Selectivity in thermal 34mCl-for-Cl exchange with chlorobenzenes

Relative reaction rates for thermal Cl exchange by 34mCl atoms, produced by the 35Cl(γ, n)34mCl reaction under high radiation dose conditions, were measured in mixtures of substituted chlorobenzenes. In I2 scavenged 1:1:1 mixtures of ortho, meta and para-C6H4 ClX (X = F, Cl, CH3, CF3), the hot substitution yields are close to statistical. Thermal 34mCl-for-Cl exchange yields exhibit high selectivity not only in mixtures of isomeric C6H4 ClX compounds, but also in mixtures of C6H5Cl and C6H4ClX. This selectivity is related to the rate constants for ipso σ-complex formation, which is a slightly electrophilic reaction with a Hammett parameter of ρ+ = -1.56. As a consequence of the strong electron withdrawal properties of the F atoms in C6F5Cl, even low concentrations of C6H5Cl can readily compete with C6F5Cl for thermal 34mCl-for-Cl exchange. (orig.)