Kinetics of Phenyl Radical Reactions with Ethane and ... - Chemistry

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Kinetics of Phenyl Radical Reactions with Ethane and ... - Chemistry

Kinetics of Phenyl RadicalReactions with Ethane andNeopentane: Reactivity ofC 6 H 5 Toward the PrimaryC-H Bond of AlkanesJ. PARK, S. GHEYAS 1 , M. C. LINDepartment of Chemistry, Emory University, Atlanta, GA 30322Received 28 February 2000; accepted 7 August 2000ABSTRACT: The kinetics of C 6 H 5 reactions with C 2 H 6 (1) and neo-C 5 H 12 (2) have been studiedby the pulsed laser photolysis/mass spectrometric method using C 6 H 5 COCH 3 as the phenylprecursor at temperatures between 565 and 1000 K. The rate constants were determined bykinetic modeling of the absolute yields of C 6 H 6 at each temperature. Another major product,C 6 H 5 CH 3 , formed by the recombination of C 6 H 5 and CH 3 , could also be quantitatively modeledusing the known rate constant for the reaction. A weighted least-squares analysis of the twosets of data gave k 1 10 11.320.05 exp[(2236 91)/T] cm 3 mol 1 s 1 and k 2 10 11.370.03exp[(1925 48)/T] cm 3 mol 1 s 1 for the temperature range studied. The result of our sensitivityanalysis clearly supports that the yields of C 6 H 6 and C 6 H 5 CH 3 depend primarily on theabstraction reactions and C 6 H 5 CH 3 , respectively. From the absolute rate constants for thetwo reactions we obtained the value for the H-abstraction from a primary C-H bond, k p -CH 10 10.400.06 exp(1790 102/T)cm 3 mol 1 s 1 . This result is utilized for analysis of other kineticdata measured for C 6 H 5 reactions with alkanes in solution as well as in the gas phase. 2000John Wiley & Sons, Inc. Int J Chem Kinet 33: 64–69, 2001INTRODUCTIONIn general, phenyl radical is more reactive than alkylradicals [1], but unlike the alkyl, there has been verylittle information available on the reactivity of C 6 H 5toward a specific type of C-H bonds of alkanes in thegas phase; namely, the primary, secondary, and tertiaryCH bonds.There had been many relative rate measurementsCorrespondence to: M. C. Lin (chemmcl@emory.edu)Contract grant sponsor: Basic Energy Sciences, Department ofEnergyContract grant number: DE-FG02-97-ER147841Current address: Chemistry Department, Temple University,Philadelphia, PA 19122 2000 John Wiley & Sons, Inc.for phenyl reactions in solution by Bridger and Russellusing C 6 H 5 CCl 4 as a reference standard [2]. Froma series of studies carried out at 333 K, they determinedthe relative reactivities of C 6 H 5 toward the primary,secondary, and tertiary C-H bonds (0.12 0.01):1.01:4.8 for aliphatic hydrocarbons larger thanC 4 . Further discussion on this and other relative ratedata will be made later on the basis of our new resultsfor the primary C-H reaction and those for secondaryand tertiary C-H reactions reported previously [3,4].This study is a continuation of our investigation onthe kinetics of C 6 H 5 radical reactions [3–9] employingthe cavity ringdown spectrometry (CRDS) [10] and/orthe pulsed laser photolysis/mass spectrometry (PLP/MS) [11]. For several systems investigated, additional


