This lesson delves into the aldol condensation catalyzed by bases, where aldols undergo dehydration to enals. As shown in Figure 1, the β-hydroxy aldehyde formed in a base-catalyzed aldol addition reaction dehydrates on heating to yield an unsaturated carbonyl product, which is commonly referred to as an enal.

Figure 1. The dehydration of a β-hydroxy aldehyde to an enal.

This process follows a two-step mechanism, as depicted in Figure 2. Here, the base first deprotonates the mildly acidic hydrogen at the α carbon to form an enolate ion intermediate. Subsequently, the hydroxide ion leaves via an E1cB elimination reaction to generate an enal as the final product. The trans stereoisomer of the enal is obtained as the major product.

Figure 2. The base-catalyzed aldol condensation mechanism for dehydration of aldol to enals.