Nucleophilic attack on the carbonyl group: Aldolisation of ethanal
The hydrogen in α position
The hydrogen in α position of aldehydes or ketones has an acidic character. here is why:
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The stabilization of the enolate ion explains the acidic character of the H in α position.
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1) Acid-base reaction, $H$ α is removed by strong alcoholate base.
2) Nucleophilic attack on the $C$ atom of the carbonyl group.
3) Acid-based proton exchange.
Aldolisation reaction is used to connect together two carbon chains!