Kinetics of hydrogen isotope exchange reactions. Part XVII. Competition between mesitylene and benzene in aromatic exchange under the influence of β-radiation

Abstract

The radiation-induced tritium uptake by mesitylene is inhibited by the addition of benzene. This inhibition is quantitatively accounted for by tritium exchange in benzene, which competes with mesitylene for the tritiating species. The total reactivity ratio mesitylene : benzene (referring to the sum of all reactions by which the tritiating species can be scavenged by these compounds) is 9; the exchange reactivity ratio (which applies only to the processes leading to tritiated mesitylene and benzene) is 8·5 ± 0·5. The enhancement of the reactivity by methyl substituents supports the formulation of the reaction mechanism in terms of radiolytically produced tritium atoms, and the agreement between the two estimates is shown to point to the relative unimportance of scavenging reactions by these aromatic molecules other than by a process which results in tritium exchange. On this basis the amount of tritium exchange in mesitylene is shown to agree with the yield of tritium atoms calculated from the known internal radiation dose to which the solutions are exposed, provided an isotope effect of ca. 10–20 (discriminating against the formation of tritium atoms relative to protium atoms) is assumed.