Decomposition of 2,3-dimethylbutane in the presence of oxygen. Part 1.—Thermochemistry of the reaction (CH3)2CHCH(CH3)2→ 2i-C3H7

Abstract

The decomposition of 2,3-dimethylbutane (DMB) in the presence of O₂ in KCl-coated vessels has been studied at 480, 500 and 540 °C over a wide range of mixture composition. At low pressures of O₂ the main initiation reaction is (CH₃)₂CHCH(CH₃)₂→ 2i-C₃H₇(10) but the reaction (CH₃)₂CHCH(CH₃)₂+ O₂→ HO₂+(CH₃)₂CHC(CH₃)₂(O) becomes dominant at the higher pressures of O₂ used.

From measurements of the yield of propene, formed in 99% yield from i-C₃H₇ radicals, accurate values of k₁₀ are obtained after extrapolation to zero O₂ and DMB pressures. Combination with Tsang's high-temperature results gives log₁₀(A₁₀/s^–1)= 16.42 ± 0.10 and E₁₀= 319.1 ± 1.5 kJ mol^–1 over the temperature range 750–1200 K. These values are compared with parameters for similar systems and a simple relation has been developed to calculate the activation energy for C—C homolysis in alkanes and alkenes.

By use of the published values for k_–10, a value of Δ_fH^⊖₂₉₈(i-propyl)= 79.6 ± 2.0 kJ mol^–1 is obtained, which corresponds to D^⊖₂₉₈(CH₃CHCH₃—H)= 401.3 ± 2.0 kJ mol^–1. With these values a self-consistent set of data is available for the heats of formation of the prototypical alkyl radicals, ethyl (117.0 ± 4.0 kJ mol^–1), i-propyl and t-butyl (37.6 ± 2.0 kJ mol^–1).

From measurements of the yield of but-2-ene in the initial stages of reaction, values of k₁₂ of (3.0 ± 0.3)× 10^–8 s^–1 at 480 °C and (1.15 ± 0.15)× 10^–7 s^–1 at 500 °C are obtained for the reaction (CH₃)₂CHCH(CH₃)₂→→(CH₃)₂CHCHCH₃+ CH₃. (12) Arrhenius parameters of log₁₀(A₁₂/s^–1)= 16.6 and E₁₂= 348 kJ mol^–1 are recommended.