Intramolecular electron transfer and dehalogenation of anion radicals. 2. Halonitroaromatic compounds

Halonitroaromatic compounds were reduced with e_aq^- or (CH₃)₂C̄OH in irradiated aqueous solutions to produce the anion radicals. The decay of these radicals follows either a bimolecular radical radical reaction or a unimolecular dehalogenation. The influence of the halogen location relative to the ring and the nitro group, and the effect of the C-halogen bond strength on the rates of the unimolecular dehalogenation were examined. p-ClC₆H₄NŌ₂^-, m-BrC₆H₄NŌ₂^-, o- and p-IC₆H₄NŌ₂^-, p-BrCH₂CH₂C₆H₄NŌ ₂^-, p-ClCH₂COOC₆H₄NŌ₂^-, p-ClCH₂CONHC₆H₄NŌ₂ ^-, and p-ClCH₂CH₂OCH₂CH₂OC ₆H₄NŌ₂^- all decayed by second-order processes. The lack of dehalogenation reaction in these nitro anion radicals is rationalized by high bond dissociation energies in the case of the halonitrobenzene radical anions, and by weak interactions of the halogen atom in the other compounds with the unpaired electron residing mainly on the nitrobenzene ring. p-ICH₂COOC₆H₄NŌ₂^- decays unimolecularly with k = 30 s^-1 reflecting the low C-I bond strength. m- and p-BrCH₂COC₆H₄NŌ₂^- decay in unimolecular processes with rates of 1.5 × 10² and 4.1 × 10⁴ s^-1, respectively. These relatively high rates are rationalized by weakening of the C-Br bond by the CO group and by an effective bridging by the CO for intramolecular electron transfer.