Atmospheric chemistry of hexa- and penta-fluorobenzene: UV photolysis and kinetics and mechanisms of the reactions of Cl atoms and OH radicals

Abstract

Photochemical reactors were used to study the kinetics and mechanisms of reactions of Cl atoms and OH radicals with hexa- and penta-fluorobenzene (C₆F₆, C₆F₅H) in 700 Torr total pressure of N₂, air, or O₂ diluent at 296 ± 2 K. C₆F₆ and C₆F₅H undergo ring-opening following 254 nm UV irradiation, but with small quantum yields (φ < 0.03). Reaction of Cl atoms with C₆F₆ proceeds via adduct formation, while the reaction of Cl atoms with C₆F₅H proceeds via hydrogen abstraction and adduct formation. C₆F₆–Cl and C₆F₅H–Cl adducts decompose rapidly (k ∼ 10⁵–10⁶ s⁻¹) reforming the reactants, and react with Cl atoms to form products. The fraction of adduct reacting with Cl atoms increases with steady state Cl atom concentration, resulting in an increasing apparent effective Cl atom rate constant. The rate constant for the H-abstraction channel for Cl + C₆F₅H is estimated at (7.3 ± 5.7) × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹. Establishment of the equilibrium between the adducts and the aromatic reactant + Cl occurs rapidly with equilibrium constants of K([adduct]/[aromatic][Cl]) = (1.96 ± 0.11) × 10⁻¹⁶ and (9.28 ± 0.11) × 10⁻¹⁷ cm³ molecule⁻¹ for C₆F₆ and C₆F₅H, respectively. Reaction of the adducts with O₂ occurs slowly with estimated rate constants of <7 and <4 × 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹ for C₆F₆–Cl and C₆F₅H–Cl, respectively. The rate constants for reaction of OH radicals with C₆F₆ and C₆F₅H were determined to be (2.27 ± 0.49) × 10⁻¹³ and (2.56 ± 0.62) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹, respectively. UV and IR spectra of C₆F₆ and C₆F₅H at 296 ± 1 K were collected and calibrated. Results are discussed in the context of available literature data for reactions of Cl atoms and OH radicals with halogenated aromatic compounds.