Aldehydes And Ketones - Carbonyl Reduction

Converting a carbonyl group fully to a CH2 group requires deoxygenation, and two classic methods achieve this under different conditions. Clemmensen reduction uses zinc amalgam and concentrated hydrochloric acid, so it works in strongly acidic medium and would damage acid-sensitive substrates such as a phenolic ring prone to side reactions. Wolff-Kishner reduction instead converts the carbonyl to a hydrazone and then treats it with a strong base like KOH in a high-boiling solvent, so it operates under basic conditions and is the correct choice when acid-sensitive groups must survive. Catalytic hydrogenation with nickel reduces the carbonyl only partially or attacks other unsaturation and does not cleanly give the methylene group here, so it is unsuitable. Sodium borohydride reduces a ketone only to a secondary alcohol, not to CH2, so it cannot accomplish the required deoxygenation. This complementary pairing of acidic Clemmensen versus basic Wolff-Kishner is stressed in NCERT and is a recurring JEE Advanced selection question. In the Wolff-Kishner route the intermediate hydrazone loses nitrogen gas on heating with base, an essentially irreversible step that drives the deoxygenation to completion. Knowing which medium each method needs lets a chemist protect sensitive groups, and here the phenolic hydroxyl and the activated ring are far better preserved under basic than under hot concentrated acid. As a check, the basic conditions of Wolff-Kishner are compatible with the acid-sensitive substrate, confirming the answer.