Issue 7, 1998

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 ΔfH298° 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.

Article information

Article type
Paper

J. Chem. Soc., Faraday Trans., 1998,94, 891-898

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

J. A. Seetula, J. Chem. Soc., Faraday Trans., 1998, 94, 891 DOI: 10.1039/A706440C

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