Intramolecular Electron Transfer in the Anion Radicals of Nitrobenzyl Halides

One-electron reduction of nitrobenzyl halides produces the anion radicals which subsequently undergo intramolecular electron transfer and decompose into nitrobenzyl radicals and halide ions. The optical absorption spectra of the initial anion radicals (λmax = 300-310 nm) and the subsequently formed nitrobenzyl radicals (λmax = 350 and 400 for the para and ortho, respectively) are quite intense (ε 104 M-1 cm-1 in most cases) and significantly different. This enables identification of the various species and measurement of the rates of intramolecular electron transfer or C-X bond scission. The rates are 4 X 103, 1.7 X 10s, and 5.7 X 105 s-1 for p-nitrobenzyl chloride, bromide, and iodide, respectively. The ortho derivatives decomposed nearly twice as rapidly while the meta decomposed much more slowly. The anion radical of p-nitrobenzyl bromide has pKa = 2.8, and the protonated form is found to undergo the intramolecular transfer ~60 times more slowly than the anion radical. The pattern of reactivity of the various anion radicals is rationalized in terms of spin density and charge distribution at the various positions on the ring and in terms of the electrophilicities of the halogens.