Free radical participation in organic chemistry: Electron Spin Resonance (ESR) studies of their structures and reactions

Electron Spin Resonance (ESR) spectroscopy is the technique sine qua non fo r establishing the geometric and electronic structures of the free radicals participating in many organic reactions and syntheses. It also allows the mechanisms of the complex network of elementary reactions constituting the overall chemical change to be determined and the rate parameters of these r eactions to be measured. Radicals can be classified into two main types sig ma- and pi-radicals. pi-radicals, exemplified by phenyl, have their free el ectron orbital projecting out from a non-planar radical centre. They are us ually more reactive than pi-radicals, exemplified by alkyl radicals in whic h the free electron resides in a carbon 2p(z) orbital perpendicular to the planar radical centre. Fluorine substitution results in a progressive chang e from a planer re-methyl radical to a tetrahedral pi-trifluoromethyl radic al. The regioselectivity and stereoselectivity of intermolecular and intram olecular free atom and radical additions to unsaturated systems are discuss ed in terms of optimal orbital overlap, steric and free valence effects as are group transfers and S-H 1 and S-H 2 homolytic substitution reactions. T he unusual reactions of aluminium and gallium atoms with alkenes, alkynes 1 ,2 and 1,3 dienes and benzene are considered. Recent controversies concerni ng the Fenton reaction and the free radical mode of action of the new antim alarial drug Artemisinin are reviewed. The potential for applications of ES R in conjuction with spin traps in heterogeneous catalysis is outlined.