Rate constants for the gas-phase reactions of chlorine atoms with 1,4-cyclohexadiene and 1,5-cyclooctadiene at 298 K

Rate Constants for theGas-Phase Reactions ofChlorine Atoms with1,4-Cyclohexadiene and1,5-Cyclooctadiene at 298 KA. SHARMA, K. K. PUSHPA, S. DHANYA, P. D. NAIK, P. N. BAJAJ

Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

Received 4 November 2011; revised 15 February 2011, 15 March 2011; accepted 15 March 2011

DOI 10.1002/kin.20567Published online 23 June 2011 in Wiley Online Library (wileyonlinelibrary.com).

ABSTRACT: Rate constants for the reactions of Cl atoms with two cyclic dienes, 1,4-cyclohexadiene and 1,5-cyclooctadiene, have been determined, at 298 K and 800 Torr of N2,using the relative rate method, with n-hexane and 1-butene as reference molecules. The con-centrations of the organics are followed by gas chromatographic analysis. The ratios of the rateconstants of reactions of Cl atoms with 1,4-cyclohexadiene and 1,5-cyclooctadiene to that withn-hexane are measured to be 1.29 ± 0.06 and 2.19 ± 0.32, respectively. The corresponding ratioswith respect to 1-butene are 1.50 ± 0.16 and 2.36 ± 0.38. The absolute values of the rate con-stants of the reaction of Cl atom with n-hexane and 1-butene are considered as (3.15 ± 0.40) ×10−10 and (3.21 ± 0.40) × 10−10 cm3 molecule−1s−1, respectively. With these, the calculatedvalues are k(Cl + 1,4-cyclohexadiene) = (4.06 ± 0.55) × 10−10 and k(Cl + 1,5-cyclooctadiene) =(6.90 ± 1.33) × 10−10 cm3 molecule−1 s−1 with respect to n-hexane. The rate constantsdetermined with respect to 1-butene are marginally higher, k(Cl + 1,4-cyclohexadiene)= (4.82 ± 0.80) × 10−10 and k(Cl + 1,5-cyclooctadiene) = (7.58 ± 1.55) × 10−10 cm3

molecule−1 s−1. The experiments for each molecule were repeated three to five times,and the slopes and the rate constants given above are the average values of these mea-surements, with 2σ as the quoted error, including the error in the reference rate con-stant. The relative rate ratios of 1,4-cyclohexadiene with both the reference molecules arefound to be higher in the presence of oxygen, and a marginal increase is observed inthe case of 1,5-cyclooctadiene. Benzene is identified as one major product in the caseof 1,4-cyclohexadiene. Considering that the cyclohexadienyl radical, a product of the hy-drogen abstraction reaction, is quantitatively converted to benzene in the presence ofoxygen, the fraction of Cl atoms that reacts by abstraction is estimated to be 0.30 ±0.04. The atmospheric implications of the results are discussed. C© 2011 Wiley Periodicals,Inc. Int J Chem Kinet 43: 431–440, 2011

Correspondence to: S. Dhanya; e-mail: [email protected]© 2011 Wiley Periodicals, Inc.

INTRODUCTION

Most of the volatile organic compounds continuouslyemitted to the earth’s troposphere from biogenic and

432 SHARMA ET AL.

anthropogenic sources undergo reactions with highlyreactive species, such as OH radical, ozone, and NO3.It is well established that oxidation reactions of unsat-urated hydrocarbons with these species play a majorrole in the tropospheric photochemistry and generationof aerosols [1,2]. Previous studies have shown that, formany organic molecules, reactions with Cl atoms arealso important in the troposphere, due to higher rateconstants of their reactions [3], and the reactions ofCl atoms with volatile organic compounds (VOCs) aresuggested to play an important role in the atmosphericchemistry of the remote marine boundary layer [4], aswell as the polluted urban areas with significant an-thropogenic emission of Cl2 [5]. A recent field studysuggests that the role of the reactions of Cl atoms inpolluted noncoastal areas may be more significant thanwhat was thought earlier [6]. Hence, there is consid-erable interest in measuring the rate constants of thereactions of Cl atoms with different VOCs, includingunsaturated molecules [7,8].

