Reaction mechanisms Part (i) Radical and radical ion reactions

Abstract

This chapter provides a summary of the 2005 literature dealing with the mechanism, and to a lesser extent kinetics, of reactions involving neutral radicals and radical ions in solution. The chemistry is organized around three major themes: substitution reactions, addition reactions and fragmentation reactions. There were a number of noteworthy developments that occurred during the period covered by this review. Things are never quite as simple as they first seem, and this has proven to be especially true for “simple” hydrogen atom abstraction processes. In addition to the “normal” mechanism, where a proton and electron are transferred simultaneously, evidence continues to mount for the importance of other mechanisms which differ in the timing of these two events, ranging from electron transfer followed by deprotonation of a radical cation, to proton transfer followed by electron transfer, to proton-coupled electron transfer (where the proton and electron are transferred simultaneously, but from “different” places). Mass spectrometry continues to emerge as a surprising new tool for studying radical and radical ion reactions, and a short section appears at the end of this review examining progress in this area. It is now possible to detect and characterize transient reactive intermediates under steady state conditions by ESI-MS. Electrochemical cells have been interfaced directly to a mass spectrometer and it is possible to monitor rearrangement reactions triggered by electron transfer by this technique, leading to a “mass voltammogram,” which is essentially a plot of MS ion intensity as a function of the potential of the electrochemical cell. Also, while not necessarily “new” in 2005, the importance of paramagnetic species in biological systems continues to be a major area of research. Major new developments elucidating the role of radicals and radical ions as reactive intermediates in enzyme-mediated reactions are reviewed.