Grignard reaction

A Grignard reaction uses an organomagnesium reagent, conventionally written $\mathrm{RMgX}$, as a carbon-based nucleophile or base. Addition to an aldehyde or ketone, followed by protonation of the alkoxide product, forms an alcohol with a new carbon-carbon bond.

The reagent is not simply a free carbanion: its bonding, aggregation, and coordination depend on its environment. Proton donors can consume it by an acid-base reaction instead of the intended carbonyl addition. This competition makes functional-group compatibility central to understanding its synthetic role.^[ref]

Two organomagnesium additions to a suitable ester give a tertiary alcohol after work-up. The first addition–elimination produces a ketone, which accepts the second carbon substituent.

Addition of an organomagnesium reagent to carbon dioxide followed by protonation gives a carboxylic acid with one additional carbon. A free acid cannot remain present during a subsequent Grignard addition without consuming reagent.