Recently, we measured the rate constants of the re-actions of Cl atoms with cyclic alkenes, namely cy-clopentene, cyclohexene, and cycloheptene, and ob-served an increase in the rate constant with the numberof carbon atoms in the ring. This was attributed tothe increase in the number of abstractable hydrogenatoms [9]. Unlike the reactions of OH radicals, unsat-uration does not appear to increase the rate constantof the Cl atom reaction considerably. To investigatethe effect of unsaturation further, we have now mea-sured the rate constants of the reactions of Cl atomswith cyclic hydrocarbons having two double bonds,namely 1,4-cyclohexadiene and 1,5-cyclooctadiene, at298 K and 1 atm pressure, using the relative ratemethod. Cyclic alkenes and dienes are constituents ofautomobile exhausts and are also emitted to the at-mosphere from various other sources such as forestfire and incinerator. Many biogenic molecules such aspinenes and carene have the same cyclic ring struc-ture as that of cyclohexene and terpenes, such as γ-terpinene, have the same ring structure as that of 1,4-cyclohexadiene.

EXPERIMENTAL

All the reactions were carried out in a quartz reactionchamber of 3-L volume, fitted with vacuum stopcocksand a sealed port for taking out samples for GC anal-ysis. Since molecular chlorine (Cl2) reacts with thesecyclodienes, Cl atoms were generated by photolysis oftrichloroacetyl chloride (CCl3COCl) at 254 nm, usinga UV lamp (Rayonett). This molecule is regularly usedas a source of Cl atoms [8], and the counter fragments

are not found to interfere with the relative rate measure-ments, even in unsaturated molecules, as indicated bythe similar rate constants estimated using Cl2, SOCl2,and CCl3COCl as Cl atom precursors [10]. The reac-tion mixture, consisting of cyclodienes (CD), whichis either 1,4-cyclohexadiene (250–350 ppm) or 1,5-cyclooctadiene (90–200 ppm), reference compound(50–100 ppm), and trichloroacetyl chloride (typically∼300 ppm), was prepared in the reaction chamber, us-ing a vacuum manifold, and the total pressure wasmaintained at 800 ± 3 Torr, by adding N2/N2–O2

mixture/air. The pressure was measured, using a ca-pacitance manometer (Pfeiffer Vacuum). Before pho-tolysis, the prepared reaction mixture was equilibratedfor 60–80 min for uniform distribution, which was con-firmed by the reproducibility of the gas chromatogram.Experiments were performed at room temperature(298 ± 2 K). The mixture was photolyzed for a pe-riod of 6–8 min, in steps of about 1 min, and aftereach photolysis step the decrease in the concentrationof the two organics, cyclodiene (CD) and the referencemolecule (R), was determined, using gas chromato-graph (Chemito GC 8610), with (5%/10%) SE 30 col-umn, in conjunction with a flame ionization detector.The stability of the reaction mixture with respect towall losses and dark reactions was checked for about 7h, which is more than the total duration of a relative ratemeasurement. Noninterference of any reaction productof CD and R in the GC analysis was also checked in-dependently. Assuming that both the molecules reactonly with chlorine atoms, the kinetic data were ob-tained from the fractional loss of CD and R, using thestandard expression

ln

((CD)t0(CD)t

)=

((kCD

kR

)ln

((R)t0(R)t

))(I)

where [R]t0 and [CD]t0 are the concentrations of R andCD, respectively, at time t0, [R]t , and [CD]t are thecorresponding concentrations at time t , and kCD and kR

are the rate constants of their reactions with chlorineatoms. In the present study, both n-hexane and 1-butenewere used as the reference molecules (R).

1,4-Cyclohexadiene (97%, from Aldrich (Stein-heim, Germany)) and 1,5-cyclooctadiene (99.5%,from Fluka (Basel, Switzerland)) were subjected tofreeze–pump–thaw cycles before use. The nitrogenand oxygen were of 99.9% purity, from INOX AirProducts, Ltd. (Mumbai, India), and zero-grade airwas from Chemtron Science Laboratories (Mumbai,India).

The products of the reaction of chlorine atomswith cyclohexadiene and cyclooctadiene, formed inthe presence of air at atmospheric pressure, were

International Journal of Chemical Kinetics DOI 10.1002/kin

RATE CONSTANTS FOR THE REACTIONS OF CHLORINE ATOMS WITH CYCLIC DIENES 433

characterized by a gas chromatograph coupled with amass spectrometer (Shimadzu), using BPX50 column(30 m × 0.25 mm × 0.25 μm) and CP WAX 52CB(30 m × 0.25 mm × 0.25 μm) columns, indepen-dently. The observed spectral features of the productswere compared with the reported mass spectra.

RESULTS

Determination of Rate Constants

To see the possible contribution from direct photolysisof CD and R molecules, these compounds were pho-tolyzed in N2, in the absence of added chlorine sourcefor about 8 min, in steps of 1 min, as in the actualmeasurements, described above in the Experimentalsection. There was no decrease in the concentration ofCD, n-hexane, and butene, which confirmed the ab-sence of any significant loss due to direct photolysis.Reactions of n-hexane, butene, and the CDs with Clatoms were also individually monitored, using GC, tosee whether any product buildup occurs that interfereswith the analysis.

