Topic overview

Start with the big picture

An aldol reaction requires at least one carbonyl partner with an α-hydrogen. Under basic conditions, an enolate attacks another carbonyl; under acidic conditions, an enol attacks a protonated carbonyl. Both routes lead to a β-hydroxy carbonyl after proton transfer. Heat and base can promote dehydration, forming a conjugated α,β-unsaturated carbonyl. Reaction choice affects selectivity: crossed aldol reactions are more manageable when one partner lacks an α-hydrogen or an enolate is prepared in advance. Intramolecular aldol reactions can form five- or six-membered rings, while retro-aldol cleavage reverses the bond-forming process. These patterns connect the reaction to synthesis and biochemical chemistry.