Question

Carboxylic acids can be made by the hydrolysis of nitriles, which in turn can be made from an alkyl halide. Draw the structures of a starting alkyl bromide and the intermediate nitrile that would be used in the synthesis of 3-methylbutanoic acid (isovaleric acid) Do not show free ions.

Answer 1

Concepts and reason

A chemical reaction involving the synthesis of a carboxylic acid (isovaleric acid) from a haloalkane (alkyl bromide) is given. The reagents are given and the structure of the starting alkyl bromide and the intermediate alkyl nitrile needs to be given. A chemical reaction involves the reaction between two or more substances. The reaction process is helped by the reagents that help in bringing about the transformation of the reactant into product.

Fundamentals

The alkyl halides undergo substitution reaction when treated with cyanide ion as follows. The alkyl cyanide further undergoes hydrolysis to give carboxylic acid as shown below:

\(\mathrm{R}-\mathrm{X} \stackrel{\mathrm{CN}}{\longrightarrow} \mathrm{R}-\mathrm{CN} \frac{\mathrm{H}_{2} \mathrm{O}}{\mathrm{H}^{\prime}} \rightarrow \mathrm{R}-\mathrm{COOH}\)

Thus, the addition of cyanide ion is a step-up reaction that is it increases the number of carbons in the carbon chain by one.

 

The structure of isovaleric acid is as follows:

Isovaleric acid is a four-carbon chain acid and is substituted at the third position by a methyl group. Its common

name is \(\beta\) -methylbutyric acid and IUPAC name is 3-methylbutanoic acid as shown below:

The structure of the alkyl bromide is as follows:

In the above reaction, the structure of alkyl halide is 1-bromo-2-methylpropane as the initial isovaleric acid has four carbons. 1-bromo-3-methylpropane undergoes substitution reaction with cyanide ion to give 3 -methylbutanenitrile. Thus, increasing the number of carbons in the chain by one.

The reaction is as follows:

 

The cyanides undergo hydrolysis to yield carboxylic acid. Therefore, the hydrolysis of the above cyanide that is 3 methylbutanenitrile gives the desired product.