Read the passage carefully and answer the questions.
Carboxylic acids are the organic compounds which comprise of carboxyl functional group, -COOH. These are generally found in nature, some higher members are known as fatty acids as they occur in natural fats as esters of glycerol. They can be prepared from oxidation of alcohols and aldehydes, hydrolysis of nitriles and amides and Grignard reagents. They are stronger acids than simple phenols and alcohols. but weaker than mineral acids. Substituents affect the acidity of aliphatic and aromatic carboxylic acids. They undergo reactions involving cleavage of -OH and C-OH bond. Carboxylic acids having an alpha hydrogen get halogenated in the presence of red phosphorus to form alpha halo carboxylic acids. The reaction is known as Hell Volhard Zelinsky reaction. Besides these aromatic carboxylic acids undergo electrophilic substitution reactions. They are versatile and are employed in the manufacture of various useful compounds. For example, methanoic acid is used in rubber, textile, dyeing, leather and electroplating industries. Higher fatty acids are used in the manufacture of soaps and detergents. Esters of Benzoic acid are used in perfumery.
Which carboxylic acid will not undergo HVZ reaction?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Benzoic acid
In the Hell-Volhard-Zelinsky (HVZ) reaction, a carboxylic acid is treated with chlorine or bromine and a little red phosphorus, and an α-hydrogen (on the carbon next to –COOH) is replaced by the halogen:
\( \displaystyle \ce{R-CH2-COOH} \xrightarrow[\text{(ii) }\ce{H2O}]{\text{(i) }\ce{X2}/\text{red P}} \ce{R-CH(X)-COOH} \)
Option (1) Ethanoic acid — Undergoes HVZ (has α-H). \( \ce{CH3COOH} \) has three α-hydrogens and gives chloroacetic or bromoacetic acid.
Option (2) Benzoic acid — Does not undergo HVZ (no α-H). In \( \ce{C6H5COOH} \) the –COOH group is attached directly to a ring carbon, which carries no hydrogen.
Option (3) Propanoic acid — Undergoes HVZ (has α-H). \( \ce{CH3CH2COOH} \) has two α-hydrogens and gives 2-halopropanoic acid.
Option (4) Phenyl acetic acid — Undergoes HVZ (has α-H). \( \ce{C6H5CH2COOH} \) has two α-hydrogens on the \( \ce{CH2} \) group between the ring and –COOH.
NCERT (2026-27 edition):
Halogenation – “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.”