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Reactions of Carboxylic Acids

Hell-Volhard-Zelinskii Reaction

The treatment of carboxylic acids with bromine and phosphorus tribromide (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 PBr3 are applied, chlorination occurs instead of bromination.

Fig.1
Halogenation of carboxylic acids in α position: Hell-Volhard-Zelinskii reaction
Fig.2
An Example of the Hell-Volhard-Zelinskii reaction
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Fig.3
Starting material
Fig.4
Product

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, 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.

Fig.5
Formation of the intermediate α-bromoacyl bromide in the Hell-Volhard-Zelinskii reaction

Analogous to acyl halides, α-bromoacyl halides may be converted into α-bromoesters, α-bromocarboxamides, and "back" to the α-bromocarboxylic acid. However, their α bromine atom may also be exchanged for another nucleophile in an SN2 reaction. The α-cyanocarboxylic acids and α-aminocarboxylic acids are, for example, available in this way.

Fig.6
Applications of α-bromoacyl halides
Fig.7
Example of an α-bromoacyl halide's conversion into an α-bromoester
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Fig.8
Starting material
Fig.9
Product

If catalytic amounts of PBr3 are applied, the intermediate α-bromoacyl bromide is attacked by another carboxylic acid molecule, yielding the corresponding anhydride. The bromide that is released during anhydride formation, then cleaves the anhydride into an α-bromocarboxylic acid - the product - and an acyl bromide. The acyl bromide reenters the reaction cycle.

Fig.10
Mechanism of Hell-Volhard-Zelinskii reaction with catalytic amounts of PBr3

Exercises

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