Name the reaction when carboxyclic acids having an a -hydrogen are halogenated at the \(\alpha \) - position on treatment with chlorine in the presence of a small amount of red phosphorus.

Hell-Volhard-Zelinsky reaction

The correct answer is option 2. Hell-Volhard-Zelinsky reaction

The reaction described, where carboxylic acids with an α-hydrogen are halogenated at the α-position in the presence of chlorine and a small amount of red phosphorus, is known as the Hell-Volhard-Zelinsky reaction (option 2).

In this reaction, the α-hydrogen of the carboxylic acid is replaced by a halogen atom (usually bromine or chlorine) to form an α-halo acid. The reaction proceeds through a radical mechanism initiated by the interaction of chlorine with red phosphorus, which generates halogen radicals. These radicals abstract a hydrogen atom from the α-position of the carboxylic acid, leading to the formation of the α-halo acid product.

A Guided Tour Through “Hell” : The Hell Volhard Zelinsky (HVZ) Reaction

The Hell-Volhard-Zelinsky reaction of carboxylic acids results in an alpha bromo carboxylic acid. The reaction proceeds in 4 steps:

1) substitution of \(OH\) for Br to give an acyl bromide,

2) keto-enol tautomerism,

3) bromination of the enol, and

4) hydrolysis of the acid bromide to give the carboxylic acid.

The resulting \(\alpha \)-bromo acids can then be transformed into alpha-amino acids.