Kinetics of the Photobromination of Fluoroethane. Estimate of the C-H Bond Dissociation Energies and the Heats of Formation of the CH3CHF and CH2CH2F Radicals

Article abstract of DOI:10.1246/bcsj.69.869

The gas-phase photobromination of fluoroethane was investigated in the temperature range 80-150°C in both the presence and absence of chloroethane as an external competitor. The rate constant for α-hydrogen abstraction in CH₃CH₂F was redetermined relative to that in CH₃CH₂Cl, and the abstraction of β-hydrogen was measured in the internal competition. The relative rates were combined with the known rate parameters for the bromination of C₂H₆ to obtain the absolute rate constants (cm³ mol^-1 s^-1): k(α) = (6.50±3.58) × 10¹² exp [ - (10360±370)/RT] and k(β) = (3.42±1.95) × 10¹² exp [ - (12950±390)/RT]. Using a justifiable approximation concerning the magnitude of the activation-energy difference for the reverse reactions between any two competitors with similar complexity the following thermochemical quantities (kcal mol^-1) have been derived: ΔH° _f(CH₃CHF) = -16.8±2, ΔH° _f(CH₂CH₂F) = -14.2±2, D° (CH₃CHF-H) = 98.2±2, D° (CH₂FCH₂-H) = 100.8±2, and D° (CH₂CH₂-F) = 45.6±2. The influence of halogen substitution on the rate parameters of hydrogen abstraction from monohaloethanes is discussed.