The experimental results, with n-hexane as the stan-dard molecule, are plotted according to Eq. (1), inFig. 1a. The plots are linear, with near zero intercept, asexpected for a valid relative rate measurement. Fromthe linear least-square fit, slopes of the lines are ob-tained, which are the ratios of the rate constants ofthese cyclodienes to that of n-hexane. Since the possi-bility of addition to double bond also exists in the reac-tions of cyclodienes with Cl atom, another unsaturatedmolecule, 1-butene, with the possibility of the similaraddition reaction, was also used as a reference for bet-ter comparison. The plots, obtained with 1-butene asreference, are given in Fig. 1b. The experiments wererepeated a minimum of three times, and the experimen-tal conditions and the slopes obtained are given in Ta-ble I. The slope and the error given for each experimentare obtained from the linear least-square fitting of thedata. The average values of these individual measure-ments are shown in bold in Table I for each set, with thequoted error of two standard deviations. The measure-ments at lower total pressures, 450 and 250 Torr, indi-cate that the slopes are independent of the total pressurein this range. In our earlier work, the rate constant ofthe reaction of Cl atom with n-hexane was consid-ered to be (3.03 ± 0.06) × 10−10 cm3 molecule−1 s−1

[3], for calculating the rate constants of cyclic alkenes[9]. In the present work, this is modified to (3.15 ±0.40) × 10−10 cm3 molecule−1 s−1, considering thebest current IUPAC estimate of the rate constant forthe Cl atom reaction with n-butane [11], the reference

0.0 0.1 0.2 0.3 0.40.0

(a)

(b)

0.1

0.2

0.3

0.4

0.5

0.6

ln((

CD

) t 0/(CD

) t t)

ln((n-hexane)t0

/(n-hexane)tt

)

0.0 0.1 0.2 0.3 0.4 0.50.0

0.2

0.4

0.6

0.8

ln((

CD

) t 0/(CD

) t t)

ln((butene)t0

/(butene)tt

)

Figure 1 Logarithmic plot of the relative decrease inthe concentration of 1,4-cyclohexadiene (�) and 1,5–cyclooctadiene (�) due to reaction with Cl atom at 298 ±2 K in N2; with (a) n-hexane and (b) 1-butene as referencecompounds.

reaction used by Atkinson et al. [3], to be (2.05 ±0.25) × 10−10 cm3 molecule−1 s−1. Similarly, (3.21 ±0.41) × 10−10 cm3 molecule−1 s−1 is used as the rateconstant for the reaction of Cl atom with 1-butene, cal-culated on the basis of the experimental relative rateratio with respect to n-hexane [12], 1.02 ± 0.02, and theabove rate constant for n-hexane. The calculated rateconstants are listed in Table I, and the errors quotedinclude the errors in the reference rate constants.

Experiments were also carried out in the presenceof air; the typical plots are shown in Fig. 2. In boththe cyclodienes, a marginal increase in the averageslope is observed consistently in the presence of air(Table I), suggesting some additional reactions in thiscondition. This increase is found to be more in the caseof cyclohexadiene. To see the effect of oxygen further,the experiments were conducted in the presence ofdifferent partial pressures of oxygen. In the case ofcyclohexadiene, a systematic increase in the relativerate ratio was observed up to 25% of oxygen, as shownin Fig. 3.


434 SHARMA ET AL.

Table I Relative Rate Measurements of Cl Atom Reactions with 1,4-Cyclohexadiene and 1,5-Cyclooctadiene underDifferent Conditions

Concentration Concentration of k (× 1010)Cyclic Diene (ppm) Reference Reference (ppm) Buffer Gas Ratio (cm3 molecule−1 s−1)

1,4 Cyclohexadiene 226 n-Hexane 72 N2 1.28 ± 0.03 4.03 ± 0.52252 n-Hexane 72 N2 1.32 ± 0.02 4.16 ± 0.53210 n-Hexane 66 N2 1.27 ± 0.04 4.00 ± 0.52

1.29 ± 0.06 4.06 ± 0.55518 1-Butene 99 N2 1.49 ± 0.06 4.78 ± 0.63549 1-Butene 78 N2 1.43 ± 0.06 4.59 ± 0.61496 1-Butene 67 N2 1.58 ± 0.09 5.07 ± 0.70

1.50 ± 0.16 4.82 ± 0.80183 n-Hexane 76 N2(400 Torr) 1.41 ± 0.16 4.44 ± 0.75316 n-Hexane 120 N2(250 Torr) 1.33 ± 0.02 4.19 ± 0.53233 n-Hexane 72 Air 1.50 ± 0.02231 n-Hexane 55 Air 1.79 ± 0.04231 n-Hexane 63 Air 1.69 ± 0.03