66 PARK ET AL.Table I Reactions and Rate ConstantsaUsed in the Modeling of the CH 6 5 CH/neo-C 2 6 5H12Reactions in thePLP/MS ExperimentReactions A n Eca Ref.1. C6H5 CH 2 6 !: CH 6 6 CH 2 52.07E 11b0.0 4443 this work2. C6H5 CH 5 12 !: CH 6 6 CH 5 11 2.32E 11 0.0 3825 this work3. C6H5 CH 3 !: CHCH 6 5 3 1.38E 13 0.0 464. C6H5 CH 6 5 !: C12H10 1.39E 13 0.0 1115. C2H5 CH 2 5 !: CH 4 10 1.08E 13 0.0 0 176. C2H5 CH 2 5 !: CH 2 4 CH 2 6 1.45E 12 0.0 0 177. C6H5 CH 2 5 !: CH 8 10 1.38E 13 0.0 46dd8. C6H5 CH 5 11 !: C11H16 1.00E 13 0.0 09. C5H11 CH 3 !: CH 6 14 2.00E 13 0.0 0d10. C5H 11 !: CH 4 8 CH3 1.07E 13 0.0 29800 1811. C5H11 CH 5 11 !: C10H22 1.26E 13 0.0 0 1912. CH3 CH 3(M) !: CH(M)2 6 2.12E 16 1.0 620LOW / 1.770E 50 9.670 6220.00/TROE / .5325 151.00 1038.00 4970.00/CH 4 /2.0/ CO/1.5/ C 2 H 6 /3.0/ C 5 H 12 /3.0/ He/ .7/ C 6 H 6 /3.0aRate constants are defined by k ATnexp(E and in units cm 3 ,1and1a/RT) mol , s ; E a is given in units of cal/mol; additional reactionsinvolving C 6 H 6 ,C 6 H 5 CH 3 , and C 6 H 5 COCH 3 can be found in ref. [3].bRead as 2.07 1011.cRef. [13] otherwise noted.dAssumed.a b cTable II Experimental Conditions, Product Yields and Modeled Rate Constants in the PLP/MS Experiment at theTemperatures Studied[C 6 H 6 ] t [C 6 H 5 CH 3 ] tTemp(K) P(Torr) [C 6 H 5 COCH 3 ] 0 [C 6 H 5 ] 0 [C 2 H 6 ] 0 Exp Model k/10 10 Exp Predicted583 3.07 2.77 0.93 533.4 0.14 0.14 0.48 0.30 0.31667 3.06 2.62 1.08 535.6 0.18 0.18 0.68 0.34 0.38765 3.06 2.85 0.86 533.9 0.22 0.22 1.09 0.28 0.28795 3.06 2.67 1.03 533.8 0.25 0.25 1.14 0.33 0.35865 3.07 2.76 0.94 536.8 0.29 0.29 1.60 0.29 0.29900 3.03 2.83 0.87 529.7 0.29 0.29 1.71 0.27 0.26955 3.04 2.85 0.85 531.2 0.33 0.33 2.03 0.23 0.231000 3.10 2.90 0.81 542.1 0.34 0.33 2.33 0.21 0.20[C 6 H 6 ] t [C 6 H 5 CH 3 ] tTemp(K) P(Torr) [C 6 H 5 COCH 3 ] 0 [C 6 H 5 ] 0 [C 5 H 12 ] 0 Exp Model k/10 10 Exp Predicted565 3.08 2.87 0.83 200.2 0.09 0.09 0.75 0.30 0.31616 3.09 2.90 0.80 200.7 0.11 0.11 1.02679 3.06 3.03 0.67 198.9 0.12 0.12 1.32709 3.07 3.11 0.59 199.9 0.13 0.13 1.66 0.21 0.21761 3.08 2.96 0.74 199.6 0.15 0.16 1.82 0.29 0.29785 3.04 2.84 0.86 196.9 0.16 0.16 1.94 0.36 0.34861 3.06 2.95 0.75 199.1 0.18 0.18 2.48 0.28 0.29927 3.07 2.56 0.84 196.8 0.21 0.22 3.00 0.28 0.33968 3.05 2.37 0.83 196.4 0.22 0.22 3.12 0.30 0.31991 3.09 2.35 0.85 201.3 0.23 0.22 3.23 0.31 0.32aAll concentrations are given in mTorr.b Product yields were measured at t 15cIn units of cm 3 1 1mol s .msec at their plateaus. Typically, 2–3 runs were carried out for each temperature.


