Reactions of hydrogen atoms with benzene and toluene studied by pulsed radiolysis: reaction rate constants and transient spectra in the gas phase and aqueous solution

Abstract

The rate constants at 25°for the gas-phase reactions of hydrogen atoms with benzene and toluene have been determined by the technique of pulsed radiolysis and are, in units of M~x sec-1, 0.37 X 108 and 1.0 X 10s, respectively. The activation energy in the case of benzene is about 3 kcal/mole. In aqueous solution, the corresponding rate constants have been determined to be 1.1 X 109 and 2.6 X 109, respectively. The limits of error on these rate constants are ca. ±20%. The rate constants were measured by following, as a function of time, the formation of the optical absorption due to radicals produced when hydrogen atoms, produced by an electron pulse, react with the aromatic compound.Benzene and toluene yield the cyclohexadienyl radical and the methylcyclohexadienyl radical, respectively, with absorption maxima at 302 and 307 mg, respectively, in the gas phase. The half-widths of the absorption peaks are about 30 mg. In aqueous solution, the absorption maxima are 311, (e311 (5.4 ± 0.5) X 103 M~l cm-1) and 315 mg, respectively, with half-widths similar to those in the gas phase. The combination of two cyclohexadienyl radicals proceeds at least as fast as once every ten collisions in the gas phase at 25°, indicating a "loose" activated complex. In the liquid phase, the combination is diffusion controlled.(1) Based on work performed under the auspices of the U. S. Atomic Energy Commission.

Kinetics and mechanism for the H‐for‐X exchange process in the H + C6H5X reactions: A computational study

“…Several sources report the rate constant of the H atom addition to toluene at room T (298-303 K) in the range (0.4-2.0)×10 −13 cm 3 /(molecule s) [34,35,44,[50][51][52]. Our calculation at HPL conditions gives k 1 = 1.4 × 10 −13 cm 3 /(molecule s), more than 80% of which falls on H-addition at o-position.…”

Generation of free radicals and electrochemiluminescence from simple aromatic molecules in aqueous solutions

“…With simple derivatives of benzene, step (9a) is generally so fast that the anion radical of step (8) has never been reliably observed (18,19), therefore the Red-Ox excitation pathway is usually blocked by production of cyclohexadienyl-type (CHD-type) radicals (14,20) …”

Effect of thermal activation on the reaction of toluene with hydrogen atoms

“…(b) Observed values of E H for benzene are 3.3-4.3 kcal/mole [5,23], and a theoretical calculation predicts a value of EH = 5 kcal/mole for toluene [3].…”

“…However, a gas-phase pulsed radiolysis study at 300°K shows that it is not possible to determine the structure of the formed radical [5]. In a recent molecular-beam study of the reaction of the F atom with various aromatic compounds [30] it was assumed that the potential barrier for rearrangement of the F atoms was much lower than that of the decomposition reactions, and that F atoms could easily hop from one carbon atom to the other on the aromatic ring.…”