In addition to the above methods, aromatic carboxylic acids can also be prepared by the oxidation of an alkyl group attached to benzene nucleus which will be oxidised to -COOH group irrespective of the size of the alkyl group.
For example.
Aromatic carboxylic acids can also be prepared by the Friedel-Craft reaction and hydrolysis of the side chain halogen derivative, e.g.,
(a) \(C_6H_6+ \underset {Carbonyl\,chloride}{COCl_2} \xrightarrow {Anhy.AlCl_3} \)
\(\underset {Benzoyl\,ehloride}{C_6H_5COCl}\xrightarrow {H_2O} C_6H_5COOH\)
Note. Carbonyl chloride should be used in excess otherwise benzophenone may be formed main product.
\(C_6H_6+COCl_2+C_6H_6 \xrightarrow {Anhy.AlCl_3} \underset {Benzophenone}{C_6H_5.C_6H_5}+2HCl\)
(b) \(\underset{Benzotrichloride}{C_6H_5CCl_3}\xrightarrow{aq.NaOH} \underset {Unstable}{C_6H_5C(OH)_3}\longrightarrow \underset {Benzonic\,acid}{C_6H_5COOH}\)
