The Clemmensen reduction is an organic reaction used to reduce an aldehyde or ketone to an alkane using amalgamated zinc and hydrochloric acid. The mechanism for the Clemmensen reduction is not yet fully understood and there are two popular proposals. The "Carbanionic mechanism", where the zinc attacks the protonated carbonyl directly, and the "Carbenoid mechanism", which is a radical process and the reduction happens on the surface of the zinc metal.
Clemmensen, E. Ber. Dtsch. Chem. Ges. 1913, 46, 1837–1843.
Clemmensen, E. Ber. Dtsch. Chem. Ges. 1914, 47, 51–63.
Clemmensen, E. Ber. Dtsch. Chem. Ges. 1914, 47, 681–687.