Iftikhar A. Awan

Isomerization and Decomposition Reactions in the Pyrolysis of Branched Hydrocarbons: 4-Methyl-1-pentyl Radical

The kinetics of the decomposition of 4-methyl-1-pentyl radicals have been studied from 927-1068 K at pressures of 1.78-2.44 bar using a single pulse shock tube with product analysis. The reactant radicals were formed from the thermal C-I bond fission of 1-iodo-4-methylpentane, and a radical inhibitor was used to prevent interference from bimolecular reactions. 4-Methyl-1-pentyl radicals undergo competing decomposition and isomerization reactions via beta-bond scission and 1, x-hydrogen migrations (x = 4, 5), respectively, to form short-chain radicals and alkenes. Major alkene products, in decreasing order of concentration, were propene, ethene, isobutene, and 1-pentene. The observed products are used to validate a RRKM/master equation (ME) chemical kinetics model of the pyrolysis. The presence of the branched methyl moiety has a significant impact on the observed reaction rates relative to analogous reaction rates in straight-chain radical systems. Systems that result in the formation of substituted radical or alkene products are found to be faster than reactions that form primary radical and alkene species. Pressure-dependent reaction rate constants from the RRKM/ME analysis are provided for all four H-transfer isomers at 500-1900 K and 0.1-1000 bar pressure for all of the decomposition and isomerization reactions in this system.

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

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.

Pressure dependence and branching ratios in the decomposition of 1-pentyl radicals: shock tube experiments and master equation modeling.

The decomposition and intramolecular H-transfer isomerization reactions of the 1-pentyl radical have been studied at temperatures of 880 to 1055 K and pressures of 80 to 680 kPa using the single pulse shock tube technique and additionally investigated with quantum chemical methods. The 1-pentyl radical was generated by shock heating dilute mixtures of 1-iodopentane and the stable products of its decomposition have been observed by postshock gas chromatographic analysis. Ethene and propene are the main olefin products and account for >97% of the carbon balance from 1-pentyl. Also produced are very small amounts of (E)-2-pentene, (Z)-2-pentene, and 1-butene. The ethene/propene product ratio is pressure dependent and varies from about 3 to 5 over the range of temperatures and pressures studied. Formation of ethene and propene can be related to the concentrations of 1-pentyl and 2-pentyl radicals in the system and the relative rates of five-center intramolecular H-transfer reactions and β C-C bond scissions. The 3-pentyl radical, formed via a four-center intramolecular H transfer, leads to 1-butene and plays only a very minor role in the system. The observed (E/Z)-2-pentenes can arise from a small amount of beta C-H bond scission in the 2-pentyl radical. The current experimental and computational results are considered in conjunction with relevant literature data from lower temperatures to develop a consistent kinetics model that reproduces the observed branching ratios and pressure effects. The present experimental results provide the first available data on the pressure dependence of the olefin product branching ratio for alkyl radical decomposition at high temperatures and require a value of = (675 ± 100) cm(-1) for the average energy transferred in deactivating collisions in an argon bath gas when an exponential-down model is employed. High pressure rate expressions for the relevant H-transfer reactions and β bond scissions are derived and a Rice Ramsberger Kassel Marcus/Master Equation (RRKM/ME) analysis has been performed and used to extrapolate the data to temperatures between 700 and 1900 K and pressures of 10 to 1 × 10(5) kPa.

A gas-phase kinetic study on the thermal decomposition of 2-chloropropene

AbstractThe gas-phase thermal decomposition of 2-chloropropene in the presence of a radical inhibitor was studied in the temperature range of 668.2–747.2 K and pressure between 11–76 Torr using the conventional static system. The dehydrochlorination to propyne and HCl was the only reaction channel and accounted for >98% of the reaction. The formation of propyne was found to be homogeneous and unimolecular and follows a first-order rate law. The observed rate coefficient is expressed by the following Arrhenius equation:

Evaluated kinetics of terminal and non-terminal addition of hydrogen atoms to 1-alkenes: a shock tube study of H + 1-butene.

k_{total} = 10^{13.05 \pm 0.46} (s^{ - 1} )\exp ^{ - 242.6 \pm 6.2({{kJ} \mathord{\left/ {\vphantom {{kJ} {mol}}} \right. \kern-\nulldelimiterspace} {mol}})/RT} .

