Kinetics of hydrogen isotope exchange reactions. Part XXII. Reactivity comparisons for a series of aromatic substrates towards radiolytically generated tritium atoms in aqueous solution

Abstract

The relative reactivities of a series of aromatic compounds towards tritium atoms, generated by β-radiolysis, have been determined in degassed aqueous solution by a competition method, involving copper(II) ions as common competitor in the radiation-induced aromatic hydrogen exchange. Under these conditions, steps subsequent to the formation of the cyclohexadienyl radical are not kinetically significant. It is shown that comparable doses of γ-irradiation induce an equivalent extent of exchange. From the effect of low concentrations of copper(II) ions on the β-radiation-induced exchange rate it is concluded, that the reversion of the intermediate cyclohexadienyl radical to aromatic compound normally involves an oxidation step [reaction with residual oxygen or copper(II)], and that bimolecular radical–radical reactions are unimportant. The rates of tritium atom attack relative to benzene are: mesitylene, 8·5; toluene, 2·2; anisole, 2·7; fluorobenzene, 1·0; chlorobenzene, 1·1; bromobenzene, 1·1; benzonitrile, 0·9; trifluoromethylbenzene, 0·35; and t-butylbenzene, ca. 2–4. Because of an intramolecular kinetic isotope effect, the rate of radiation-induced tritium uptake by mesitylene is ca. 1·5 ± 0·2 times larger than that by [2,4,6-²H₃]mesitylene. The extinction coefficients at the u.v. absorption maxima for these substrates in aqueous solution have been redetermined. The solubility of t-butylbenzene in water at 25° was found to be (1·9 ± 0·1)× 10^–2 g dm^–3. The rate constant for the reaction of hydrogen (tritium) atoms with iodide ion is ca. 5·5 × 10⁷ dm³ s^–1 mol^–1.