The $C$ atom of the carbonyl group is activated as a result of its high positive partial charge. It is ready for nucleophilic attack (Nu = nucleophile):
In acidic medium, the $O$ atom of the carbonyl group can pick up a $H^+$ ion and thus increase the electrophilicity of the carbon neighbor:
A very strong base (here ethanolate anion) can remove a hydrogen atom in α to produce an anion stabilized by mesomerism. This anion has a marked nucleophilic character on the α carbon :
