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

Abstract

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 ] cm 3 /(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 cm 3 /(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 Δ H ° 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 Δ f H °(C 2 H 5 ) = 28.40 ± 0.25 kcal/mol and the bond dissociation enthalpy, DH °(C 2 H 5 −H) = 100.5 ± 0.3 kcal/mol.