91Ó°ÊÓ

A chemical reaction in which benzene derivatives are reduced to non-conjugated cyclohexa-1,4-dienes by treatment with sodium (Na) or lithium (Li) in a mixture of liquid ammonia and alcohol is known as birch reduction.

A solution of sodium in liquid ammonia contains solvated electrons that can add to the benzoate ion, forming a radical anion. The strongly basic radical anion abstracts a proton from ethanol in the solvent, giving a radical. The radical quickly adds another solvated electron to form an anion.

Electron-withdrawing substituents stabilize the carbanions, whereas electron-donating substituents destabilize them. Therefore, reduction takes place on carbon atoms bearing electron-withdrawing substituents EWG (such as those containing carbonyl groups) and not on carbon atoms bearing electron-releasing substituents (such as alkyl and alkoxy groups).

A carbon-bearing electron-withdrawing carbonyl group is reduced with the addition of a hydrogen atom.

A carbon-bearing and electron-releasing alkoxy group is not reduced.

Substituents that are strongly electron-releasing (-OCH₃ , for example) deactivate the aromatic ring toward Birch reduction.

In these deactivated systems, lithium is used with the solvent (THF) and a weaker proton source (tert-butyl alcohol).

The stronger reducing agent and a weak proton source enhance the reduction.