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

The gas-phase photobromination of fluoroethane was investigated in the temperature range 80-150 degrees C in both the presence and absence of chloroethane as an external competitor. The rate constant for alpha-hydrogen abstraction in CH3CH2F was redetermined relative to that in CH3CH2Cl, and the abstraction of beta-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 (cm(3) mol(-1) s(-1)): k(alpha) = (6.50 +/- 3.58) x 10(12) exp [-(10360 +/- 370)/RT] and k(beta) = (3.42 +/- 1.95) x 10(12) 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: Delta H degrees(f)(CH3CHF) = -16.8 +/- 2, Delta H degrees(f)(CH2CH2F) = -14.2 +/- 2, D degrees(CH3CHF-H) = 98.2 +/- 2, D degrees(CH2FCH2-H) = 100.8 +/- 2, and D degrees(CH2CH2-F) = 45.6 +/- 2. The influence of halogen substitution on the rate parameters of hydrogen abstraction from monohaloethanes is discussed.