Kinetics and Mechanism of the Gas-Phase Reaction of Cl Atoms with Benzene

Abstract

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 Cl2, C6H6, and a reference compound in 120−700 Torr of N2 diluent at 296 K. Reaction of Cl atoms with C6H6 proceeds via two pathways; (a) H-atom abstraction and (b) adduct formation. At 296 K the rate constant for the abstraction channel is k 1a = (1.3 ± 1.0) × 10-16 cm3 molecule-1 s-1. Phenyl radicals produced via H-atom abstraction from C6H6 react with Cl2 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,1b = [C6H6−Cl]/[C6H6][Cl] and lies in the range (1−2) × 10-18 cm3 molecule. Flash photolysis experiments conducted using C6H6/Cl2 mixtures in 760 Torr of either N2 or O2 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 O2 and an upper limit of k(C6H6−Cl + O2) < 8 × 10-17 cm3 molecule-1 s-1 was established. As part of this work a value of k(Cl + CF2ClH) = (1.7 ± 0.1) × 10-15 cm3 molecule-1 s-1 was measured. These results are discussed with respect to the available literature concerning the reaction of Cl atoms with benzene.

Studies of the Kinetics and Thermochemistry of the Forward and Reverse Reaction Cl + C6H6 = HCl + C6H5

“…The room-temperature values of k 1 determined by Shi and Bernhard 5 and Sokolov et al 6 divided by our k -1 yield K eq and thus ∆G, which together with our calculated ∆S yields ∆H. Their results imply D 298 (C 6 H 5 -H) ) 466 ( 1 and 472 ( 3 kJ mol -1 , respectively.…”

Kinetic studies of the reaction of atomic chlorine with chlorobenzene

“…This indicates that the determination of Shi et al [9], which is 200 times larger, is likely too high. This is in accord with arguments of Sokolov et al [8] concerning the reaction Cl + C6H6 → products (4) who noted that impurities or secondary chemistry might have been important in earlier work. Sokolov et al observed no formation of HCl and set an upper limit of k1 < 2.5 x 10 -16 cm 3 molecule -1 s -1 , which agrees with extrapolation of our data.…”

“…By analogy with Cl addition to benzene [8], adduct formation between Cl and C6H5Cl will be thermodynamically unfavorable at elevated temperatures. The estimated thermochemistry for adduct formation between Cl and benzene [8] implies that, if similar interactions occur between Cl and chlorobenzene, then under our conditions less than 1 part in 10 6 of atomic Cl would be bound at equilibrium, and thus formation of dichlorocyclohexadienyl may be neglected here. The corresponding lifetime for this adduct at 700 K is less than 20 ns.…”

“…Sokolov et al determined an upper limit of 2.5 x 10 -16 cm 3 molecule -1 s -1 for HCl formation at room temperature [8]. This is below a prior measured total rate constant of 9.8 x 10 -16 cm 3 molecule -1 s -1 by Shi and Bernhard [9].…”

Chlorine chemistry in urban atmospheres: a review

“…One exception to the increased reactivity of chlorine is the reaction with benzene, for which the ratio k Cl /k OH is on the order of 10 À4 . [11,16] . [17] The oxidation of the common biogenic, isoprene, by chlorine radicals takes place primarily through chlorine addition, with ,10 % of the reaction proceeding through the hydrogen abstraction pathway.…”