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= 10^{15.3 ± 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= 10^11.6 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= 10^{13.5 ± 0.9} exp[(–61.8 ± 15.0) kJ mol^–1/RT] and k₄/cm³ mol^–1 s^–1= 10^{13.9 ± 0.4} exp[(–92.8 ± 6.7) kJ mol^–1/RT]. These results are in good agreement with our previous measurements.