Kinetics and Mechanism of the Gas-Phase Reaction of Cl Atoms and OH Radicals with Fluorobenzene at 296 K

Abstract

Smog chamber/FTIR techniques were used to study the kinetics and mechanism of the reaction of Cl atoms and OH radicals with fluorobenzene, C6H5F, in 700 Torr of N2 or air diluent at 296 K. Reaction of Cl atoms with C6H5F proceeds via two pathways: H-atom abstraction to give HCl and the C6H4F radical and adduct formation to give the C6H5F−Cl adduct. At 296 K the rate constant for the abstraction channel is k5a(Cl + C6H5F) = (1.1 ± 0.1) × 10-17 cm3 molecule-1 s-1. The C6H5F−Cl adduct undergoes rapid (k ∼ 108 s-1) decomposition to reform C6H5F and Cl atoms and reaction with Cl atoms via a mechanism which, at least in part, leads neither to production of C6H5Cl nor to reformation of C6H5F. As the steady-state Cl atom concentration is increased, the fraction of the C6H5F−Cl adduct undergoing reaction with Cl atoms increases causing an increase in the effective rate constant for the reaction of C6H5F with Cl atoms. The equilibrium between Cl atoms, C6H5F, and the C6H5F−Cl adduct is established rapidly and has an equilibrium constant estimated to be K5b=[C6H5F−Cl]/[C6H5F][Cl] = (3.2 ± 2.4) × 10-18 cm3 molecule-1. An upper limit of k9 < 6 × 10-17 cm3 molecule-1 s-1 was established for the reaction of the C6H5F−Cl adduct with O2. The reaction of OH radicals with C6H5F was studied and a rate constant of k(OH + C6H5F) = (7.9 ± 2.2) × 10-13 cm3 molecule-1 s-1 was determined. The results are discussed with respect to the available literature concerning reaction of Cl atoms and OH radicals with aromatic compounds. As part of this work, rate constants for reaction of OH radicals with 2-, 3-, and 4-fluorophenol of (6.3 ± 1.3) × 10-12, (2.3 ± 0.5) × 10-11, and (2.5 ± 0.5) × 10-11 cm3 molecule-1 s-1 were determined.