1.66 ± 0.30215 1-Butene 75 Air 1.87 ± 0.02112 1-Butene 120 Air 1.78 ± 0.06

1.82 ± 0.101,5-Cyclooctadiene 141 n-Hexane 77 N2 2.34 ± 0.26 7.37 ± 1.24

115 n-Hexane 84 N2 2.17 ± 0.24 6.84 ± 1.15117 n-Hexane 84 N2 1.99 ± 0.07 6.09 ± 0.80117 n-Hexane 77 N2 2.36 ± 0.12 7.43 ± 1.01119 n-Hexane 88 N2 2.07 ± 0.05 6.52 ± 0.84

2.19 ± 0.32 6.90 ± 1.33155 1-Butene 99 N2 2.23 ± 0.06 7.16 ± 0.93137 1-Butene 78 N2 2.58 ± 0.13 8.28 ± 1.13132 1-Butene 67 N2 2.26 ± 0.07 7.25 ± 0.94

2.36 ± 0.38 7.58 ± 1.55127 n-Hexane 48 Air 2.64 ± 0.08

89 n-Hexane 43 Air 2.64 ± 0.06183 n-Hexane 29 Air 2.45 ± 0.08146 n-Hexane 57 Air 2.39 ± 0.03190 n-Hexane 59 Air 2.40 ± 0.03

2.50 ± 0.26

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.2

0.4

0.6

0.8

1.0

1.2

ln((

CD

) t 0/(CD

) t t)

ln((reference)t0

/(reference)tt

)

Figure 2 Logarithmic plot of the relative decrease in theconcentration of 1,4-cyclohexadiene, ◦: with respect to n-hexane, �: with respect to 1-butene and �: 1,5-cyclooctadienewith respect to n-hexane due to reaction with Cl atom at298 ± 2 K in air.

Stable Product Analysis in the ChlorineAtom Initiated Oxidation

Attempts were made to characterize the stable productsformed during the chlorine atom initiated oxidation ofboth the dienes in atmospheric conditions. For this,a mixture of cyclodiene/CCl3COCl/air, at a total pres-sure of 800 Torr, was photolyzed at 254 nm, for a periodranging from 1 to 8 min. The products were analyzedby GC-MS, with different columns. Figure 4 shows thetotal ion chromatogram obtained in the case of cyclo-hexadiene. The only products clearly identified here arebenzene and phenol. The mass spectrum of one prod-uct, seen in both carbowax (retention time of 12.2 min)and BPX50 columns (retention time of 10.4 min),matches with that of an unsaturated acyclic alcohol, ei-ther 2,4-hexadiene-1-ol or 5-hexyn-1-ol. The intensity



0.0 0.2 0.4 0.60.0

0.5

1.0

1.5

0 20 40 60 80 1001.0

1.5

2.0

2.5

Obs

erve

d ra

te c

onst

ant r

atio

O2 %

ln((

c-he

xadi

ene)

t 0/(c-h

exad

iene

) t t)

ln((n-hexane)t0

/(n-hexane)tt

)

Figure 3 Logarithmic plot of the relative decrease inthe concentration of 1,4-cyclohexadiene with respect to n-hexane at varying partial pressure of oxygen due to reactionwith Cl atom at 298 ± 2 K. �: 5%, ◦: 10%, and •: 20% oxy-gen. Inset: Variation of the observed rate constant ratio ofcyclohexadiene, with reference to that of hexane at differentpartial pressures of oxygen.

ratios for 35Cl and 37Cl isotopes in the recorded massspectra of the products, with retention times of 11.2

0 40 80 120 160−10

0

10

20

30

40

50

For

mat

ion

of b

enze

ne (p

pm)

Depletion of c-hexadiene (ppm)

Figure 5 Typical plots of amounts of benzene formedagainst the amounts of 1,4-cyclohexadiene reacted with Clatom. � and �: in air; •: in N2, shifted by –5 units for clarity.

and 11.9 min, indicate that these are formed by additionof Cl atom to cyclohexadiene, but the exact structurecould not be assigned. In the case of cyclooctadienealso, the products could not be characterized, withoutambiguity, by matching with the mass spectra availablein the library. The depletion of cyclohexadiene and for-mation of benzene were followed quantitatively in thepresence of air, and two typical plots of their lineardependence are shown in Fig. 5. From 10 indepen-dent experiments, the average yield of benzene was

6 8 10 12 14 16 18 200

20

40

60

80

100B

ba

Phenol

Inte

nsity

(arb

t. un

it)

Time (min)

3 4 5 6 7 8 9 10 11 12 13 14 15 160

10

20

30

40

50

ca b

Benzene

A

Inte

nsity

(arb

t. un

it)

Figure 4 Total ion chromatograms of the products of Cl atom initiated oxidation of 1,4-cyclohexadiene with (A) CP WAXcolumn; a, unphotolyzed; b, photolyzed sample, both at 30◦C isothermal; and c, photolyzed with temperature programming,maintained at 100◦C for 10 min and raised the temperature to 150◦C at the rate of 20◦C per min (plots shifted vertically forclarity) and (B): BPX50 column with temperature programming, maintained at 50◦C for 5 min and with the temperature raisedto 180◦C at the rate of 10◦C/min; a, unphotolyzed and b, photolyzed.


