The reaction is named after three chemists, the German chemists Carl Magnus von Hell — and Jacob Volhard — and the Russian chemist Nikolay Zelinsky — An approach using a Strecker synthesis  was described as "excellent but tedious"  and so an alternative starting with propionic acid was developed. In its first step, a combination of bromine and phosphorus tribromide catalyst is used to prepare 2-bromopropanoic acid ,  which is then converted to a racemic mixture of the amino acid product by ammonolysis. 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. If an aqueous solution is desirable, a full molar equivalent of PBr3 must be used as the catalytic chain is disrupted.
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For help asking a good homework question, see: The reaction is initiated by addition of a catalytic amount hell-volhard-zelinnsky PBr 3after which one yell-volhard-zelinsky equivalent of Br 2 is added. In other projects Wikimedia Commons. PBr 3 replaces the carboxylic OH with a bromide, resulting in a carboxylic acid bromide. The mechanism for the exchange between an alkanoyl bromide and a carboxylic acid is below. How do you think the alpha-bromine is incorporated into the acyl bromide?
If an aqueous solution is desirable, a full molar equivalent of PBr 3 must be used as the catalytic chain is disrupted. This question appears to be off-topic. This page hell-volhard-zelinsly last edited on 29 August hell-voolhard-zelinsky, at Acyl halides lack the carboxylic acid hydrogen.
Unlike other halogenation reactions, this reaction takes place in the absence of a halogen carrier. Sometimes, phosphorus tribromide is not enough to form the acyl halide but, since there is also bromine in the media, phosphorus pentabromide is formed which can form acyl halides even of solid acids.
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Reazione di Hell-Volhard-Zelinsky