Comparison of hydrogen atom and hydroxyl radical reactions with simple aromatic molecules in aqueous solution

Abstract

The reactions of hydrogen atoms and hydroxyl radicals with benzene and 10 monosubstituted benzenes were investigated by measuring transient absorption spectra, calculating Gibbs free energies, absorption spectra of intermediates formed and determining the rate constants of H reactions. The basic reaction is addition to the aromatic ring forming cyclohexadienyl type radicals. Most radical exhibit absorption spectra with maxima in the 300–340 nm range, these maxima for molecules with strong electron withdrawing groups are shifted to 340–420 nm. The spectra of H and OH adducts are similar suggesting similar intermediates. In substituted benzenes the absorption band is due to several isomers. Ortho and para additions are important in reaction with all molecules studied. Both, H and OH can react with the –Cl, –Br and –NO2 substituents in exchange reaction without cyclohexadienyl radical. OH reacts with the aromatic ring of benzyl chloride in radical addition, while H eliminates Cl from the substituent.