61.

Explain why :

Acetaldehyde undergoes aldol condensation, but formaldehyde does not.

Aldol condensation involves the addition of an aldehyde (or ketonic) group of one molecule of the carbonyl compound (aldehyde or ketone) with the α-hydrogen atoms of the other. Acetaldehyde has α-hydrogen atoms and undergoes aldol condensation. Benzaldehyde, which has no α-hydrogen atom, does not.