Mechanism of Aldol Condensation Reaction

The Aldol Condensation reaction involves the reaction of an aldehyde or ketone having at least one α-hydrogen with another molecule of the same or different aldehyde or ketone in the presence of a dilute base to form a β-hydroxy aldehyde (aldol) or β-hydroxy ketone. Here is the step-by-step mechanism:

1. Formation of Enolate Ion: The base abstracts an α-hydrogen from the aldehyde or ketone to form an enolate ion.
2. Nucleophilic Addition: The enolate ion then attacks the carbonyl carbon of another molecule of aldehyde or ketone, forming an alkoxide ion.
3. Protonation: The alkoxide ion abstracts a proton from water or another molecule of the solvent to form a β-hydroxy aldehyde or β-hydroxy ketone.
4. Dehydration (if applicable): In some cases, the β-hydroxy aldehyde or ketone can undergo a dehydration reaction to form an α,β-unsaturated aldehyde or ketone.

An example from the NCERT syllabus is the reaction of acetaldehyde (CH₃CHO) to form 3-hydroxybutanal (aldol) and subsequently crotonaldehyde (α,β-unsaturated aldehyde).