Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Radical ion

A radical ion is a free radical species that carries a charge. Radical ions are encountered in organic chemistry as reactive intermediates and in mass spectrometry as gas phase ions. Positive radical ions are called radical cations whereas negative radical ions are called radical anions.

Notation

In organic chemistry, a radical ion is typically indicated by a superscript dot followed by the sign of the charge: $M^{\bullet +}$ and $M^{\bullet -}$. In mass spectrometry, the sign is written first, followed by the superscripted dot: $M^{+\bullet }$ and $M^{-\bullet }$.

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.

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.

An example of a non-carbon radical anion is the superoxide anion, formed by transfer of one electron to an oxygen molecule.

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 anti-aromatic. 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 fragments into a complex mixture of ions and uncharged radical species. For example the methanol 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.