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 ΔfH⊖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.