Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl more efficiently than in the former.
The major product formed in the following reaction is
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → B only
Zinc amalgam and concentrated hydrochloric acid on heating is the Clemmensen reduction: the carbonyl group of an aldehyde or ketone is reduced to a \( \ce{CH2} \) group, so –CHO becomes –\( \ce{CH3} \). The hot concentrated acid also acts on the secondary benzylic alcohol group: it is protonated and loses water to give a benzylic carbocation, which loses \( \ce{H^{+}} \) from the next carbon to give the alkene that is conjugated with the benzene ring (the most stable alkene).
\( \ce{C6H5-CH(OH)-CH(CH3)-CH2-CHO} \) → \( \ce{C6H5-CH(OH)-CH(CH3)-CH2-CH3} \) (Clemmensen reduction)
\( \ce{C6H5-CH(OH)-CH(CH3)-CH2-CH3} \) → \( \ce{C6H5-CH=C(CH3)-C2H5} \) + \( \ce{H2O} \) (dehydration)
A. \( \ce{C6H5-CH(OH)-CH(CH3)-CH2-CH3} \) — Incorrect. This is only the product of the first step; in hot concentrated HCl the benzylic alcohol does not survive but is dehydrated.
B. \( \ce{C6H5-CH=C(CH3)-C2H5} \) — Correct. CHO is reduced to \( \ce{CH3} \) and the OH is lost with the H of the next carbon, giving the alkene conjugated with the ring (2-methyl-1-phenylbut-1-ene).
C. \( \ce{C6H5-C(CH3)=CH-C2H5} \) — Incorrect. Here the methyl group sits on the carbon attached to the ring; in the starting compound it is on the next carbon, and no step moves it.
D. 2-phenyl-3-methyltetrahydrofuran — Incorrect. A ring ether would need the OH group to add to the aldehyde carbon, but the CHO group is reduced to \( \ce{CH3} \), so no ring can close.
NCERT (2026-27 edition):
Reduction to Hydrocarbons – “The carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc-amalgam and concentrated hydrochloric acid [Clemmensen reduction] or with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).”