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H Atom Attack on Propene Claudette M ARTICLE pubs.acs.org/JPCA H Atom Attack on Propene Claudette M. Rosado-Reyes,* Jeffrey A. Manion, and Wing Tsang National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899, United States bS Supporting Information d ABSTRACT: The reaction of propene (CH3CH CH2) with hydrogen atoms has been investigated in a heated single-pulsed shock tube at temperatures between 902 and 1200 K and pressures of 1.5À3.4 bar. Stable products from H atom addition and H abstraction have been identified and quantified by gas chromatography/flame ionization/mass spectrometry. The reaction for the H addition channel involving methyl displacement from propene has been determined relative to methyl displacement from 1,3,5-trimethylbenzene þ f d þ (135TMB), leading to a reaction rate, k(H propene) H2C CH2  13 À 3 CH3) = 4.8 10 exp( 2081/T)cm/(mol s). The rate constant for the abstraction of the allylic hydrogen atom is determined to be k(H þ propene f d þ  13 À 3 CH2CH CH2 H2) = 6.4 10 exp( 4168/T)cm /(mol s). The reaction of H þ propene has also been directly studied relative to the reaction of H þ propyne, and the relationship is found to be log[k(H þ propyne f acetylene þ þ f þ À ( þ CH3)/k(H propene ethylene CH3)]=( 0.461 0.041)(1000/T) (0.44 ( 0.04). The results showed that the rate constant for the methyl displacement reaction with propene is a factor of 1.05 ( 0.1 larger than that for propyne near 1000 K. The present results are compared with relevant earlier data on related compounds. ’ INTRODUCTION chemical processes. For H-atom attack on propene the reaction Liquid fuels are comprised of unsaturated and saturated— channels in Scheme 1 should be considered. normal, branched, and cyclic—hydrocarbons. An alkyl radical is Reaction 1 involves terminal H-addition to propene to formed when a fuel molecule decomposes upon radical attack or produce an isopropyl radical. Previous work of the unimolecular thermal excitation. The alkyl radical can breakdown via β-bond decomposition of iso-C3H7 has shown that at high temperatures relevant to combustion, reaction 1 is reversible, favoring back scission to produce another alkyl radical and a 1-alkene molecule. À 1 Propene is the simplest substituted of the 1-alkenes. As the fuel decomposition into propene and H (log KÀ1 (1000 K) = 15.63). breakdown progresses via unimolecular reactions and oxidation This channel is invisible to our experiments. Reaction 2 consists steps, H atoms are produced. They are among the most of the addition of H atom to the central carbon atom with important intermediates in hydrocarbon combustion. subsequent displacement of the methyl moiety to produce This paper is concerned with the reaction of H atoms with ethylene. This is actually a chemically activated decomposition. propene, studied under shock-tube conditions. It is an extension The thermochemistry favors the methyl displacement process of earlier measurements of the mechanism and rate constants for overwhelmingly. Thus the measured rate constant refers to the addition process. the reaction of H atoms with other unsaturated hydrocarbons. ff These reactions are of particular importance with respect to the H-atom abstraction reactions can take place in three di erent mechanisms associated with the formation of the initial aromatic ways, abstraction of allylic H (reaction 3), vinylic H (reaction 4), ring and subsequent soot formation. H-atom reaction with larger and methylvinylic H (reaction 5). Reaction 3 is a source of allyl unsaturated hydrocarbons leads to resonance stabilized radicals, radicals. A possible sink for allyl radicals in our experiments is such as allyl, benzyl, and propargyl radicals. They are intermedi- their recombination reaction to form 1,5-hexadiene and their ates in the cyclization steps that initiate the production of reaction with methyl radicals to form 1-butene. Reactions 4 and 5 aromatic rings, as precursors of polycyclic aromatic hydrocar- produce 1- and 2-propenyl radicals. 1-Propenyl radicals decom- pose into acetylene and methyl radical, while the decomposition bons (PAHs) and soot. Under fuel-rich conditions, the concen- þ trations of these radicals can build up in combustion systems. of 2-propenyl radicals favors the propyne H channel. These The reactivity of H-atoms is such that their attack on any channels are thermodynamically less favorable than the abstrac- hydrocarbon of even moderate complexity leads to multiple reaction tion of an allylic H-atom. channels. In the case of unsaturated hydrocarbons (always present in high temperature rich systems) there are contributions Received: December 2, 2010 from abstraction as well as addition channels. Their relative Revised: February 18, 2011 importance dictates the reaction pathways for subsequent Published: March 15, 2011 This article not subject to U.S. Copyright. 