Name the reaction when carboxylic acids having an \(\alpha\)-hydrogen are halogenated at the \(\alpha\)-position on treatment with chlorine in the presence of a small amount of red phosphorus.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Hell-Volhard-Zelinsky reaction
An α-halocarboxylic acid is formed when a carboxylic acid with an α-hydrogen is treated with chlorine or bromine and a little red phosphorus: the Hell-Volhard-Zelinsky (HVZ) reaction.
\( \ce{RCH2COOH} \xrightarrow[\text{(ii) }\ce{H2O}]{\text{(i) }\ce{X2}\text{/red P}} \ce{RCH(X)COOH} \) (X = Cl, Br)
Why the other options are incorrect
Option (1) Kolbe electrolysis — Incorrect. Electrolysis of an aqueous solution of the sodium or potassium salt of a carboxylic acid gives an alkane with twice the alkyl group; no halogen is involved.
\( \ce{2RCOONa + 2H2O} \xrightarrow{\text{electrolysis}} \ce{R-R + 2CO2 + H2 + 2NaOH} \)
Option (3) Friedel Crafts reaction — Incorrect. Alkylation or acylation of an aromatic ring with anhydrous \( \ce{AlCl3} \); the α-carbon of an acid is not halogenated.
\( \ce{C6H6 + CH3COCl} \xrightarrow{\text{anhyd. }\ce{AlCl3}} \ce{C6H5COCH3 + HCl} \)
Option (4) Esterification — Incorrect. A carboxylic acid and an alcohol give an ester with an acid catalyst; no halogen is introduced.
\( \ce{RCOOH + R^{\prime}OH} \xrightarrow{\ce{H+}} \ce{RCOOR^{\prime} + H2O} \)
NCERT (2026-27 edition):
Halogenation
