The electron is transferred from the alkali metal ion to an unoccupied antibonding p-p п* orbital of the aromatic molecule. This transfer is usually only energetically favorable if the aprotic solvent efficiently solvates the alkali metal ion. Effectiveness for this is in the order diethyl ether < THF < 1,2-dimethoxyethane < HMPA. In principle any unsaturated molecule can form a radical anion, but the antibonding orbitals are only energetically accessible in more extensive conjugated systems. Ease of formation is in the order benzene < naphthalene < anthracene < pyrene, etc. On addition of a proton source, the structure of the resulting hydrogenated molecule is defined by the charge distribution of the radical anion. For instance, the anthracene radical anion forms mainly (but not exclusively) 9,10-dihydroanthracene.
A very effective way to remove any traces of water from THF is by reflux with sodium wire in the presence of a small amount of benzophenone. Benzophenone is reduced to the ketyl radical anion by sodium which gives the THF solution an intense blue color. However, any trace of water in THF will further reduce the ketyl to the colourless alcohol. In this way, the color of the THF signals the dryness and the distilled THF contains less than 10 ppm of water This treatment also effectively removes any peroxides in the THF. Radical anions of this type are also involved in the Acyloin condensation.
Cyclooctatetraene is reduced by elemental potassium directly to the dianion (skipping the radical anion state) because the 10 electron system is an aromat. Quinone is reduced to a semiquinone radical anion. Semidiones are derived from the reduction of dicarbonyl compounds.