Carboxylic acids with alpha-hydrogens undergo halogenation at the alpha-position using chlorine or bromine in the presence of red phosphorus, a reaction known as Hell-Volhard-Zelinsky reaction.
How do carboxylic acids with alpha-hydrogens undergo halogenation, and what is the specific reaction known as?
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Carboxylic acids with alpha-hydrogens undergo halogenation via a reaction known as the Hell-Volhard-Zelinsky (HVZ) reaction. In the presence of a halogen, typically bromine or chlorine, and a catalytic amount of phosphorus, the alpha-hydrogen of the carboxylic acid is replaced by a halogen atom. The mechanism involves the formation of a reactive carboxylic acid-halogen complex, followed by nucleophilic attack at the alpha-position. The HVZ reaction allows the introduction of halogen substituents at the alpha-carbon of carboxylic acids, providing a route to α-halo-carboxylic acids with potential synthetic utility.