Decomposition of 2,3-dimethylbutane in the presence of oxygen. Part 2.—Elementary reactions involved in the formation of products

Abstract

A detailed product analysis has been carried out over a wide range of mixture composition on the decomposition of 2,3-dimethylbutane (DMB) in the presence of O₂ in KCl-coated vessels between 480 and 540 °C. Propene, 2-methylbut-2-ene, CH₄, HCHO and 2,3-dimethylbut-1-ene are the major initial products, with smaller amounts of 2,3-dimethylbut-2-ene, 3-methylbut-1-ene, but-2-ene and, at high O₂ pressures, tetramethyloxirane. By extrapolation of product yields to zero O₂ and DMB pressures to remove contributions from the chain processes, values of k₀ of 0.0211 ± 0.0027 dm³ mol^–1 S^–1 at 500 °C and 0.163 ± 0.011 dm³ mol^–1 S^–1 at 540 °C are obtained: (CH₃)₂ CHCH(CH₃)₂+ O₂→(CH₃)₂ CHC(CH₃)₂+ HO₂(0), No other values of k₀ available for DMB or any analogous reaction involving a hydrocarbon, despite the importance of this type of reaction in combustion. The recommended Arrhenius parameters are log₁₀(A₀/dm³ mol^–1 S^–1)= 10.31 ± 0.25 and E₀= 173 ± 3.5 kJ mol^–1. These parameters are consistent with a dissociation energy of 387 kJ mol^–1 for the tertiary C—H bond in DMB, in excellent agreement with recent values for that in i-butane.

Detailed studies of the relative yields of the products have permitted rate constants to be obtained, and Arrhenius parameters to be recommended, for a number of the elementary steps in the DMB + O₂ system. Patterns in the variation of rate constants for different alkyl radical homolyses and alkyl + O₂ reactions are discussed.