Radical anion

In organic chemistry, a radical anion is a free radical species that carries a negative charge. Radical anions are encountered in organic chemistry as reduced derivatives of polycyclic aromatic compounds, e.g. sodium naphthenide. An example of a non-carbon radical anion is the superoxide anion, formed by transfer of one electron to an oxygen molecule. Radical anions are typically indicated by . Many aromatic compounds can undergo one-electron reduction by alkali metals. 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. Effective solvents are those that bind to the alkali metal cation: 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. Salts of the radical anions are often not isolated as solids but used in situ. They are usually deeply colored. Naphthalene in the form of Lithium naphthalene is obtained from the reaction of naphthalene with lithium. Sodium naphthalene is obtained from the reaction of naphthalene with sodium. Sodium 1-methylnaphthalene and 1-methylnaphthalene are more soluble than sodium naphthalene and naphthalene, respectively. biphenyl as its lithium salt. acenaphthylene is a milder reductant than the naphthalene anion. anthracene in the form of its alkali metal salts. pyrene as its sodium salt. Perylene in the form of its alkali metal (M = Li, Na, Cs) etherates. Cyclooctatetraene is reduced by elemental potassium to the dianion. The resulting dianion is a 10-pi electron system, which conforms to the Huckel rule for aromaticity. Quinone is reduced to a semiquinone radical anion. Semidiones are derived from the reduction of dicarbonyl compounds.