Temperature Coefficients of Rates of Ethyl Radical Reactions with HBr and Br in the 228−368 K Temperature Range at Millitorr Pressures

The rates of the reactions of ethyl radicals with HBr (k 7) and with Br atoms (k 8) have been measured in the temperature range 228−368 K at millitorr pressures using the very low pressure reactor (VLPR) technique. The Arrhenius function for the H atom abstraction reaction is found to be k 7 = (1.43 ± 0.06) × 10-12 exp[−(444 ± 26)/RT] cm3/(molecule s), while the ethyl radical disproportionation with Br atom shows no temperature dependence. Its average value over the entire temperature range is k 8 = (1.18 ± 0.05) × 10-11 cm3/(molecule s). Reaction 7 is significantly slower than has been reported in the only other two direct measurements, both finding a negative activation energy for k 7 of from −1.0 to −1.1 kcal/mol. The small positive activation energy found in this work for k 7 fits standard models for H atom metathesis. Combination with all known kinetic information for ethane bromination gives an average reaction enthalpy of ΔH°7 = 13.0 ± 0.2 kcal/mol using both the second- and third-law thermochemical calculations. It sets the heat of ethyl radical formation to Δf H°(C2H5) = 28.40 ± 0.25 kcal/mol and the bond dissociation enthalpy, DH°(C2H5−H) = 100.5 ± 0.3 kcal/mol.