KINETICS OF PHENYL RADICAL REACTIONS WITH ETHANE AND NEOPENTANE 67Significantly, as shown in Table II, the absoluteyields of toluene can be quantitatively predicted withthe rate constant k 3 1.38 10 13 exp(23/T) cm 3mol 1 s 1 determined previously from three independentsets of experiments on C 6 H 5 H 2 [12], CH 4 [13],and i-C 4 H 10 [3]. This is reassuring and important becausethe concentration of C 6 H 5 CH 3 depends stronglyon those of CH 3 and C 6 H 5 . Our ability in predictingthe toluene yields in this and other systems, includingthe recently completed C 6 H 5 CH 2 O study [14], suggeststhat the concentration of C 6 H 5 , generated by thepulsed photolysis of acetophenone and measured byNO titration, is quite reliable. In addition, in our previousstudy on the C 6 H 5 CH 4 reaction [13], wesimultaneously measured the yields of C 2 H 6 andC 6 H 5 CH 3 for several temperatures; both yields couldbe quantitatively modeled also. The results presentedin Figure 1 give rise toFigure 2 Comparison of various C 6 H 5 abstraction rateconstants.k1 10 11.320.053 1 1exp[(2236 91)/T] cmmol s (I)k2 10 11.370.033 1 1exp[(1925 48)/T] cmmol s (II)based on a weighted least-squares analysis. The activationenergies thus determined, 4.43 0.18 and 3.83 0.09 kcal/mol, respectively, for C 2 H 6 and C 5 H 12 areconsiderably higher than the corresponding values forthe abstraction reactions, C 6 H 5 toluene, 2.04 0.07kcal/mol, and xylenes, 1.05 0.05 kcal/mol. Thelarge difference between these two classes of reactionsis fully expected because of the much weaker primaryC-H bonds in the CH 3 -substituted benzenes. Comparisonof the present results with other C-H abstractionreactions involving alkanes will be made later.Figure 2 compares the absolute rate constants forthe present C 2 H 6 and neo-C 5 H 12 reactions with thosefor H 2 ,CH 4 , and i-C 4 H 10 referred to earlier. It shouldbe stressed that both H 2 and CH 4 data include theshock-tube results of Troe and co-workers [15] above1000 K.Sensitivity AnalysisWe have carried out sensitivity analysis for the twokey products, C 6 H 6 and C 6 H 5 CH 3 , using the SENKINprogram[16]. The results are presented in Figures 3and 4 for C 2 H 6 and C 5 H 12 , respectively. Those reactionsthat have mole fraction (X i ) sensitivity coefficients,defined by S ij (X i /k j )(k j /X i ), less than 0.1are not included in the figures. As is evident fromtheresults, only three reactions in each of the systemshave a significant effect on the measured concentra-Figure 3 Sensitivity analyses for C 6 H 6 (a) and C 6 H 5 CH 3(b) in the C 6 H 5 C 2 H 6 reaction at 900 K. Reaction conditionsare given in Table II and the sensitivity coefficientsevaluated in terms of mole fractions are as defined in thetext.


68 PARK ET AL.analysis convoluting all errors fromindividual measurementsgives the following expression with relativelysmall standard deviations:kp-CH 10 10.400.06exp[(1790 102)/T] cmmol3 1s1(III)for the temperature range 565–1000 K.Equation (III) allows us to estimate the rate constantsfor the C 6 H 5 attack on a secondary and a tertiaryC-H bond, k s-CH and k t-CH, respectively, with Bridgerand Russell’s relative rates obtained at 333 K in solutionfor aliphatic reactions, 0.12:1.01:4.8 [2], ascited in the Introduction,k 1.0 109 (aliphatic) cmmol3 1s1s-CHk 4.7 109 (aliphatic) cmmol3 1s1t-CHFigure 4 Sensitivity analyses for C 6 H 6 (a) and C 6 H 5 CH 3(b) in the C 6 H 5 neo-C 5 H 12 reaction at 927 K. Reactionconditions are given in Table II and the sensitivity coefficientsevaluated in terms of mole fractions are as defined inthe text.tions of C 6 H 6 and C 6 H 5 CH 3 ; they areCH RH : CH R (1,2)6 5 6 6CH CH : CHCH (3)6 5 3 6 5 3CH CH : C H (4)6 5 6 5 12 10The rate constants for these reactions except (1) and(2) have been reliably determined [11–13]. Accordingly,k 1 and k 2 can be individually determined withreliability on account of the larger S ij values as shown.This result may be compared with other specific examplesthat have also been measured by Bridger andRussell [2] at 333 K in solution using the C 6 H 5 CCl 4reaction as the reference standard. Combining with ourabsolute rate constant for the CCl 4 reaction measuredin the temperature range 298–523 K [4], their relativerate constants can be converted to k p-CH, k s-H and k t-CHfor comparison with our absolute rate constants measuredin the gas phase, typically over a range of temperaturesoverlapping 333 K. Table III summarizedthese specific examples.As is evident fromthese results, the agreement betweenthe relative rate data measured in solution andour more elaborate direct measurements of absoluterate constants in the gas phase is reasonable, exceptthe reactions with cyclopentane and 2,3,4-trimethylpentane. The reason for the deviation in these twocases is not clear and should be examined further. Inaddition, we notice a significant molecular-size dependenceamong in the three tertiary C-H abstraction reactionsin the gas phase fromisobutane to 2,3,4-trimethylpentane. Such dependence was, however, notobserved in solution for the two larger molecules studied,as shown in Table III.Relative Reactivities of C 6 H 5 Toward p-, s-,and t-CH BondsThe absolute rate constants for the C 2 H 6 and neo-C 5 H 12 reactions given by Eqs. (I) and (II) can be usedto calculate the rate constant per primary C-H bond,k p-CH, by dividing the values of k 1 and k 2 by 6 and 12,respectively. The result is presented in the inset of Figure1. The two sets of the data agree within the combinederrors as indicated. A weighted least-squaresCONCLUSIONThe kinetics of the C 6 H 5 reactions with ethane andneopentane have been measured by the pulsed laserphotolysis/mass spectrometry method employing acetophenoneas the radical source. Kinetic modeling ofthe benzene formed in these systems over the temperaturerange 565–1000 K yielded the following rateconstants, given in units of cm 3 mol 1 s 1 , for the re-


