Kinetics and thermochemistry of the R+HBr⇄RH+Br (R=C₂H₅ or β-C₂H₄Cl) equilibrium An abinitio study of the bond energies in partly chlorinated ethanes and propanes

Abstract

The kinetics of the reaction of ethyl and β-chloroethyl radicals with HBr have been investigated under pseudo-first-order conditions in a heatable tubular reactor. The pressure-independent rate constants determined were fitted to the following Arrhenius expression (error limits stated are 1σ+Student's t values, units in cm³ molecule^-1 s^-1): k(C₂H₅)=(1.87±0.14)×10^-12 exp[+(3.7±0.2) kJ mol^-1/RT] and k(β-C₂H₄Cl)=(5.7±1.6)×10^-13 exp[+(2.2±0.8) kJ mol^-1/RT]. The kinetic data were used in a second-law procedure to calculate the entropy and enthalpy of formation values for the radicals studied at 298 K (entropy in J K^-1 mol^-1 and enthalpy in kJ mol^-1): 244±6, 120.7±2.1 (C₂H₅) and 271±7, 93.0±2.4 (β-C₂H₄Cl). The enthalpy of formation values of chloroethyl radicals were used in group additivity calculations to obtain Δ_fH₂₉₈^° values for six monochlorinated propyl and butyl radical isomers. Extensive abinitio molecular orbital calculations at the MP4/6-311G(d,p) level were used to determine all bond energies in monochlorinated ethane and propane, and in dichlorinated ethane molecules. The global minimum structures of open- and closed-shell species needed for calculations were determined at the MP2/6-31G(d,p) level. The calculated values are in close agreement with experimentally determined bond enthalpies. The calculations show a significant effect of chlorine atom(s) on the structure of chlorinated free radicals and on the bond energies of chlorinated molecules.

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