Reaction mechanisms Part (i) Radical and radical ion reactions

Abstract

Though still somewhat neglected in most textbooks of organic chemistry, the importance of reactions involving radicals and radical ions as intermediates is widely recognized. These reactions are of interest to synthetic chemists because they can be used to introduce structural features that often cannot be achieved readily with conventional two electron (polar) processes. In addition to their utility in organic and polymer synthesis, these reactions are also important in a number of other areas including biochemistry, medicine, food science, atmospheric chemistry—just to name a few. Through the popular press, even the general public is aware of radicals, mainly in the context of health and nutrition, as evidenced by the increased interest in antioxidants. This chapter reviews some of the more significant and interesting developments in 2007 dealing with the mechanism of radical and radical ion reactions in solution, several developments for which are especially worth noting. For example, “simple” hydrogen atom transfer reactions have been thoroughly studied since the 1940s, because among other reasons, they are fundamentally important to understanding the oxidation of virtually all organic-based materials. Yet, recent work has shown that there are at least four different mechanisms via which a hydrogen atom can be transferred. It is also noteworthy that modern, high-level molecular orbital calculations are now commonly used to probe the mechanisms of radical reactions—often dealing with issues that are impossible to address experimentally. Finally, the number of publications describing radical-type reactions in biology and medicine continued to grow in 2007, with the increasing recognition of these intermediates as critical in numerous biological processes.