Kinetics of the reactions of hydroxyl radicals with benzene, benzene-d6 and naphthalene.
Abs. rate consts. for the reactions of hydroxyl radicals with benzene (1), benzene-d6 (2), and naphthalene (3) have been obtained using excimer laser photolysis to generate OH and time resolved resonance fluorescence as its monitor. Reaction (1) was studied at total pressures between 1.5 and 112 mbar at 298 K using Ar as a diluent gas, k1 increases with pressure up to .apprx.40 mbar, but is essentially pressure independent at still higher pressures. The temp. variation of k1 was studied at 244-524 K. At temps. below 330 K normal Arrhenius behavior is obsd., corresponding to the addn. reaction (1) OH + C6H6 -> C6H6OH. Between 330 and 420 K a sharp decrease of k1 with increasing temp. is noted, which is attributed to reaction (1) becoming reversible. The stabilization energy of C6H6OH was (79 +- 6) kJ/mol. Above 420 K a transition of k1 to a pos. temp. dependence, corresponding to the abstraction reaction (1') OH + C6H6 -> H2O + C6H5 is noted. Rate consts. for the reaction of OH with benzene-d6 (2) are essentially identical to those of reaction (1) except for temps. above 420 K, where k2 falls considerably below k1. This isotope effect is a confirmation of the (1') abstraction route. Reaction (3) was studied at total pressures between 6 and 128 mbar (He) where it is essentially pressure independent. Its temp. variation was studied between 337 and 525 K. The stabilization energy of the OH-naphthalene adduct is estd. to be (95 +- 6) kJ/mol.
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