13.11: Preparation of Carboxylic Acids: Carboxylation of Grignard Reagents
Carboxylic acids can be prepared by the carboxylation of Grignard reagents (RMgX). This method is convenient for converting alkyl (primary, secondary or tertiary), vinyl, benzyl, and aryl halides to carboxylic acids with one additional carbon than the starting RMgX.
The carboxylation mechanism involves two steps. In the first step, the nucleophilic attack of the Grignard reagent on the electrophilic site of the carbon dioxide generates the magnesium salt of the carboxylate ion. In the subsequent step, it undergoes protonation by aqueous acid to yield carboxylic acid.
For the second step, the proton source is added after completion of the first step (reaction between the Grignard reagent and carbon dioxide) because of the less compatibility of the proton source with the Grignard reagent.
The major drawback of this method is the presence of substituents incompatible with the Grignard reagent (such as –OH, –NH, –SH, or –C=O) on the aryl or alkyl halides.