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.