Clemmensen reduction

Sometimes you do not want to turn a carbonyl into an alcohol; you want to erase it completely, replacing the whole C=O with a plain CH2 group (a methylene). The Clemmensen reduction does exactly that under strongly acidic conditions: it strips an aldehyde or ketone all the way down to a hydrocarbon (R-CO-R' becomes R-CH2-R').

The recipe is zinc amalgam (zinc treated with mercury, written Zn(Hg)) in concentrated hydrochloric acid, with heating. The detailed mechanism is debated and messy (it likely involves the metal surface and radical or carbanion-like intermediates rather than a clean discrete carbocation), but the net transformation is simple and reliable: both C-O bonds are removed and replaced by C-H bonds. Think of it as the carbonyl being scrubbed off the carbon entirely.

Its value is mostly as one half of a pair. Because it runs in strong acid, the Clemmensen reduction is the choice when your molecule contains acid-tolerant but base-sensitive groups. Its partner, the Wolff-Kishner reduction, does the same job in strong base, so you pick whichever set of conditions your other functional groups can survive. A classic use: after a Friedel-Crafts acylation puts a ketone on a benzene ring (acylation avoids the rearrangements of direct alkylation), Clemmensen reduces that ketone to a clean straight-chain alkyl group.