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Hell-Volhard-Zelinskii Reaction

The treatment of carboxylic acids with bromine and phosphorus tribromide ($\ce{PBr3}$) or a mixture of bromine and phosphorus, yields the α-bromocarboxylic acid. Phosphorus tribromide is often applied only as a catalyst. Thus, if chlorine and $\ce{PBr3}$ are applied, chlorination occurs instead of bromination.

Halogenation of carboxylic acids in α position: Hell-Volhard-Zelinskii reaction

Example

Keto–enol tautomerism

In organic chemistry, keto–enol tautomerism refers to a chemical equilibrium between a keto form (a ketone or an aldehyde) and an enol (an alcohol). The enol and keto forms are said to be tautomers of each other. The interconversion of the two forms involves the movement of an alpha hydrogen and the shifting of bonding electrons; hence, the isomerism qualifies as tautomerism.

The reaction is known as the Hell-Volhard-Zelinskii reaction (after J. Volhard, C.M. Hell, and N.D. Zelinskii). In the frist reaction step, $\ce{PBr3}$ converts the carboxylic acid into the corresponding acyl bromide. The acyl bromide is in equlibrium with the corresponding enol. A nucleophilic attack of the enol π bond on a bromine molecule yields HBr and an α-bromoacyl bromide, which may be isolated

Sodium benzoateSodium benzoate is a preservative. It is bacteriostatic and fungistatic under acidic conditions. It is most widely used in acidic foods such as salad dressings (vinegar), carbonated drinks (carbonic acid), jams and fruit juices (citric acid), and pickles (vinegar)
Sodium benzoate is produced by the neutralization of benzoic acid with sodium hydroxide

Preparation of Benzene