Theatment of a ketone or aldehyde with a Grignard reagent, RMgX, yields an alcohol by nucleophilic addition of a carbon anion, or carbanion (Section 17.6). The C-Mg bond in the Grignard reagent is so strongly polarized that a Grignard reagent acts for all practical purposes as R:- +MgX.
Acid-base complexation of Mg2+ with the carbonyl oxygen atom first serves to make the carbonyl groubp a better acceptor, and nucleophilic addi-tion of R:_ then produces a tetrahedral magnesium alkoxide intermediate. Protnation by addition of water or dilute aqueous acid in a separate step yields the neutral alcohol (Figure 19.6). Unlike the nucleophilic additions
Of water and HCN, Grignard additions are irreversible because a carbanion is too poor a leaving group to be expelled in a reversal step.