436 SHARMA ET AL.

measured to be 30 ± 4% of cyclohexadiene consump-tion. The dependence in the absence of air, also shownin Fig. 5, was found to deviate marginally from lin-earity. Owing to very low yield of phenol, quantitativeestimation was not possible in the present experiments.

DISCUSSION

The rate constants for the reactions of Cl atom with1,4-cyclohexadiene and 1,5-cyclooctadiene have notbeen reported so far. The reaction of Cl atom withcyclohexadiene has been used as a source of cyclo-hexadienyl radicals [13], but there is no mention of therate constant of the reaction. The values of the rate con-stants obtained in the present work, with n-hexane and1-butene as references, are found to differ marginally,the latter being always higher. We have considered therate constants determined with n-hexane as the refer-ence molecule for comparison of different cyclic unsat-urated compounds, since it was also used as the refer-ence in our earlier study on cycloalkenes [9]. However,the reported rate constants in [9] are modified, using thecorrected rate constant value for n-hexane, wheneverquoted here.

As mentioned in the Introduction, the results of ourearlier measurements of the rate constants of the reac-tions of Cl atom with cyclopentene, cyclohexene, andcycloheptene, indicated an increase in the rate constantwith increasing number of carbon atoms [9]. Unlike thereactions of OH radicals, where the rate constant in-creases by an order of magnitude with unsaturation, therate constants of the reactions of Cl atoms with cyclicalkenes were only marginally higher than those of thecorresponding saturated cyclic alkanes. The productsobserved in the reactions of cyclic alkenes with Clatoms indicated the contribution of both addition andabstraction reactions [9]. This implied that the rateconstants for addition and abstraction reactions arecomparable in the case of the Cl atom reaction withcyclic alkenes. The present results show that the rateconstant for the reaction of Cl atom with cyclohexa-diene, (4.06 ± 0.55) × 10−10 cm3 molecule−1s−1, isvery close to that of cyclohexene [9] and cyclohex-ane [3], (4.13 ± 0.68) × 10−10 and (3.26 ± 0.42)× 10−10 cm3 molecule−1s−1, respectively. This fur-ther confirms that replacing two CH2 groups with onedouble bond in these cyclic molecules has no pro-nounced effect on the rate constant of their reactionwith Cl atom. However, in cyclooctadiene, with twomore CH2 groups, there is an increase in the numberof abstractable hydrogen atoms and hence there is anincrease in the rate constant as compared to that ofcyclohexadiene.

Another reason for the very close rate constantsof the reactions of cyclohexane, cyclohexene, and 1,4-cyclohexadiene with Cl atoms could be that these reac-tions proceed with very small or almost no activationbarrier and hence could be collision controlled [10].The bimolecular rate constant for collision, k, is givenby

k = νσ (II)

where ν is the speed at which the Cl atom and the cy-cloalkene approach each other and σ is the collisioncross section in a hard sphere model, given by πd2 andwhere d is the collision diameter, 1/2 (d1 + d2), whered1 and d2 are the diameters of Cl atom and the alkene,respectively. The rate constant is expected to be pro-portional to d2/μ1/2, where μ is the reduced mass of Cland alkene. Thus, the higher rate constant in the case ofcyclooctadiene as compared to the six-membered ringscould be due to a larger molecular size. The calculatedcollision-controlled rate constant for the reaction of Clatom with cyclopentene is 2.2 × 10−10 cm3 molecule−1

s−1 and for the reactions with cyclohexane, cyclohex-ene, and cyclohexadiene, is about 18% higher, 2.6 ×10−10 cm3 molecule−1 s−1. Even though these calcu-lated values are marginally lower than the experimentalvalues [8,9 and the present work], the increase observedin the experimental value for cyclohexene as comparedto that for cyclopentene is about 15%, comparable tothe increase in the calculated collision-controlled rateconstants. However, the percentage increase calculatedin the collision-controlled rate constant for cycloocta-diene as compared to the six-membered rings is lessthan that observed. The rate constants of the reactionof Cl atoms with biogenic molecules such as α-pinene,β-pinene, and limonene [10], which have a basic struc-ture of a six-membered unsaturated ring with alkylgroups attached, are also reported to be high, >5 ×10−10 cm3 molecule−1 s−1, as in the case of cyclo-heptene and cyclooctadiene. Here also, the percentageincrease in the rate constant as compared to cyclohex-ene or cyclohexadiene is higher than that expected forcollision-controlled rate constants.

