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Organic Chemistry

22.4 Alpha Bromination of Carboxylic Acids

Organic Chemistry22.4 Alpha Bromination of Carboxylic Acids

22.4 • Alpha Bromination of Carboxylic Acids

The α bromination of carbonyl compounds by Br2 in acetic acid is limited to aldehydes and ketones because acids, esters, and amides don’t enolize to a sufficient extent. Carboxylic acids, however, can be α brominated by a mixture of Br2 and PBr3 in the Hell–Volhard–Zelinskii (HVZ) reaction.

Heptanoic acid reacts with bromine, phosphorus tribromide, to form 2-bromoheptanoic acid in 90 percent yield

The Hell–Volhard–Zelinskii reaction is a bit more complex than it looks and actually involves α substitution of an acid bromide enol rather than a carboxylic acid enol. The process begins by reaction of the carboxylic acid with PBr3 to form an acid bromide plus HBr (Section 21.4). The HBr then catalyzes enolization of the acid bromide, and the resultant enol reacts with Br2 in an α-substitution reaction to give an α-bromo acid bromide. Addition of water hydrolyzes the acid bromide in a nucleophilic acyl substitution reaction and yields the α-bromo carboxylic acid product.

Carboxylic acid reacts with phosphorus tribromide to form two interconvertable acid bromide intermediates, which react with bromine to yield an alpha-bromo acid bromide, which further reacts with water to form an alpha-bromocarboxylic acid.
Problem 22-6

If methanol rather than water is added at the end of a Hell–Volhard–Zelinskii reaction, an ester rather than an acid is produced. Show how you would carry out the following transformation, and propose a mechanism for the ester-forming step.

3-methyl pentanoic acid reacts with unknown reagent represented by question mark to form six-carbon chain with carbonyl group at C 2, bromine at C 3, and methyl at C 4.
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