Reactions

The reaction proceeds via two successive single-electron transfer (SET) steps. First, a dissolved electron from the alkali metal (here, Na) is transferred to the aromatic ring, generating a radical anion. Then, the radical anion is protonated by the alcohol, giving a cyclohexadienyl radical. Subsequently, a second solvated electron adds to the radical, generating a cyclohexadienyl carbanion. Finally, the carbanion is protonated by a second molecule of alcohol, yielding the final 1,4-cyclohexadiene.

It is important to note that the Birch reduction shows a regioselectivity depending on whether the ring bears an electron-donating group (EDG) or an electron-withdrawing group (EWG). With an EDG, reduction (i.e. protonation) occurs at the unsubstituted positions, so the double bonds stay conjugated with the substituent. With an EWG, reduction occurs at the substituted positions, so the double bonds remain in the unsubstituted part of the ring.