The reduction of aromatic compounds by alkali metals in liquid ammonia represents an important method for the preparation of partially unsaturated six-membered rings. The reaction was discovered by Wooster and Godfrey, but the major development resulted from the efforts of A. J. Birch, and the reaction has since come to bear his name. Although a variety of metals can be used, the most common are sodium and lithium, and, to a lesser extent, potassium. Cosolvents such as ether or tetrahydrofuran (THF) are often used to improve solubility, and weak acids such as alcohols may be employed during the reaction as proton sources. The latter are necessary for the reduction of benzene and its unactivated derivatives. Improvement in experimental procedures by Wilds and Nelson, the application to polynuclear compounds by Hückel and later by Harvey, and the development of methods for the alkylation of the anions generated in this process (i.e., reductive alkylation) have made this reaction an important approach to the synthesis of a wide variety of organic compounds.