Hvz Reaction Hell Volhard Zelinsky Reaction — the NEET Chemistry reaction: mechanism, reagents, conditions, structures and exam traps.
HVZ Reaction (Hell-Volhard-Zelinsky Reaction) The Hell-Volhard-Zelinsky (HVZ) reaction is a halogenation reaction that substitutes an alpha-hydrogen atom of a carboxylic acid with a halogen atom (chlorine or bromine). This reaction requires the presence of at least one alpha-hydrogen on the carboxylic acid. Disappearance of the halogen color (e.g., reddish-brown for bromine, greenish-yellow for chlorine). Evolution of hydrogen halide gas (HCl or HBr), which can be observed as white fumes in moist air. Generally an exergonic reaction, driven by the formation of the more stable alpha-halo carboxylic acid and hydrogen halide (HX). The carboxylic acid reacts with a catalytic amount of phosphorus and halogen ( X 2 ) (or phosphorus trihalide, e.g., PCl 3 ) to form an acyl halide ( RCH 2COX ). The acyl halide tautomerizes to its enol form (enolization), which is more reactive towards electrophilic attack. The enol form reacts with X 2 (halogen) at the alpha-carbon to form an alpha-halo acyl halide ( RCHXCOX ). The alpha-halo acyl halide undergoes exchange with another molecule of the starting carboxylic acid or is hydrolyzed during workup to yield the alpha-halo carboxylic acid ( RCHXCOOH ), regenerating the acyl halide ( RCH 2COX ) to continue the catalytic cycle. Forgetting the requirement of an alpha-hydrogen: Carboxylic acids without alpha-hydrogens (e.g., benzoic acid, pivalic acid) do not undergo the HVZ reaction. Over-halogenation: If excess halogen is used and multiple alpha-hydrogens are present, multiple halogens can be substituted (e.g., acetic acid can form dichloroacetic acid). Incorrectly assuming direct halogenation of the carboxylic acid, missing the crucial acyl halide intermediate. Confusing the HVZ reaction with free radical halogenation or other types of halogenation reactions. Failing to identify phosphorus or phosphorus trihalides as essential catalysts for acyl halide formation.