Identify the name of the following reaction from the given options.
\( \displaystyle \ce{R-CH2-COOH} \xrightarrow[\text{(ii) }\ce{H2O}]{\text{(i) }\ce{X2}\text{/Red Phosphorus}} \ce{R-CH(X)-COOH} \)
X = Cl or Br
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Hell-Volhard-Zelinsky reaction
The four options are named reactions from different families:
Hell-Volhard-Zelinsky (HVZ) reaction: α-halogenation of a carboxylic acid with \( \ce{X2} \)/red phosphorus.
\( \displaystyle \ce{R-CH2-COOH} \xrightarrow[\text{(ii) }\ce{H2O}]{\text{(i) }\ce{X2}\text{/red P}} \ce{R-CH(X)-COOH} \)
Kolbe’s reaction: sodium phenoxide is heated with CO2 under pressure and then acidified, giving salicylic acid.
\( \displaystyle \ce{C6H5ONa} \xrightarrow[\text{(ii) }\ce{H^+}]{\text{(i) }\ce{CO2}\text{, 400 K, 4-7 atm}} \ce{o-HOC6H4COOH} \)
Fehling’s test: an aldehyde reduces the \( \ce{Cu^{2+}} \) of Fehling’s solution to a red-brown precipitate of Cu2O.
\( \displaystyle \ce{R-CHO + 2Cu^2+ + 5OH^- -> RCOO^- + Cu2O v + 3H2O} \)
Gabriel phthalimide synthesis: potassium phthalimide is alkylated with an alkyl halide and the product is hydrolysed, giving a primary amine.
\( \displaystyle \ce{C6H4(CO)2NK + R-X -> C6H4(CO)2N-R + KX} \)
\( \displaystyle \ce{C6H4(CO)2N-R + 2NaOH(aq) -> C6H4(COONa)2 + R-NH2} \)
The given reaction converts \( \ce{R-CH2-COOH} \) into R–CH(X)–COOH with \( \ce{X2} \)/red phosphorus, followed by water: it is the Hell-Volhard-Zelinsky reaction.
NCERT (2026-27 edition):
Substitution Reactions in the Hydrocarbon Part
