13.13 : Carboxylic Acids to Methylesters: Alkylation using Diazomethane

Carboxylic acids react with diazomethane in an ether solvent via alkylation at the carboxylate oxygen atom to give methyl esters of the corresponding acid with excellent yields.

Diazomethane is a yellow gas having a boiling point of −23 °C. It is conveniently prepared by the action of a base on N-methyl-N-nitrosourea or N-methyl-N-nitrosotoluenesulphonamide.

The esterification mechanism involves the protonation of diazomethane by the carboxylic acid to yield a carboxylate salt and methyldiazonium cation. The methyldiazonium cation is highly unstable due to the presence of N≡N, which functions as an excellent leaving group.

In the final alkylation step, an S_N2 attack on the methyldiazonium ion by the carboxylate anion produces a methyl ester, along with the elimination of N₂ gas.

However, diazomethane is a potentially explosive gas and requires precautions during its handling and storage. It undergoes spontaneous detonation upon contact with rough or scratched surfaces. Therefore, flame-polished glassware and a blast shield are often recommended in reactions involving diazomethane. Moreover, diazomethane is highly toxic as well as carcinogenic.

Diazomethane decomposes in the presence of heat or light to generate carbenes and eliminate nitrogen.