Rate Constants for the Reactions of OH Radicals with Aromatics: Benzene, Phenol, Aniline, and 1,2,4‐Trichlorobenzene

Abstract

Abstract Reactions of OH radicals with aromatics were investigated in the presence of He at pressures ranging from 5 to 800 mbar using pulsed vacuum uv photolysis of H 2 O for the production of OH and time resolved resonance fluorescence for the detection of OH. At room temperature (296 K) in the presence of 133 mbar He the rate constants, k , were observed to be in the high pressure limit. The rate constants, ( k ± Δ k )/10 −12 cm 3 s −1 , were determined to be: (1.0 ± 0.2) for benzene; (120 ± 24) for aniline; (28 ± 6) for phenol, (0.50 ± 0.10) for 1,2,4‐trichlorobenzene. — For the temperature dependence of the rate constant for 1,2,4‐trichlorobenzene the Arrhenius expression ( k ± 3 σ ) = (2.3 ± 1) · 10 −12 exp [ — (430 ± 125) K/ T ] cm 3 s −1 is obtained from measurements at 273 to 368 K. — The rate constants at room temperature can be correlated with the ionization potential, I , of the aromatics, yielding the equation: log k /cm 3 s −1 = 0.74–1.36 · I /eV. This equation is seen to represent the rate data on 18 aromatics to within a precision of a factor of two; with some precautions it might be used to predict rate constants for further substituted benzene aromatics. Outliers in such a diagram are benzaldehyde and the cresols (being more reactive than predicted), and diphenyl and chlorinated aromatics (being less reactive).