Hell-Volhard-Zelinsky halogenation

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The Hell-Volhard-Zelinsky halogenation reaction halogenates carboxylic acids at the α carbon.

[edit] Scheme

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.

[edit] References

• Hell C. (1881). "". Berichte 14: 891.
• Volhard J. (1887). "". Annalen der Chemie 242: 141.
• Zelinsky N. (1887). "". Berichte 20: 2026.
• Allen C. F., Kalm M. J. (1958). "2-Methylenedodecanoic Acid". Organic Syntheses 38: 47.