Chapter 17: Q11P (page 810)

Show how the following compounds can be prepared from the given starting material:
a.
b.

Short Answer

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Step by step solution

Halogenation of the -carbon of carboxylic acids

The halogenation of the -carbon of carboxylic acids is known as the Hell-Volhard-Zelinski reaction.In this reaction, the carboxylic acids are treated with ${PBr}_{3}$ and ${Br}_{2}$ , and after hydrolysis, the $α$ -carbon of carboxylic acid gets brominated.

Preparing compound from starting material shown in (a)

a.) When a carboxylic acid is treated with ${PBr}_{3}$ , it forms an acyl bromide. This acyl bromide is in equilibrium with its enol. Now, enol bromination forms a protonated -brominated acyl bromide. In the second step, this protonated $α$ -brominated acyl bromide gets hydrolyzed and forms $α$ -brominated carboxylic acid. By adding an excess of ${}^{-}{OH}$ , the bromine is replaced by OH. The reaction is shown below.

Formation of $α$ -hydroxy carboxylic acid

Preparing compound from starting material shown in (b)

b.) When acetic acid(carboxylic acid) is treated with ${PBr}_{3}$ and ${Br}_{2}$ , it brominates the -carbon, and this reaction is known as the Hell-Volhard-Zelinski reaction. Now, the addition of ${({CH}_{3})}_{2} CuLi$ (Gilman reagent) replaces the bromine by methyl ( ${- CH}_{3}$ ) group.