Carboxylic acids having an α-hydrogen are halogenated at the α-position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give α-halocarboxylic acids. The reaction is known as Hell-Volhard-Zelinsky reaction.
Carboxylic acids having an α-hydrogen undergo substitution at the α-position. In aromatic carboxylic acids, substitution can also take place in the benzene ring. Which of the following reactions does benzoic acid NOT undergo?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Friedel-Crafts reaction in the presence of anhydrous AlCl3
In benzoic acid the carboxyl group is attached directly to the ring. It withdraws electrons from the ring, so it is a deactivating and meta-directing group. Benzoic acid still undergoes electrophilic substitution, but more slowly, and the new group enters the meta position.
Option (1) Nitration with a mixture of concentrated HNO3 and concentrated H2SO4 — Takes place. The product is m-nitrobenzoic acid:
\( \displaystyle \ce{C6H5COOH + HNO3} \xrightarrow{\text{conc. }\ce{H2SO4}} \ce{O2N-C6H4-COOH + H2O} \quad \text{(m-nitrobenzoic acid)} \)
Option (2) Bromination with Br2 in the presence of FeBr3 — Takes place. The product is m-bromobenzoic acid:
\( \displaystyle \ce{C6H5COOH + Br2} \xrightarrow{\ce{FeBr3}} \ce{Br-C6H4-COOH + HBr} \quad \text{(m-bromobenzoic acid)} \)
Option (3) Friedel-Crafts reaction in the presence of anhydrous AlCl3 — Does not take place. Two things stop it: the carboxyl group makes the ring less reactive, and the catalyst AlCl3, a Lewis acid, gets bonded to the carboxyl group, so it is not free to act as the catalyst.
Option (4) Reaction with sodium hydrogencarbonate, with evolution of CO2 — Takes place. Carboxylic acids react even with weak bases such as hydrogencarbonates and give off carbon dioxide; this is the usual test for a carboxyl group:
\( \displaystyle \ce{C6H5COOH + NaHCO3 -> C6H5COONa + H2O + CO2 ^} \)
NCERT (2026-27 edition):
8.9.4 Substitution Reactions in the Hydrocarbon Part – “2. Ring substitution
Aromatic carboxylic acids undergo electrophilic substitution reactions in which the carboxyl group acts as a deactivating and meta-directing group. They however, do not undergo Friedel-Crafts reaction (because the carboxyl group is deactivating and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxyl group).”