Discovered in the mid-20th century by an Australian chemist, the Birch reduction demonstrates a beautiful and effective way of reducing one of benzene’s double bonds by using an alkali metal (sodium, lithium, or potassium) in liquid ammonia, and an alcohol. The mechanism of this organic reaction follows a radical pathway and some simple rearrangements that occur, and the product of the reduction is 1,4-cyclohexadiene.
Here is the reaction for benzene, the simplest aromatic compound with a benzenoid ring, undergoing Birch reduction, and a depiction of the mechanism follows:
In the mechanism, the radicals are generated by the donation of solvated electrons from the metal dissolved in liquid ammonia, and it seems like two equivalents of the alcohol are needed in order to obtain the desired product.
The Birch reduction works not only for benzene, but any aromatic compound that contains the benzene-ring structure, which is a feature found in many polycyclic aromatic hydrocarbons, or PAHs (ex. naphthalene, anthracene, phenanthrene, etc.). The Birch reduction also works for substituted benzene, like benzoic acid or anisole. In terms of regioselectivity, there is still debate about what effects electron-donating and -withdrawing groups have on the final product.