Some important methods for the preparation of carboxylic acids are as follows :

1. From Primary alcohols and aldehydes

Primary alcohols and aldehydes can easily be oxidised to the corresponding carboxylic acids with oxidising agents such as potassium permanganate (in acidic or alkaline medium), potassium dichromate (in acidic medium)

Example

(O)

H+/ K2Cr2O7

(O)Ethyl alcohol Acetaldehyde Acetic acid CH3CH2OH CH3CHO CH3COOH

2. Hydrolysis of Nitriles

Nitriles yield carboxylic acids when subjected to hydrolysis with an acid or alkali.

Example

CH3 C N + 2H2O CH3COOH + NH3 H+

Methyl cyanide Acetic acid

3. Acidic hydrolysis of esters Esters on hydrolysis with dilute mineral acids yield corresponding carboxylic acid

Example

CH3 C OC2H5 + H2O CH3 H+

Ethyl acetate Acetic acid

C OH + C2H5OH

Ethyl alcohol

O O

4. From Grignard reagent

Grignard reagent reacts with carbon di oxide (dry ice) to form salts of carboxylic acid which in turn give corresponding carboxylic acid after acidification with mineral acid.

Example

C O + CH3MgBr

O dry ether

H2O

H+CH3 C

O

OMgBr CH3 C

O

OH + Mg OH

Br Methyl Magnessium bromide

Acetic acid

C O +

O dry ether H2O

H+

C

O

OMgBr C

O

OH + Mg OH

X Benzoic acid

MgBr

Phenyl Magnesium Bromide

XII U12-Carbonyl Compounds.indd 170 2/19/2020 5:15:17 PM

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Formic acid cannot be prepared by Grignard reagent since the acid contains only one carbon atom 5. Hydrolysis of acylhalides and anhydrides

a) Acid chlorides when hydrolysed with water give Carboxylic acids.

Example

CH3 C Cl + H2O

Acetyl chloride

O

CH3 C OH + HCl

O Acetic acid

b) Acid anhydride when hydrolysed with water give corresponding carboxylic acids.

CH3 C O C CH3 + H2O 2CH3 C OH

O O O Acetic anhydride Acetic Acid

C6H5 C

Benzoic anhydride

O

2C6H5 C OH

O Benzoic acid

O C C6H5 + H2O

O

6. Oxidation of alkyl benzenes Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzene with

chromic acid or acidic or alkaline potassium permanganate. The entire side chain is oxidised to –COOH group irrespective of the length of the side chain.

Example

CH3

OH-/KMnO4

COOH

Toluene Benzoic acid

3(O)

Evaluate yourself

1. What happens when n-propyl benzene is oxidised using H+ / KMnO4?

2. How will you prepare benzoic acid using Grignard reagent.