Question

Aldehydes that do not undergo aldol condensation are
1. propanal
2. trichloroethanal
3. 2-phenylethanal
4. ethanal
5. benzaldehyde

• A. 3 and 4 only
• B. 2 and 5 only
• C. 1, 2 and 5 only
• D. 2, 3 and 5 only

Answer 1

The correct option is B 2 and 5 only

An aldol condensation is a condensation reaction in which an enol or an enolate ion reacts with a carbonyl compound to form a $\beta$-hydroxyaldehyde or $\beta$-hydroxyketone, followed by dehydration to an $\alpha ,\beta$-unsaturated aldehyde.
Aldol condensation occurs in aldehydes having $\alpha$-hydrogen.
2 and 5 do not have $\alpha$-hydrogen thus they will undergo aldol condensation. Hence correct option is (b).