Mechanism and rate parameters for the pyrolysis of 2,3-dimethylbutane

Abstract

The pyrolysis of 2,3-dimethylbutane (DMB) was investigated in the ranges of 667-770K and 10-181 Torr (1.3-24kPa) at up to 3% decomposition. A Rice-Herzfeld chain terminated by recombinations of methyl and isopropyl radicals is postulated with self-inhibition due to the abstraction of hydrogen atoms from the major olefin product, 2-methyl-2-butene (2M2B), giving resonance-stabilized 2-methyl-2-butenyl radicals which participate in termination reactions. DMB---- >2i-C 3 H 7 (1a) DMB----> CH 3 + C 5 H 11 (1b) can be represented for the temperature range investigated, by k 1 = k 1a + k 1b — 10 16.7 exp ( — 322 kj mol -1 / RT ) s -1 which is in agreement with a previous measurement, with thermochemical calculation and with the results of the present investigation. The values of the ratios of rate constants for the reactions i-C 3 H 7 — -►H + C 3 H 6 (3a) i-C 3 H 7 — —> CH 3 + C 2 H 4 (3b) 2,3-dimethylbutyl —->i-C 3 H 7 + C 3 H 6 (5a) 2,3-dimethylbutyl ——> CH 3 + 3-methyl-1-butene (5b) are estimated to be k 3b / k 3a = 0.040 and k 5a / k 5a = 55.6, independent of temperature. The values of the rate constants for the reactions CH 3 + DMB----> CH 4 + C 6 H 13 (7) i-C 3 H 7 + DMB -> C 3 H 8 + C 6 H 13 (4) CH 3 + 2M2B -> CH 4 + C 5 H 9 (12) are estimated to be k 7 = 10 13.3 exp ( — 60 kJ mol -1 / RT ) cm 3 mol -1 s -1 , k 4 = 10 13.2 exp (— 89 kJ mol -1 / RT ) cm 3 mol -1 s -1 , and, k 12 = 10 13.7 exp (— 63 k J mol -1 / RT ) cm 3 mol -1 s -1 . The differential activation energy for abstraction of primary/tertiary hydrogen atoms from DMB is found to be 21 k J mol -1 . The results confirm also the value 10 11.6 cm 3 mol -1 s -1 for the rate constant of the mutual recombination of isopropyl radicals.