Decomposition and Isomerization of 5-Methylhex-1-yl Radical

Abstract

The decomposition and isomerization reactions of the 5-methylhex-1-yl radical (1-5MeH) have been studied at temperatures of 889-1064 K and pressures of 1.6-2.2 bar using the single pulse shock tube technique. The radical of interest was generated by shock heating dilute mixtures of 5-methylhexyl iodide to break the weak C-I bond, and the kinetics and reaction mechanism deduced on the basis of the olefin cracking pattern observed by gas chromatographic analysis of the products. In order of decreasing molar yields, alkene products from 1-5MeH decomposition are ethene, isobutene, propene, 3-methylbut-1-ene, but-1-ene, E/Z-hex-2-ene, 4-methylpent-1-ene, and hex-1-ene. The first three products account for almost 90% of the carbon balance. The mechanism involves reversible intramolecular H-transfer reactions that lead to the formation of the radicals 5-methylhex-5-yl (5-5MeH), 5-methylhex-2-yl (2-5MeH), 5-methylhex-4-yl (4-5MeH), 5-methylhex-6-yl (6-5MeH), and 5-methylhex-3-yl (3-5MeH). Competitive with isomerization reactions are decompositions by beta C-C bond scission. The main product forming radical is 5-5MeH, which is formed by intramolecular abstraction of the lone tertiary H in the radical. This reaction is deduced to be a factor of 4.0 +/- 0.7 faster on a per hydrogen basis than the analogous abstraction of a secondary hydrogen in 1-hexyl radical. The estimated uncertainty corresponds to 1 standard deviation. The following relative rates have been deduced under our reaction conditions: k(4-5MeH --> C(2)H(5) + 3-methylbut-1-ene)/k(4-5MeH --> CH(3) + Z-hex-2-ene) = 10((0.39+/-0.12)) exp[(675 +/- 270)K/T]; k(4-5MeH --> C(2)H(5) + 3-methylbut-1-ene)/k(4-5MeH --> CH(3) + E-hex-2-ene) = 10((-0.10+/-0.09)) exp[(1125 +/- 210)K/T]; k(3-5MeH --> iso-C(3)H(7) + but-1-ene)/(k)(3-5MeH --> CH(3) + 4-methylpent-1-ene) = 10((0.26+/-0.55)) exp[(1720 +/- 1300)K/T]. Observed olefin distributions depend on the relative rate constants and the interplay of chemical activation and falloff behavior as the energy distributions of the various radicals relax to steady-state values. A kinetic model using an RRKM/master equation analysis has been developed, and absolute rate expressions have been deduced. The model was used to extrapolate the data to temperatures between 500 and 1900 K and pressures of 0.1-1000 bar, and results for 12 isomerization reactions and 10 beta C-C bond scission reactions are reported.