One-electron reduction

Overview
A one-electron reduction in organic chemistry involves the transfer of an electron from a metal to an organic substrate. It serves to differentiate between true organic reductions and other reductions such as hydride transfer reactions that actually involve two-electron species.

The first intermediate in a one-electron reduction is often a radical anion which then engages in secondary reactions. In the Birch reduction the secondary reaction is proton abstraction from an alcohol. This reaction type is also called a dissolving metal reduction. Alkyne reduction to an alkene in the liquid ammonia / sodium system follows the same theme. The first radical anion intermediate abstracts a proton from ammonia to the free radical. A second one-electron transfer leads to the anion which also abstracts a proton to the neutral alkene.

In the Wurtz reaction two radical intermediates dimerize in a coupling reaction. Likewise acetone is converted to pinacol with a magnesium - mercury amalgam in a pinacol coupling reaction. Acyloin condensation couples two carboxylic acids to a α-hydroxyketone. Reactions of this type are also called reductive couplings. In the Clemmensen reduction of ketones to alkanes with zinc-mercury amalgam the intermediate is an organozinc carbenoid. Electron rich organic molecules like tetrakis(dimethylamino)ethylene (TDAE) are effective reducing agents capable of generating the anion from alkyl halides such as 5-chloromethyl-6-nitrobenzo[1,3]dioxole 1.