Dissolving-metal reduction of an aromatic ring to a 1,4-cyclohexadiene is the Birch reduction. A solvated electron, generated from an alkali metal in liquid ammonia or an amine solvent, adds to the arene to give a radical anion; protonation by an alcohol, followed by a second electron transfer and protonation, yields the unconjugated diene ^[1]. Electron-donating substituents direct reduction to positions that leave those substituents on the remaining double bonds, whereas electron-withdrawing groups invert the regioselectivity. The process is a dissolving-metal, single-electron reduction, not a hydride transfer from lithium aluminum hydride or a related complex hydride. A scalable variant uses lithium and ethylenediamine in tetrahydrofuran while retaining dissolving-metal reduction of the arene ^[2]. A separate ammonia-free protocol likewise preserves sequential electron and proton transfers ^[3]. The conceptual boundary is partial reduction of an aromatic π system to a nonconjugated diene; full saturation, benzylic hydrogenolysis, and carbonyl reduction are separate dissolving-metal reactions. Reversible biological reductions that generate analogous radical-anion intermediates are mechanistically related but are not the laboratory Birch process ^[1].