An approximate linear correlation has been ob-served between the rate constants of reactions with OHand Cl atoms for many unsaturated and saturated hy-drocarbons, including many halocarbons [14–16] andchlorofluoroesters [17], with different slopes for satu-rated and unsaturated compounds. Such a linear cor-relation indicates similarity of the mechanisms of thereaction of these hydrocarbons with the OH radicaland Cl atom. Since addition as well as abstraction isinvolved in the reactions of the unsaturated molecules,the parameters are different from those of the saturated



1E−10 1E−91E−13

1E−12

1E−11

1E−10k O

H /

cm3 m

olec

ule−1

s−1

kCl

/ cm3 molecule−1 s−1

Figure 6 Correlation plots between the rate constants forthe reactions of Cl and OH radicals with unsaturated hy-drocarbons at room temperature. - - -: For molecules andrate constants taken from [16] shown as � (C2H4, C2H3Cl,1,1-C2H2Cl2, cis-C2H2Cl2, trans-C2H2Cl2, C2Cl4, C3H6,1-C4H8, and 1- C5H10). —: After including the data forunsaturated cycloalkenes from the present work, [9], [10],shown as • (cyclopentene, cyclohexene, cycloheptene, 1,4-cyclohexadiene, 3-carene, α-pinenes, β-pinene, limonene,isoprene, and myrcene).

molecules. It was observed that the linearity in thecase of unsaturated molecules holds good within theerror limits, even after inclusion of the three cyclicalkenes [9]. The rate constants of reactions of OH andCl atom with unsaturated molecules, including 1,4-cyclohexadiene and biogenic unsaturated molecules[6,18] are plotted in Fig. 6. (Since the rate constantfor the reaction of the OH radical with cyclooctadieneis not reported, this molecule could not be included.) Itis observed that even in these cases, where the rate con-stants with Cl atoms are going higher than the calcu-lated collision-controlled limits, the linear correlationbetween the constants of Cl and OH is maintained. Thecorrelation obtained for unsaturated molecules consid-ered in [16] yields a straight line,

log kOH = (2.4 ± 0.3) log kCl + (12.9 ± 2.8) (III)

with a correlation coefficient of 0.95. After inclusion ofcyclopentene, cyclohexene, cycloheptene, cyclohexa-diene, and biogenic molecules reported in [10], this ismodified marginally to

log kOH = (2.1 ± 0.1) log kCl + (9.6 ± 1.2) (IV)

with a correlation coefficient of 0.97.In all the cyclic alkenes, the measured relative rate

ratios of the sample and standard molecules, which givethe ratios of the rate constants, are found to increasemarginally in the presence of air [9]. In the case ofcyclohexadiene, where two double bonds are present,

the increase observed in air is more clearly seen. Incyclooctadiene, considering the large error bars, theincrease in air can be considered marginal (Table I),similar to cyclopentene, cyclohexene, and cyclohep-tene. Earlier studies on the reactions of Cl atoms withacyclic alkenes have shown that the observed rate con-stant ratios are identical, within experimental error, inair and nitrogen [12,19]. In the case of the reactionsof Cl atom with biogenic molecules such as limonene,3-carene, β-pinene, and myrcene the relative rate ra-tios were found to be marginally higher in the presenceof air but were still considered as identical within ex-perimental errors except in the case of myrcene [10].However, as seen from Fig. 3, our study shows a sys-tematic increase in the observed relative rate ratio forthe reaction of Cl atom with cyclohexadiene, with in-creasing concentration of oxygen. Very recently, dur-ing the measurement of the rate constant of the Cl atomreaction with methacrolein with reference to propane,addition of 2–20% oxygen was found to increase therelative ratio by a statistically significant amount [20].Such difference in the rate constant in the presence ofair was suggested to be due to interference of the OHreaction, generated during the reactions of peroxy radi-cals with HO2, as observed in the Cl-initiated oxidationof acetaldehyde [21,22]. Similar reactions may also beresponsible for the marginal increase observed in thepresent study. Detailed understanding of the reactionmechanism, based on the product studies, can probablyconfirm the occurrence of such reactions. The presentstudy, where benzene and phenol only could be con-firmed as products, is not able to clearly explain theoxygen dependence of the relative rates.

