When 1,3-Diphenylprop-2-en-1-one is formed by the reaction of benzaldehyde and acetophenone in the presence of dil. NaOH followed by heating, which is the correct option?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Acetophenone acts as Nucleophile
The formation of 1,3-diphenylprop-2-en-1-one (benzalacetophenone, chalcone) is a cross aldol condensation (Claisen-Schmidt reaction). In dilute NaOH only the carbonyl compound with an α-hydrogen can form a carbanion (enolate):
Acetophenone, \( \ce{C6H5COCH3} \), has three α-hydrogens; \( \ce{OH^{-}} \) removes one to give the enolate \( \ce{C6H5COCH2^{-}} \), which is the nucleophile.
Benzaldehyde has no α-hydrogen; its carbonyl carbon is attacked, so it is the electrophile.
The β-hydroxy ketone formed loses water on heating to give the conjugated α,β-unsaturated ketone:
\( \displaystyle \ce{C6H5CHO + CH3COC6H5} \xrightarrow[\text{heat}]{\text{dil. }\ce{NaOH}} \ce{C6H5CH=CHCOC6H5 + H2O} \)
Why the other options are incorrect
Option (1) Acetophenone acts as Electrophile — Incorrect. It is the component that forms the carbanion, so it attacks rather than being attacked.
Option (2) Benzaldehyde acts as Nucleophile — Incorrect. Without an α-hydrogen benzaldehyde cannot form an enolate, so it can only accept the nucleophile.
Option (4) Benzaldehyde has an α-hydrogen — Incorrect. The carbon next to the CHO group of benzaldehyde is a ring carbon with no hydrogen; this is why benzaldehyde undergoes the Cannizzaro reaction with concentrated alkali.
NCERT (2026-27 edition):
Reactions due to α-Hydrogen – “Acidity of α-hydrogens of aldehydes and ketones: The aldehydes and ketones undergo a number of reactions due to the acidic nature of α-hydrogen. The acidity of α-hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base. …
(ii) Cross aldol condensation: When aldol condensation is carried out between two different aldehydes and / or ketones, it is called cross aldol condensation. If both of them contain α-hydrogen atoms, it gives a mixture of four products. … Ketones can also be used as one component in the cross aldol reactions.”