KINETICS OF ALPHA-HYDROGEN AND BETA-HYDROGEN ABSTRACTION FROM C2H5CL BY BR ATOMS - ESTIMATE OF C-H BOND-DISSOCIATION ENERGIES AND HEATS OF FORMATION OF CH3CHCL AND CH2CH2CL RADICALS

The gas-phase photobromination of ethyl chloride has been investigated over an extended temperature range in the presence and absence of ethane as external competitor. The rate constant for α-hydrogen abstraction in CH3CH2Cl was redetermined relative to C2H6 between 40 and 150°C. The abstraction of β-hydrogen was measured in the internal competition in the range 70-150°C by use of electron capture detection. The relative rates were combined with the known rate parameters for the bromination of C2H6 [log (A/cm3 mol-1 s-1) = 14.14 ± 0.06, E = 13.66 ± 0.14 kcal mol-1] to obtain the absolute rate constants (cm3 mol-1 s-1) k(α) = 1012.71±0.06 exp[-(9.95 ± 0.14)/RT] and k(β) = 1012.60±0.07 exp[-(14.14 ± 0.15)/RT], where the activation energies are expressed in kcal mol-1. From an assessment of thermochemical and kinetic data the following radical heats of formation and C-H bond dissociation energies (kcal mol-1) in C2H5Cl have been obtained: ΔHf° (CH3ĊHCl) = 17.6 ± 1, D°(CH3CHCl-H) = 96.5 ± 1 and ΔHf° (ĊH2CH2Cl) = 21.8 ± 1, D°(CH2ClCH2-H) = 100.7 ± 1. These results compare satisfactorily with recent determinations by the monoenergetic electron impact method. © 1990 American Chemical Society.