1,2-Propanediol(Propylene glycol)
1,2,3-Propanetriol(Glycerol, Glycerin)
Nomenclature of Alcohols
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Unsaturated alcohols– the double and triple bond are shown by the infix -en-
and -yn-– the hydroxyl group is shown by the suffix -ol– number the chain to give OH the lower number
52
OH1346
trans-3-hexene-1-ol(E)-3-hexene-1-ol
IUPAC Nomenclature of Alcohols
HOH
5-Hexny-1-ol
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Alcohols are further classified as:
1. Methyl alcohol (CH3OH)
2. primary (1o)alcohol
2. secondary (2o)alcohol
3. tertiary (3o)alcohol
Classification of Alcohols
H OH
R
H
R' OH
R
H
R' OH
R
R"
(1o) (2o) (3o) primary secondary tertiary
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Alcohols are further classified as:
5. allylic alcohol
6. benzylic alcohol
Classification of Alcohols
CH3CH CHCH2OH
H
CH3
H
OH CH2OH C-CH3
OH
H
C-CH3
OH
CH3
Benzylic alcohols (1o) (2o) (3o)allylic alcohol
(1o primary)
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Alcohols are polar compounds both the C-O and O-H bonds are polar covalent
-
+
+O
HH
H
C
H
Alcohols – Physical Properties
12
Alcohols associate in the liquid state by hydrogen bonding. Hydrogen bonding: the attractive force between a partial
positive charge on hydrogen and a partial negative charge on a nearby oxygen, nitrogen, or fluorine atom.
– the strength of hydrogen bonding in alcohols is approximately 2 to 5 kcal/mol.
– hydrogen bonds are considerably weaker than covalent bonds (for example, 110 kcal/mol for an O-H bond).
– hydrogen bonding can have a significant effect on physical properties.
Alcohols – Physical Properties
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– association of ethanol molecules in the liquid state, with two of the three possible hydrogen bonds to the upper oxygen are shown as follows:.
Alcohols – Physical Properties
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– alcohols have higher boiling points and are more soluble in water than hydrocarbons
CH3CH2CH2OHCH3CH2CH2CH3
CH3OHCH3CH3
CH3CH2OH
CH3CH2CH3
CH3CH2CH2CH2CH2OH
HOCH2CH2CH2CH2OHCH3CH2CH2CH2CH2CH3
Structural Formula Name
MolecularWeight(g/mol)
BoilingPoint(°C)
Solubilityin Water
methanol 32 65 infiniteethane 30 -89 insoluble
ethanol 46 78 infinitepropane 44 -42 insoluble
1-propanol 60 97 infinitebutane 58 0 insoluble
1-pentanol 88 138 2.3 g/100 g
1,4-butanediol 90 230 infinitehexane 86 69 insoluble
Alcohols – Physical Properties
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Acidity Constants:
According to Bronsted-Lowry definitions:
an acid is a hydrogen ion donor.
a base is a hydrogen ion acceptor.
The strength of the acid is usually compared measuring their degree of ionization in water.
Relative Acidity of Alcohols
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Most alcohols are about the same or slightly weaker acids than water
– pKa = -logKa– aqueous solutions of alcohols have the same pH as
that of pure water– Alcohols are neutral compounds!
CH3O H O HH
[CH3O- ][H3O+]
[CH3OH]
CH3O H OH
H+
Ka =
+ +
= 3.2 x 10-16
pKa = 15.5
Acidity of Alcohols
17
In the presence of strong acids, the oxygen atom of an alcohol can behave as a weak base – proton transfer from the strong acid forms an oxonium
ion
– thus, alcohols can function as both very weak acids and weak bases
– Alcohols can lose a proton to a very strong base, e.g.
NaOH + ROH RO- Na+ + H2
CH3CH2-O-H H OH
H OH
HH2SO4
CH3CH2-O HH
CH3CH2-O HH HH
O H
Ethyloxonium ion(pKa -2.4)
••
Hydronium ion(pKa -1.7)
Ethanol
++
+++
••+
++
Basicity of Alcohols
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– pKa values for several low-molecular-weight alcohols
(CH3)3COH
(CH3)2CHOH
CH3CH2OH
H2O
CH3OH
CH3COOH
HCl
Compound pKa
-7
15.5
15.7
15.9
17
18
4.8
hydrogen chloride
acetic acid
methanol
water
ethanol
2-propanol
2-methyl-2-propanol
Structural Formula
Stronger acid
Weaker acid
*Also given for comparison are pKa values for water, acetic acid, and hydrogen chloride.
