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

Abstract

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.

The problematic C2H4+F2 reaction barrier

“…71 For the fluoroethyl radical, Miyokawa et al 72 reported an experimental ⌬ f H 298°͑ C 2 H 4 F͒ = −14.2Ϯ 2 kcal mol −1 , but the uncertainty in this value is too large for our purposes. The Gurvich tables recommend ⌬ f H 298°͑ C 2 H 4 ͒ = 12.52Ϯ 0.12 kcal mol −1 and ⌬ f H 298°͑ F͒ = 18.97Ϯ 0.07 kcal mol −1 .…”

Accurate Theoretical Thermochemistry for Fluoroethyl Radicals

“…Four years later Tschuikow and Salomon studied the photobromation of C 2 H 5 Cl, and analyzing the kinetics and thermochemical data of related compounds they got a somewhat higher value, −74.1 ± 4.6 kJ/mol, for Δ f H 298 ° (CH 3 –CHF). Then, in 1996, an even higher experimental value appeared when Miyokawa et al researched the photobromination of fluoroethane and determined −70.3 ± 8.4 kJ/mol for Δ f H 298 ° (CH 3 –CHF).…”

“…Four years later Tschuikow and Salomon 49 studied the photobromation of C 2 H 5 Cl, and analyzing the kinetics and thermochemical data of related compounds they got a somewhat higher value, −74.1 ± 4.6 kJ/ mol, for Δ f H 298 °(CH 3 −CHF). Then, in 1996, an even higher experimental value appeared when Miyokawa et al 47 Chen and his associates 50 using HF/6-31G(d) geometries and frequencies in conjunction with MP2/6-311G(d,p) energies [MP2/6-311G(d,p)//HF/6-31G(d)] determined…”

“…The only available experimental value, Δ f H 298 ° (CH 2 F–CH 2 ) = −59.4 ± 8.4 kJ/mol, is that of Miyokawa and associates, who studied the kinetics of gas phase photobromination of CH 3 –CH 2 F …”

“…CH 2 F−CH 2 . The only available experimental value, Δ f H 298 °(CH 2 F−CH 2 ) = −59.4 ± 8.4 kJ/mol, is that of Miyokawa and associates, who studied the kinetics of gas phase photobromination of CH 3 −CH 2 F. 47 Chen and his associates, 37 ) = −59.9 ± 2.8 kJ/ mol. Although our error bar overlaps with that of ref 46, the 3.5 kJ/mol discrepancy seems fairly large and requires an explanation because the two computational approaches are similar.…”