Wolff-Kishner reduction
/ VOLF KISH-ner /
The Wolff-Kishner reduction does the same end job as the Clemmensen, erasing a carbonyl down to a CH2 (methylene) group, but it works under strongly basic conditions instead of acidic ones. It is the base-loving partner you reach for when your molecule cannot survive strong acid.
It runs in two conceptual stages. First the aldehyde or ketone is condensed with hydrazine (H2N-NH2) to form a hydrazone, C=N-NH2 (this first step is itself an imine-type nucleophilic addition followed by loss of water). Then a strong base such as potassium hydroxide, with heat, drives off the nitrogen as N2 gas and replaces the carbon's bonds to nitrogen with bonds to hydrogen. The escaping, very stable N2 is a big part of what makes the reaction go. The popular modern variant, the Huang Minlon modification, uses a high-boiling solvent like ethylene glycol so the whole thing can be done in one pot at high temperature.
As with the Clemmensen, the real lesson is selectivity by condition. If your substrate has base-stable but acid-sensitive groups, use Wolff-Kishner; if it has acid-stable but base-sensitive groups, use Clemmensen. Together they let you reduce a carbonyl to a methylene under whichever pH the rest of the molecule tolerates.
Cyclohexanone heated with hydrazine and KOH in ethylene glycol (the Huang Minlon conditions) gives cyclohexane, releasing N2 gas as the carbonyl becomes a CH2.
Via a hydrazone, the carbonyl is reduced to CH2 and nitrogen leaves as N2, all under base.
Clemmensen (acid) and Wolff-Kishner (base) are a complementary pair for the very same transformation; the deciding factor is which conditions the rest of your molecule can withstand, not which reduction is 'better'.