Dynamics of endoergic aromatic substitution reactions

Abstract

The endoergic substitution reactions Br +o-, m-, p-CH₃C₆H₄Cl →o-, m-, p-CH₃C₆H₄Br + Cl., (ΔH₀°≈ 15 kcal mol^–1)., have been studied using the crossed beams method in the collision energy range 20–30 kcal mol^–1. o-Chlorotoluene was found to be more reactive than p-chlorotoluene at the highest energy but the reverse was true below 25 kcal mol^–1. No reaction was observed for the meta isomer. An explanation for the lower reactivity of m-chlorotoluene is offered in terms of possible features of the potential-energy surface. In all of the reactions observed, the products are largely forward scattered, indicating that the majority of collision complexes survive for less than one rotational period. This is understandable in light of the ca. 2 kcal mol^–1 endoergicity to Br addition that results from the loss in resonance stabilization energy. Very little of the energy available to the products of these reactions is channeled into translation. The experimental product translational energy distributions and excitation functions suggest that, in those complexes that decompose through Cl elimination, only a few vibrational degrees of freedom in the vicinity of the collision are involved in energy redistribution.