Kinetics of the gas-phase thermal decomposition of 3-bromopropene

The hydrogen halide elimination is believed to proceed through a semipolar four-membered cyclic transition state. The presence of a methyl group on the α-carbon atom lowered the activation energy by 47 kJ mol−1. The experimentally observed pressure dependence of the rate constant is compared with the theoretically predicted values that are obtained by RRKM calculations.

The decomposition of 2-pentyl and 3-pentyl radicals

2-Chloropropene and 2-bromopropene have been decomposed in single pulse shock tube experiments. The only products under all conditions are propyne and allene. The high pressure rate expressions are k(2-BrC3H5 ⇒ propyne/allene + HBr) = 1014.9exp(−32 830/RT) s−1k(2-ClC3H5 ⇒ propyne/allene + HCl) = 1014.8exp(−34 200/RT) s−1 in the temperature range 1100 to 1250 K and pressures of 150 to 800 kPa. The propyne to allene ratios are 1.8 and 1.6 for the brominated and chlorinated compounds respectively with minimal temperature dependence. Results are compared with those for the alkyl compounds and ab-initio calculations on 2-chloropropene. Differences in energy transfer efficiencies for 2-chloropropene and 4-methylcyclohexene decompositions are explored.

Shock tube study of the decomposition of cyclopentyl radicals

Single-pulse shock tube methods have been used to thermally generate hydrogen atoms and investigate the kinetics of their addition reactions with 1-butene at temperatures of 880 to 1120 K and pressures of 145 to 245 kPa. Rate parameters for the unimolecular decomposition of 1-butene are also reported. Addition of H atoms to the π bond of 1-butene results in displacement of either methyl or ethyl depending on whether addition occurs at the terminal or nonterminal position. Postshock monitoring of the initial alkene products has been used to determine the relative and absolute reaction rates. Absolute rate constants have been derived relative to the reference reaction of displacement of methyl from 1,3,5-trimethylbenzene (135TMB). With k(H + 135TMB → m-xylene + CH3) = 6.7 × 10(13) exp(-3255/T) cm(3) mol(-1) s(-1), we find the following: k(H + 1-butene → propene + CH3) = k10 = 3.93 × 10(13) exp(-1152 K/T) cm(3) mol(-1) s(-1), [880-1120 K; 145-245 kPa]; k(H + 1-butene → ethene + C2H5) = k11 = 3.44 × 10(13) exp(-1971 K/T) cm(3) mol(-1) s(-1), [971-1120 K; 145-245 kPa]; k10/k11 = 10((0.058±0.059)) exp [(818 ± 141) K/T), 971-1120 K. Uncertainties (2σ) in the absolute rate constants are about a factor of 1.5, while the relative rate constants should be accurate to within ±15%. The displacement rate constants are shown to be very close to the high pressure limiting rate constants for addition of H, and the present measurements are the first direct determination of the branching ratio for 1-olefins at high temperatures. At 1000 K, addition to the terminal site is favored over the nonterminal position by a factor of 2.59 ± 0.39, where the uncertainty is 2σ and includes possible systematic errors. Combining the present results with evaluated data from the literature pertaining to temperatures of <440 K leads us to recommend the following: k∞(H + 1-butene → 2-butyl) = 1.05 × 10(9)T(1.40) exp(-366/T) cm(3) mol(-1) s(-1), [220-2000 K]; k∞(H + 1-butene → 1-butyl) = 9.02 × 10(8)T(1.40) exp(-1162/T) cm(3) mol(-1) s(-1) [220-2000 K]. Analogous rate constants for other unbranched 1-olefins should be very similar. Despite this, a factor of three discrepancy in the branching ratio for terminal and nonterminal addition is noted when comparing the present values with recommendations from a recent model of the important H + propene reaction. This difference is suggested to be well outside of the possible experimental errors of the present study or the expected differences with 1-butene. There thus appear to be inconsistencies in the current model for propene. In particular the addition branching ratio from that model should not be used as a reference value in extrapolations to other systems via rate rules or automated mechanism generation techniques.