Question

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

a.

b.

Short answer

a.

b.

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 ${\text{PBr}}_{\text{3}}$and ${\text{Br}}_2$, and after hydrolysis, the $\alpha$-carbon of carboxylic acid gets brominated.

Preparing compound from starting material shown in (a)

a.) When a carboxylic acid is treated with ${\text{PBr}}_{\text{3}}$, 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 $\alpha$-brominated acyl bromide gets hydrolyzed and forms $\alpha$-brominated carboxylic acid. By adding an excess of ${}^{\text{-}}\text{OH}$, the bromine is replaced by OH. The reaction is shown below.

Formation of $\alpha$-hydroxy carboxylic acid

Preparing compound from starting material shown in (b)

b.) When acetic acid(carboxylic acid) is treated with ${\text{PBr}}_{\text{3}}$and ${\text{Br}}_2$, it brominates the -carbon, and this reaction is known as the Hell-Volhard-Zelinski reaction. Now, the addition of ${\left({\text{CH}}_{\text{3}}\right)}_{\text{2}}\text{CuLi}$(Gilman reagent) replaces the bromine by methyl (${\text{- CH}}_{\text{3}}$) group.

Hell-Volhard-Zelinski reaction and Gilman reagent addition