11 Reaction mechanisms Part (i) Radical and radical ion reactions

Abstract

This chapter provides an overview of the 2004 literature pertaining to the mechanism and kinetics of reactions involving neutral radicals and radical ions in solution. This overview begins by highlighting significant contributions to our understanding of fundamental classes of reactions: substitution, addition, and fragmentation. Afterwards, rearrangement reactions, which are intramolecular variants of these fundamental classes, are examined. With regard to a specific subclass of substitution reactions, Evans and Polanyi provided a foundation for understanding structure–reactivity relationships in hydrogen atom abstraction processes in 1938. Hence, it is particularly noteworthy that in 2004, a great deal of research effort was still directed towards improving our understanding of this fundamental class of radical reactions. There are many reasons for this continued interest: radical reactions are important in biological systems, and a number of fundamental processes (e.g., enzyme-catalyzed oxidations, lipid oxidations, etc.) involve hydrogen atom abstraction. Another facet of this chemistry is related to antioxidants, natural and synthetic compounds whose purpose is to protect synthetic materials (e.g., organic polymers), the foods we eat, and for that matter, our bodies (vitamin supplements, disease treatment) from the harmful effects of radicals. Also, the recent discovery that certain nitroxyl radicals can selectively abstract hydrogen, and that this chemistry can be used in industrially relevant processes, has driven a great deal of research reported in 2004.