Kinetics and mechanism of the gas-phase reaction of Cl atoms with benzene

The gas-phase reaction of Cl atoms with benzene has been studied using both experimental and computational methods. The bulk of the kinetic data were obtained using steady-state photolysis of mixtures containing Cl-2, C6H6, and a reference compound in 120-700 Torr of N-2 diluent at 296 K. Reaction of Cl atoms with C6H6 proceeds via two pathways; (a) I-I-atom abstraction and (b) adduct formation. At 296 K the rate constant for the abstraction channel is k(1a) = (1.3 +/- 1.0) x 10(-16) cm(3) molecule(-1) s(-1). Phenyl radicals produced via H-atom abstraction from C6H6 react with Cl-2 to give chlorobenzene. The main fate of the C6H6-Cl adduct is decomposition to reform C6H6 and Cl atoms. A small fraction of the C6H6-Cl adduct undergoes reaction with Cl atoms via a mechanism which does not lead to the production of C6H5Cl, or the reformation of C6H6. As the steady-state Cl atom concentration is increased, the fraction of the C6H6-Cl adduct undergoing reaction with Cl atoms increases causing an increase in the effective rate constant for benzene removal and a decrease in the chlorobenzene yield. Thermodynamic calculations show that a rapid equilibrium is established between Cl atoms, C6H6, and the C6H6-Cl adduct, and it is estimated that at 296 K the equilibrium constant is K-c,K-1b = [C6H6-Cl]/[C6H6][Cl] and lies in the range (1-2) x 10(-18) cm(3) molecule.(1) Flash photolysis experiments conducted using C6H6/Cl-2 mixtures in 760 Torr of either N-2 or O-2 diluent at 296 K did not reveal any significant transient UV absorption; this is entirely consistent with results from the steady-state experiments and the thermodynamic calculations. The C6H6-Cl adduct reacts slowly (if at all) with O-2 and an upper limit of k(C6H6-Cl + O-2) < 8 x 10(-17) cm(3) molecule(-1) s(-1) was established. As part of this work a value of k(Cl + CF2ClH) = (1.7 +/- 0.1) x 10(-15) cm(3) molecule(-1) s(-1) was measured. These results are discussed with respect to the available literature concerning the reaction of Cl atoms with benzene.