An EPR study of electron-gain and -loss: Radicals and radical ions from halogenated acetophenones

Abstract

This study is concerned with the fundamental reactions of electron -gain and -Joss by molecules in condensed phases. Using EPR spectroscopy attempts have been made to characterise primary radical - ions and hence to deduce mechanistic details. We have used chloro- and bromo- derivative with the halogen atom in the 2'-position in the benzene ring, and in the a-position in the side-chain (i.e., PhCOCH₂hal). Their reactions nicely illustrate the range of mechanisms available. Radical-anions have been specifically generated by exposing dilute solutions of the parent compounds to ionizing radiation at 77K, using solvents such as methanol (CD₃OD) which give good glasses on freezing, and which scavenge electron-Joss centres, leaving the ejected electrons to react with the solute molecules. Acetophenone gives the π*-anion under these conditions. The major species formed from 2'-halogen derivatives is the PhCOCH₂ radical. We suggest that this is formed by initial addition to the benzene ring, followed by electron transfer into the C-hal σ*-orbital and subsequent loss of the halide ions. Hydrogen-atom transfer from the methyl group gives the species detected by EPR spectroscopy. The α-halo derivatives also undergo dissociative electron-capture, again giving PhCOCH₂ radicals as the main product. Radical-cations are formed in the same way, but using CFCl₃ as an electron-capture solvent. In this case, the holes are mobile and react with the solute molecules to give the primary radical-cations. For the 2'-bromo derivative the major species, having a very large hyperfine coupling to bromine, is identified as a σ*-radical formed by bonding between bromine and the carbonyl oxygen. A similar species is detected for the α-chloro-derivative, but in this case the major cation is the normal aromatic π-cation. For the α-halo compounds, the species detected are the α-halo radicals, PhCOCH hal. A major conclusion from this work is that σ*- radicals are important intermediates in radical -reactions that must always be considered.