A Grignard reaction uses an organomagnesium reagent, conventionally written , 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]
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