Question

Acetaldehyde shows aldol condensation but benzaldehyde does not

Answer 1

benzaldehyde does not contain alpha hydrogen

Answer 2

Acetaldehyde has molecular formula CH3CHO.It is clear that it contains α hydrogen which is being abstructed by base. But in benzaldehyde there is no α hydrogen which can be abstructed by base.

For this reason acetaldehyde shows aldol condensation but benzaldehyde does not.

In case of acetaldehyde we get β-hydroxy aldehyde whicn on heating gives crotonaldehyde which is α,β-unsaturated aldehyde.