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A radical ion is a free radical species that carries a charge. Radical ions are encountered in organic chemistry as reactive intermediates.
radical anions
Many aromatic compounds can undergo one-electron reduction by alkali metals. For example the reaction of naphthalene with sodium in an aprotic solvent yields the naphthalene radical anion - sodium ion salt. In a ESR spectrum this compound shows up as a quintet of quintets (25 lines). In the presence of a proton source the radical anion is protonated and effectively hydrogenated like in the Birch reduction.
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 *. 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.
Radical cations
Cationic radical species do also exist but are much less stable. They appear prominently in mass spectroscopy. When a gas-phase molecule is subjected to electron ionization one electron is abstracted by an electron in the electron beam to create a radical cation M+.. This species represents the molecular ion or parent ion and will tell the precise molecular weight. On a typical mass spectrum more signals show up because the molecular ion fragmentates into a complex mixture of ions and uncharged radical species. For example the ethanol radical cation fragments into a methyl cation CH3+ and a hydroxyl radical. In naphthalene the unfragmented radical cation is by far the most prominent peak in the mass spectrum. Secondary species are generated from proton gain (M+1) and proton loss (M-1).
Polarons and bipolarons are radical cations encountered in doped conducting polymers.
"Radical ion"