Rate Constants for the Initiation of n-Butane Pyrolysis and for the Recombination of Ethyl Radicals

Abstract

Study of the pyrolyses of n-butane and of ethane in the temperature range 895–981 K permits the evaluation of the rate constant, k1/s–1= 1015.3 ± 0.9 exp[(–323 ± 17) kJ mol–1/RT] assuming that k1= 3k2. n-C4H10→22H5˙(1), n-C4H10→CH3˙+ n-C3H7˙(2) This leads via the use of thermochemical data to the result k–1/cm3 mol–1 s–1= 1011.6 over the whole temperature range of the present work, which is in remarkable agreement with other recent estimates made using widely differing techniques at temperature in the range 350–600 K. The present value of k-1 is much lower than previously thought and the possibility that this discrepancy stems from errors in the thermochemistry of ethyl is discussed in detail. It is concluded that this possibility can be discounted and that the current thermochemistry is reliable.

Values of the rate constants for the reactions CH3˙+ n-C4H10→CH4+ C4H9˙(3) and C2H5˙+ n-C4H10→C2H6+ C4H9˙(4) are obtained: k3/cm3 mol–1 s–1= 1013.5 ± 0.9 exp[(–61.8 ± 15.0) kJ mol–1/RT] and k4/cm3 mol–1 s–1= 1013.9 ± 0.4 exp[(–92.8 ± 6.7) kJ mol–1/RT]. These results are in good agreement with our previous measurements.