2727 Published 2011 by the American Chemical Society dx.doi.org/10.1021/jp111476q | J. Phys. Chem. A 2011, 115, 2727–2734 The Journal of Physical Chemistry A ARTICLE Scheme 1 In a recent study we have examined the analogous reaction residence times, leading to the possibility of studying primary mechanism for H-atom reaction with propyne.2 This furnishes an chemical processes, assuring the absence of surface reactions, and interesting basis for comparison, involving the effect of the degree minimizing contributions from chain reactions and secondary unsaturation on nonterminal addition and the consequence for chemistry. (Certain commercial materials and equipment are abstraction in going from a propargyl-H to an allylic-H. identified in this paper in order to specify adequately the The available literature data on the reaction of H atoms with experimental procedure. In no case does such identification propene and other relevant unsaturated hydrocarbons are com- imply recommendation of endorsement by the National Institute piled in Table 1. Arrhenius parameters have been measured for of Standard and Technology, nor does it imply that the material the overall reaction at temperatures of 298À445 K, resulting in or equipment is necessarily the best available for the purpose.) the rate expression of 1.32  1013 exp(À785/T)cm3/(mol 3 s),3 Table 2 contains a listing of the mixtures used in these studies and at temperatures of 177À473 K leading to a rate expression of along with the temperature and pressure conditions achieved 6.13  1012 exp(À609/T)cm3/(mol 3 s).4 These are mostly due with each. Note particularly the large difference in concentration to terminal addition. Little work has been done at room between the radical source and the target molecules. This temperature.5 Estimates on the ratio of rate constants of non- guarantees that radicals will not attack radical source or reaction terminal versus terminal H atom addition have been made at products. Two sources are used to generate H atoms. One of 6 À room temperature. The rate constant available for abstraction is them is hexamethylethane (HME, (CH3)3C C(CH3)3). Its an estimate. Particularly important is the absence of direct thermal decomposition takes place via, HME f 2 t-butyl f2 þ measurements of the individual rate constants for hydrogen i-C4H8 2H. The reaction is monitored by the disappearance of atom attack on propene. A direct measure of methyl displace- HME and the formation of i-butene. The i-butene concentration ment and allyl formation may be of particular importance in is a measure of the number of H atoms released into the system. simulation studies due to the difference in reactivity of these The extent of the reaction of HME decomposition is also used to two radicals. The present study is carried out with this aim in determine the temperature of the experiment by means of its rate À mind. It will also lead to a better basis of estimates of related expression k(HME f 2 t-butyl) = 1016.40 exp(À34400/T)s 1.8 reaction processes. To cover lower temperature regions, experiments have been carried out using t-butylperoxide (tBPO, (CH3)3CO-OC- ≈ (CH3)3) in an excess of H2 ( 20%) to generate H atom, ’ EXPERIMENTAL METHODS f f þ (CH3)3CO-OC(CH3)3 2(CH3)3CO 2CH3C(O)CH3 þ f þ Experiments were carried out in a heated single pulsed shock 2CH3, 2CH3 2H2 2CH4 2H. In this system, chlorocy- tube. Details of the apparatus and general procedures have been clopentane is used as a temperature standard. The thermal previously published.7 For the present purposes, it can be decomposition of chlorocyclopentane into cyclopentene and regarded as a pulse reactor with heating time, derived from hydrogen chloride is characterized by the rate expression À recorded pressure traces, of 500 μs. The unique features of the 4.47  1013 exp(À24570/T)s 1.9 Experimental pressures are instrument are the generation of high temperatures and short calculated from the temperature and mixture composition, 2728 dx.doi.org/10.1021/jp111476q |J. Phys. Chem. A 2011, 115, 2727–2734 The Journal of Physical Chemistry A ARTICLE Table 1. Summary of Relevant Data on the Reaction of Hydrogen Atoms with Alkenes and Alkynes reaction rate constant, cm3/(mol 3 s) T (K) P (bar) reference Addition H þ propene f products 1.33  1013 exp(À785/T) 298À445 0.067 Harris et al. (1982)3 þ f  12 À À 4 H propene i-C3H7 6.13 10 exp( 609/T) 177 473 0.067 Kurylo et al. (1971) 5.3  1011 298 1.01  105 Munk et al. (1986)5 1.32  1013 exp(À785/T) 300À2500 ¥ Tsang, W. (1991, 1992)16,17,a 5.70  109 T1.16 exp(À440/T) 177À910 133À1200 Seakins et al.
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