Forskning ved Københavns Universitet - Københavns Universitet


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

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  • O. Sokolov
  • M. D. Hurley
  • T. J. Wellington
  • E. W. Kaiser
  • J. Platz
  • Nielsen, Ole John
  • F. Berho
  • M. T. Rayez
  • R. Lesclaux

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 k1a = (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 Kc,1b = [C6H6-Cl]/[C6H6][Cl] and lies in the range (1-2) × 10-18 cm3 molecule.1 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.

TidsskriftJournal of Physical Chemistry A
Udgave nummer52
Sider (fra-til)10671-10681
Antal sider11
StatusUdgivet - 24 dec. 1998

ID: 225754002