Hell-Volhard-Zelinsky halogenation

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The Hell-Volhard-Zelinsky halogenation reaction halogenates carboxylic acids at the α carbon. The reaction is named after three chemists, the German chemists Carl Magnus von Hell (1849-1926) and Jacob Volhard and the Russian chemist Nikolay Zelinsky ^{[1] [2] [3] [4]} .

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The reaction is initiated by addition of a catalytic amount of PBr₃, after which one molar equivalent of Br₂ is added.

PBr₃ replaces the carboxylic OH with a bromide, resulting in a carboxylic acid bromide. The acyl bromide can then tautomerize to an enol, which will readily react with the Br₂ to brominate a second time at the α position.

In neutral to slightly acidic aqueous solution, hydrolysis of the α-bromo acyl bromide occurs spontaneously, yielding the α-bromo carboxylic acid in an example of a nucleophilic acyl substitution. If an aqueous solution is desirable, a full molar equivalent of PBr₃ must be used as the catalytic chain is disrupted.

If little nucleophilic solvent is present, reaction of the α-bromo acyl bromide with the carboxylic acid yields the α-bromo carboxylic acid product and regenerates the acyl bromide intermediate. In practice a molar equivalent of PBr₃ is often used anyway to overcome the slow reaction kinetics.

The mechanism for the exchange between an alkanoyl bromide and a carboxylic acid is at below. The α-bromoalkanoyl bromide has a strongly electrophilic (δ+) carbonyl carbon because of the electron-withdrawing effects of the two bromides. The carboxylic acid will act as a nucleophile that the carbonyl oxygen of the carboxylic acid has a partial negative charge (δ-).

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