Kinetics and mechanism of the reaction of hydrogen and deuterium atoms with benzene and benzene-d 6

Abstract

The kinetics of the gas phase reactions of H (and D) atoms with benzene were investigated using fast flow techniques. Specific rate constants were obtained using two separate systems, one with ESR detection to monitor the decrease in atomic reactants and the other with mass spectrometry to follow the change in benzene concentration. The results from these two systems are in essential agreement with one another when discussed in the light of the proposed reaction mechanisms. The mass spectrometric studies lead to a recommended specific rate constant of (5.7 ± 1.0) × 1010 cm3 mole−1 · sec−1 at 300°K for the H+C6H6 reaction. The activation energies for the D and H atom reactions with C6H6 are equal, within experimental error, with the value of 2.70 ± 0.20 kcal/mole reported. For reasons discussed in this paper, it is concluded that this activation energy refers to the addition of the atomic reactant to the benzene ring. Since isotopic exchange reactions have been observed in these systems, the over-all rate of the D+C6H6 reaction obtained from the ESR studies is higher than the rate obtained mass spectrometrically for the H + C6H6 reaction. Competitive experiments using C6H6/C6D6 mixtures in reactions with H and D atoms revealed the absence of any primary kinetic isotope effect on reaction rates. Thus the rate determining step does not involve hydrogen (or deuterium) atom abstraction from the benzene ring. From the products observed mass spectrometrically, it is clear that the reaction of hydrogen atoms with benzene is an over-all complex process and must include reactions leading to (a) isotopic exchange, (b) net addition to the benzene ring leading ultimately to cyclohexane formation, and (c) some cracking of the benzene ring with formation of C2H6 and CH4 as well as probable other low-molecular weight saturated molecules.