The formation of benzene can be understood by con-sidering the probable reaction scheme as given below.The reaction of Cl atom with cyclohexadiene leads tocyclohexadienyl radical as well as chlorocyclohexenylradical. The cyclohexadienyl radical gives benzene bydisproportionation reaction (reaction (3)) in the ab-sence of oxygen [23,24]. In the presence of oxygen,reactions (4) and (5) are possible, of which formationof benzene along with a HO2 radical, by reaction (4),is identified as the dominant channel [13,25,26]:

H

+ HCl+ Cl(1)

+ Cl

Cl

(2)


438 SHARMA ET AL.

H

+

H

+(3)

H

+ HO2+ O2(4)

H

+ O2

O2H

(5)

In the case of the reaction of OH with cyclohexa-diene, which also leads to cyclohexadienyl radical,a yield of 15% is reported for benzene, independentof the partial pressure of oxygen varying from 5 to760 Torr, by Ohta [26], and 12.5%, by Tuazon et al[27]. Thus, considering that cyclohexadienyl radical isquantitatively converted to benzene and that additionreaction does not lead to benzene formation, the per-centage contribution of the abstraction reaction to thetotal reaction of OH radicals with 1,4-cyclohexadienewas considered to be around 15% [25]. In the presentstudy of the Cl atom reaction, the yield of benzene is30%, higher than that for the OH radical reaction. Con-sidering this as the yield of cyclohexadienyl radical,percentage contribution toward hydrogen abstractionis 30% in this case. Our measured total rate constantfor the reaction of Cl atom with 1,4-cyclohexadiene,(4.06 ± 0.55) × 10−10 cm3 molecule−1 s−1, consists ofabstraction of one of the four allylic hydrogen atoms(two CH2 groups), along with addition to either of thetwo double bonds. Thus, the rate constant of the hy-drogen abstraction reaction from each allylic group inthe symmetric 1,4-cyclohexadiene is calculated to be(6.1 ± 1.2) × 10−11 cm3 molecule−1 s−1. In a simi-lar manner, the rate constant of the addition reactionto each double bond is calculated to be (1.4 ± 0.3)× 10−10 cm3 molecule−1 s−1. The rate constant forallylic abstraction is higher, whereas that for additionis considerably lower, than those estimated for sec-ondary allylic hydrogen abstraction (∼ 4 × 10−11) andaddition (∼ 3 × 10−10 ) in simple noncyclic alkenes[19,28]. From the rate constant of reaction of Cl atomwith cyclohexane [3], the rate constant of the hydro-gen abstraction reaction from nonallylic CH2 groupscan be estimated to be 5.4 × 10−11 cm3 molecule−1 s−1,very similar to that of the allylic hydrogen abstractionreaction, calculated above. These values confirm thatreplacing two CH2 groups by CH CH changes therate constant values only marginally. In our previous

study [9], considering the increase in the rate constantof cyclopentene, cyclohexene, and cycloheptene to bedue to the increase in the number of abstractable hy-drogen atoms, we had estimated the rate constant forthe reaction of Cl atom per CH2 group to be (8.7 ±1.6) × 10−11 cm3 molecule−1 s−1. This approximateestimation, which does not differentiate between allyland nonallyl CH2 groups, is close to that estimated nowfor allyl CH2 groups in cyclohexadiene.

The reason for the predominance of reaction (4), ascompared to the formation of a peroxy radical (reaction(5)), normally observed in other cases, is the formationof a stable aromatic ring. In the case of molecules likecyclooctadiene, formation of the peroxy radical and itssubsequent reactions are expected to be more important[27]. Hence, it is possible that subsequent reactions ofHO2 radicals, formed significantly in cyclohexadieneonly, are responsible for the dependence of the ob-served relative rate ratio on the oxygen concentration,which is also observed to be more significant in cyclo-hexadiene. However, as mentioned earlier, the limiteddata do not lead to a clear understanding of this oxygendependence. It was also difficult to assign the reactionsresponsible for the formation of phenol, due to a lackof quantitative data.

Atmospheric Lifetimes

The main sink of these cyclic dienes is expected to bereactions with tropospheric oxidants, such as ozone,OH radical, and NO3, because these are sparingly sol-uble in water and do not absorb considerably at wave-lengths above 320 nm, as would be required for pho-tolysis in the troposphere. The reactions of Cl atomare not very important in ambient conditions becausethe upper limit for the chlorine atom concentration isas low as 103 atoms cm−3 in the northern hemisphere[29]. However, the peak concentration during sunrisein the marine boundary layer is reported [4] to be ashigh as 1.3 × 105 atoms cm−3 and the concentrationin the polluted noncoastal areas is also expected to behigh. To understand the contribution of the Cl atomreaction in the atmospheric oxidation of cyclic dienes,tropospheric lifetimes (τX) of these cyclic dienes withrespect to removal by species X, which can be OH, O3,NO3, and Cl, were estimated from the respective rateconstants, kX, as

τ = 1

kX[X](V)

where [X] is the concentration of the above reactivespecies at ambient conditions or marine boundary layerconditions. The calculated values are shown in Table II.



