When a carbonyl compound is treated with a strong base, the α position gets deprotonated to give a resonance-stabilized intermediate called an enolate. Enolates are ambident nucleophiles because they possess two nucleophilic sites that can attack an electrophile owing to the delocalization of the negative charge between the α carbon and oxygen atoms. When the oxygen atom attacks an electrophile, it is called O-attack, whereas electrophilic attack via the α carbon is known as C-attack.
C-attack is much more common than O-attack despite the negative charge being more localized on the oxygen atom. (Recall that for a set of contributing structures with a negative charge on different atoms, the contributing structure with the negative charge on the most electronegative atom will usually most closely resemble the true structure.) As a result, there are two distinct conventions in drawing the C-attack mechanism:
The carbanion mechanism convention is more simplistic in representation because it needs fewer curved arrows, but both conventions are widely used.
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