Kinetic Isotope Effects in the Unimolecular Reactions of Chemically Activated Chloroethane-d and -d5 and 1,2-Dichloroethane-d and -d4 Molecules

The nonequilibrium kinetic isotope effects for H–Cl and D–Cl elimination from chemically activated (average energy of ∼91 kcal mole⁻¹) chloroethane‐d₀ and‐d₅ and 1,2‐dichloroethane‐d₀ and ‐d₄ have been experimentally measured as 3.3 ± 0.4 and 3.5 ± 0.1, respectively. The chemically activated molecules were generated by the combination reactions of methyl and chloromethyl radicals or their deuterated equivalents at room temperature. These isotope effects and also some recently measured thermal activation isotope rate data were interpreted using RRKM computations based upon a four‐centered model for the reaction. Rather good agreement between experimental results and the model calculations was obtained. Even all the kinetic data for chloroethane including the new isotope results were not sufficient to permit the definition of a unique model; however, both the thermal and chemical activation isotope measurements were best fitted by models with the hydrogen weakly bound in the four‐centered ring. Suggestions are made for the relative contributions of the in‐ring deuterium atom and of out‐of‐ring deuterium atoms to the over‐all nonequilibrium isotope effect for C₂D₅Cl. Calculated isotope effects are presented for chloroethane $d_1$ , chloroethane‐d₄, and some tritiated chloroethanes on the basis of the optimized models.