15.15 : 塩基触媒によるアルドール付加反応

In aqueous sodium hydroxide, two aldehyde molecules undergo an addition reaction to form a β-hydroxy aldehyde. This is a base-catalyzed aldol addition reaction with a multistep mechanism.

The reaction begins with an enolization step, where the hydroxide ion reversibly deprotonates the aldehyde at the α carbon to generate a resonance-stabilized enolate ion.

Next, the nucleophilic addition of the enolate's α carbon to the carbonyl group of the unreacted aldehyde yields an alkoxide ion intermediate.

The subsequent protonation of the alkoxide ion generates a β-hydroxy aldehyde as the aldol addition product. Here, the equilibrium is more favorable in the forward direction.

However, for α-substituted aldehydes, the equilibrium shifts backwards to the precursors due to the steric hindrance at the reaction site. 

In ketones, the equilibrium favors the ketone reactant rather than the corresponding aldol addition product.