Table II Tropospheric Lifetimes (τ ) Calculated for the Cyclodienes with Respect to Reactions with Different Species

1,4-Cyclohexadiene 1,5-Cyclooctadiene

kCl (cm3 molecule−1 s−1) (present work) 4.06 × 10−10 6.9 × 10−10

τCl (ambient conditions) 11.4 days 6.7 daysτCl (marine boundary layer) 5.3 h 3.1 hkOH (cm3 molecule−1 s−1) [7] 9.9 × 10−11 Not availableτOH 2.8 h –kNO3 (cm3 molecule−1 s−1) [7] 6.6 × 10−13 Not availableτNO3 1.7 h –kO3 (cm3 molecule−1 s−1) [7] 4.6 × 10−17 1.4 × 10−16

τO3 8.6 h 2.8 h

The ambient concentrations used for Cl [31], OH, NO3, and O3 [32] are 2.5 × 103, 1 × 106, 2.5 × 108, and 7 × 1011 molecules cm−3,respectively.

The typical ambient concentrations (in units ofmolecule cm−3) used are 5 × 103, 2 × 106, 5 ×108, and 7 × 1011 for Cl [30], OH, NO3, and O3

[31], respectively. The concentrations of Cl and OH,present only in the daytime, are 12-h daytime averageconcentrations and that of NO3 is a 12-h nighttime av-erage. Hence, these are corrected by dividing by 2, toget the average 24-h concentration, whereas the con-centration of O3, given above is 24-h average and isused directly. The rate constants obtained in nitrogen,with n-hexane as the reference, were used for theseestimations. From Table II, it can be seen that in thecase of 1,4-cyclohexadiene, removal by NO3 radical isvery efficient in the nighttime. The daytime removal byOH is comparable with that by O3 for this molecule,because the rate coefficient of reaction of ozone withthis molecule is slower than that with other cyclic di-enes [7]. In marine boundary layer conditions, whereconcentration of Cl atom is 50 times higher than thatin ambient conditions, the reaction with Cl atoms be-comes competitive with the reactions with OH andozone, for both cyclohexadiene and cyclooctadiene.

CONCLUSION

The rate constants for the reactions of Cl atoms with1,4-cyclohexadiene and 1,5-cyclooctadiene have beendetermined at room temperature, 298 ± 2 K, using therelative rate method. The rate constants determined innitrogen are (4.06 ± 0.55) × 10−10 and (6.90 ± 1.33)× 10−10 cm3 molecule−1 s−1 with respect to n-hexaneand (4.82 ± 0.80) × 10−10 and (7.58 ± 1.55) × 10−10

cm3 molecule−1 s−1 with respect to 1-butene. The ob-served depletion ratio of the cyclic diene to that of thereference molecule is found to increase in the presenceof oxygen, the effect being more prominent in the caseof 1,4-cyclohexadiene. Benzene, one major product in

the case of reaction of Cl atom with cyclohexadiene,is estimated quantitatively, and the yield is found to be(30 ± 4)% of the cyclohexadiene consumption. Basedon this, the rate constant for abstraction of H atom fromthe allylic group of cyclohexadiene is calculated to be(6.1 ± 1.2) × 10−11 cm3molecule−1 s−1, assumingthat cyclohexadienyl radicals formed by H atom ab-straction are quantitatively converted to benzene. Sim-ilarly, the rate constant for addition of Cl atom is esti-mated to be (1.4 ± 0.3) × 10−10 cm3 molecule−1 s−1

in 1,4-cyclohexadiene. The results also indicate thatthe reaction with Cl atom becomes a competitive chan-nel for both cyclohexadiene and cyclooctadiene, in theconditions of the marine boundary layer and pollutedurban industrial areas, especially in the case of 1,4-cyclohexadiene, which reacts slowly with ozone.

The authors are thankful to Dr. S. K. Sarkar, Head, Radiationand Photochemistry Division, and Dr. T. Mukherjee, Direc-tor, Chemistry Group, for their keen interest and encourage-ment during the course of this work. The authors thank H. D.Alwe and M. Gurav for assistance in the experiments.

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Documents

Rate constants for the gas-phase reactions of chlorine atoms with 1,4-cyclohexadiene and 1,5-cyclooctadiene at 298 K