The Birch reduction is an organic reaction where aromatic rings undergo a 1,4-reduction to provide unconjugated cyclohexadienes. The reduction is conducted by sodium or lithium metal in liquid ammonia and in the presence of an alcohol. The mechanism begins with a single electron transfer(SET) from the metal to the aromatic ring, forming a radical anion. The anion then picks up a proton from the alcohol which results in a neutral radical intermediate. Another SET, and abstraction of a proton from the alcohol results in the final cyclohexadiene product and two equivalents of metal alkoxide salt as a byproduct. In the case of substituted aromatic rings, the regiochemistry can be predicted using Birch's empirical rules.
Birch, A. J. J. Chem. Soc. 1944 430–436.