In an aldol addition reaction, the β-hydroxy aldehyde formed contains mildly acidic hydrogens at the α carbon.

The heating of the aldol product in a strong base shifts the equilibrium forward, enabling a dehydration reaction to form an enal.

As for the dehydration mechanism, it occurs in two distinct steps: enolization and elimination.

In the first step, the base abstracts the hydrogen atom at the α carbon generating an enolate ion intermediate — the conjugate base.

The second step involves the loss of the hydroxide ion via an E1cB elimination reaction to form a stable enal as the final product. Here, 'E' corresponds to elimination, '1' represents a unimolecular reaction, and 'cB' denotes a conjugate base.

Stereochemistry plays a crucial role. Among the cis and trans stereoisomers of the enal, the formation of the trans isomer is favorable due to the fewer steric interactions.

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.

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