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

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-center dot 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-center dot and (OH)-O-center dot 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-center dot and (OH)-O-center dot can react with the -Cl, -Br and -NO2 substituents in exchange reaction without cyclohexadienyl radical. (OH)-O-center dot reacts with the aromatic ring of benzyl chloride in radical addition, while H-center dot eliminates Cl from the substituent.