Acidity of Alcohols
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Alcohols can be prepared by the following methods:
i. Fermentation of carbohydrates
ii. Hydration of alkenes
iii. Nucleophilic substitution reaction
iv. Reduction of aldehydes and ketones
Preparation of Alcohols
20
Alcohols can be prepared by:
i. Fermentation of carbohydrates:
Fermentation is the decomposition of complex organic compounds into simpler compounds by the action of biological catalysts known as enzymes.
Fermentation of glucose give ethanol:…
Preparation of Alcohols
C6H12O6 CH3CH2OHenzymes
Glucose ethanol(a sugar) (an alcohol)
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Alcohols can be prepared by:
ii. Hydration of alkenes:
Treatment of alkenes with water in the presence of a strong acid as a catalytic results in the addition of water to the double bond (hydration reaction):….
+ H2OCH2 CH2 CH3 CH2OH
Preparation of Alcohols
Cyclohexane cyclohexanol
H+
Strong acid (conc. H2SO4)
+ OHH2O
H+
Strong acid (conc. H2SO4)Ethylene Ethanol
b.
a.
22
Alcohols can be prepared by:
iii. Nucleophilic Substitution Reactions: Reaction of an alkyl halide in hydroxide ion is a
nucleophilic substitution reaction. Primary alcohols are prepared by heating primary
alkyl halides in aqueous sodium hydroxide (NaOH), this reacion proceed by SN2 mechanism:…
Since 2o and 3o alkyl halides are likely to give elimination reaction products, this method is not very useful for preparing alcohols.
Preparation of Alcohols
CH3CH2CH2Br CH3CH2CH2OH + NaBr+ NaOHaqheat
1-Bromopropane 1-Propanol (a 1o alkyl halide) (a 1o alcohol)
23
Alcohols can be prepared by:
iv. Reduction of Aldehydes and Ketones:
a. Alcohols are prepared from carbonyl compounds by reduction reactions in which hydrogen atoms are added to the carbonyl group by NaBH4 or LiAlH4 followed by H2O or dilute acid hydrolysis:…..
Preparation of Alcohols
R CH2CHO R CH2CH2OHNaBH4
OrLiAlH4An Aldehyde A 1o alcohol
R COCH3 R CHCH3
OHA Ketone A 2o alcohol
NaBH4
OrLiAlH4
Reagents: Sodium borohydride: (NaBH4) Lithium aluminum hydride (LiAlH4)
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Alcohols can be prepared by:
iv. Reduction of Aldehydes and Ketones:
b. By Catalytic Hydrogenation involves the addition of Hydrogen to aldehydes or ketones under pressure to produce 1o and 2o Alcohols respectively:…..
Preparation of Alcohols
R CH2CHO R CH2CH2OH+ H2 Ni
PressureAn Aldehyde hydrogen A 1o alcohol
R COCH3R CHCH3
OH
+ H2
A Ketone hydrogen A 2o alcohol
Reagents: Nickle (Ni); Hydrogen gas (H2)
Ni
Pressure
25
Alcohols can be prepared by:
iv. Reduction of Aldehydes and Ketones:
c. By Grignard addition to an Aldehyde or Ketone followed by H2O or dilute acid hydrolysis to produce 1o, 2o and 3o
Alcohols respectively.
Grignard Reagent is prepared by reacting Magnesium metal with an Alkyl halide in diethyl ether as solvent.
Preparation of Alcohols
R X R Mg X+ Mgdiethyl ether
(as solvent)Alkyl halide Magnesium Grignard reagent
R = CH3; CH3CH2; or Ph;
X = Cl; Br; IIn this reaction, the “R” of Grignard reagent acts as Nucleophile
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Alcohols can be prepared by: Formaldehyde is reduced to 1o alcohol:..
Aldehydes are reduced to 2o alcohols:…
Ketones are reduced to 3o alcohols:…
Preparation of Alcohols
H CH
O
H C-H
R"
O- +MgX+ R"MgX H C-H
R"
OH
Formaldehyde Grignard reagent Magnesium salt of alcohol 1 o alcohol
R COCH3 R C-CH3
R"
O- +MgX
+ R"MgX R C-CH3
R"
OH
R' CHO R' C-H
R"
O- +MgX
+ R"MgX R' C-H
R"
OH
H2O
H+
Aldehyde Grignard reagent Magnesium salt of alcohol 2 o alcohol
H2O
H+
H2O
H+
Ketone Grignard reagent Magnesium salt of alcohol 3 o alcohol
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