KINETICS OF PHENYL RADICAL REACTIONS WITH ETHANE AND NEOPENTANE 69Table III Comparison of the Rate Constants per C-H bond, k p-CH, k s-CH , and kt-CHfor Reactions Studied in the GasPhase and in Solution at 333 Kak p-CHk s-CHk t-CHReactantGasSolnGasSolnGasSolnneopentane 0.12 b 0.22 ctetramethyl butane0.22 ccyclopentane 0.55 d 1.9 ccyclohexane 1.4 d 1.7 ccycloheptane 3.5 d 3.1 ccyclooctane 3.4 d 3.5 cisobutane3.0 e2,3-dimethyl butane 13 e 11 c2,3,4-trimethyl pentane 62 e 6.2 cak is given in units of 10 9 cm 3 mol1 s1x.bThis work.cRef. [2].dRef. [4].eRef. [3].actions with C 2 H 6 and neo-C 5 H 12 , respectively:k 101k 10211.320.0511.370.03exp[(2236 91)/T]exp[(1925 48)/T]These results provide for the first time the absolute rateconstant for the reaction of the phenyl radical with asingle primary C-H bond in the 560–1000 K range,k p-CH 10 10.400.06 exp[(1790 102)/T] cm 3 mol 1s 1 . The data can be used to extract rate constants forC 6 H 5 attacks on secondary and tertiary C-H bondsfromavailable total rate constants.The authors are grateful for the support of this work fromthe Basic Energy Sciences, Department of Energy, undercontract no. DE-FG02-97-ER14784.BIBLIOGRAPHY1. Yu, T.; Lin, M. C. Combust Flame 1995, 100, 169.2. Bridger, R. F.; Russell, G. A. J AmChemSoc 1963,85, 3754.3. Park, J.; Gheyas, S. I.; Lin, M. C. Int J ChemKinet1999, 31, 645.4. Yu, T.; Lin, M. C. J Phys Chem1995, 99, 8599.5. Yu, T.; Lin, M. C. J AmChemSoc 1993, 115, 4371.6. Yu, T.; Lin, M. C. J AmChemSoc 1994, 116, 9571.7. Yu, T.; Lin, M. C.; Melius, C. F. Int J ChemKinet 1994,26, 1096.8. Park, J.; Lin, M. C. J Phys ChemA 1997, 101, 14.9. Park, J.; Chakraborty, D.; Bhusari, D. M.; Lin, M. C. JPhys ChemA 1999, 103, 4002.10. Park, J.; Lin, M. C. Cavity-Ring-Down Spectrometry-A New Technique for Trace Absorption Measurements;ACS Symposium Series 720: Am. Chem. Soc., Washington,D.C., 1999; Chapter 13, p 196.11. Park, J.; Lin, M. C. Recent Research Development inPhysical Chemistry; Transworld Research Network: India,1998; Chapter 2, p 965.12. Park, J.; Dyakov, I. V.; Lin, M. C. J Phys ChemA 1997,101, 8839.13. Tokmakov, I. V.; Park, J.; Gheyas, S. I.; Lin, M. C. JPhys ChemA 1999, 103, 3636.14. Choi, Y. M.; Xia, Wensheng; Park, J.; Lin, M. C. J PhysChemA 2000, 104, 7030.15. Heckmann, E.; Hippler, H.; Troe, J. 26th Symp (Int) onCombustion; The Combustion Institute: Pittsburgh, PA,1996; p 543.16. Lutz, A. E.; Lee, R. K.; Miller, J. A. SENKIN: A FOR-TRAN Programfor Predicting Homogeneous Gas-Phase Chemical Kinetics with Sensitivity Analysis;Sandia National Laboratories Report No. SANDIA 89-8009, 1989.17. Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Esser, C.;Frank, P.; Just, Th.; Kerr, J. A.; Pilling, M. J.; Troe, J.;Walker, R. W.; Warnatz, J. J Phys ChemRef Data 21,1992, 411.18. Tsang, W. J AmChemSoc 1985, 107, 2872.19. Nielsen, O. J.; Ellermann, T.; Wallington, T. J. ChemPhys Lett 1993, 203, 302.

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