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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
18
– 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
19
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)
21
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:…..