Rate Constants for the Reaction of OH Radicals with a Series of Alkanes and

Alkenes at 299 It 2 K

ROGER ATKINSON,* SARA M. ASCHMANN, ARTHUR M. WINER, and JAMES N. PITTS, JR.

Statewide Air Pollution Research Center, Uniuersity of California, Riuerside, California 92521

Abstract

Using methyl nitrite photolysis in air as a source of hydroxyl radicals, relative rate constants for the reaction of OH radicals with a series of alkanes and alkenes have been determined at 299 f 2 K. The rate constant ratios obtained are: relative to n-hexane = 1.00, neopentane 0.135 f 0.007, n-butane 0.453 f 0.007, cyclohexane 1.32 f 0.04; relative to cyclohexane = 1.00, n-butane 0.341 f 0.002, cyclopentane 0.704 f 0.007,2,3-dimethylbutane 0.827 f 0.004, ethene 1.12 f 0.05; relative to propene = 1.00,2-methyl-2-butene 3.43 f 0.13, isoprene 3.81 f 0.17, 2,3-dimethyl-2-butene 4.28 f 0.21. These relative rate constants are placed on an absolute basis using previous absolute rate constant data and are compared and discussed with liter- ature data.

Introduction

Organic compounds emitted into the atmosphere are degraded by three major gas-phase reaction pathways, namely, photolysis, reaction with ozone and reaction with the hydroxyl radical [l-31. For the majority of organics, reaction with the OH radical is the predominant loss process in the lower atmosphere.

While rate constants for the reaction of OH radicals with a large number of organics have been determined [4], only for a few of the LC2 organics have the rate constants been accurately established by more than one absolute rate constant determination-examples being ethane [4], l,l,l-trichloro- ethane [5,6], and n-butane [4,7] among the alkanes and haloalkanes; pro- pene and 1-butene [4,8] among the alkenes; and benzene, toluene, and the xylenes [4,9,10] among the aromatics. For most of the organics studied, rate constants have been obtained either from a single absolute rate con- stant determination or from relative rate measurements, the latter typically having estimated error limits of -420% [4].

* Author to whom correspondence should be addressed.

International Journal of Chemical Kinetics, Vol. 14,507-516 (1982) 0 1982 John Wiley & Sons, Inc. CCC 0538-8066/82/050507-10$02.00

508 ATKINSON ET AL.

Thus for many organics there is a need to determine OH radical rate constants more precisely, either through additional absolute rate constant determinations (with error limits of at least &lo%), or via relative rate constant studies using a technique which is accurate to a few percent.

In this work the photolysis of methyl nitrite (CH3ONO) has been used to yield large concentrations of OH radicals, and relative rate constants have been determined for the reaction of OH radicals with a series of alkanes (neopentane, n-butane, cyclopentane, n-hexane, cyclohexane, and 2,3- dimethylbutane) and alkenes (ethene, propene, 2-methyl-2-butene, iso- prene, and 2,3-dimethyl-2-butene) at 299 f 2 K. n-Butane and propene, for which accurate OH radical rate constants are available from absolute rate constant studies [4,7,8], were included in order to place the relative rate constant data on an absolute and self-consistent basis.

Experimental

OH radicals were generated by the photolysis (A 2 290 nm) of methyl nitrite (CH30NO) in air at part-per-million concentrations (1 ppm = 2.37 X l O I 3 molec/cm3 at 299 K and 735 torr total pressure) [ll]:

CH30NO + h~ + CH30 + NO

CH30 + 0 2 - HCHO + HO2

H02 + NO + OH + NO2

In order to minimize the formation of 0 3 during these irradiations, NO was added to the reaction mixtures, which had initial concentrations of CH30NO,4-15 ppm; NO, -5 ppm; and 0.5-1.0 ppm each of the reference organic and the reactant organic(s). Ultra-zero air (Liquid Carbonic, <0.1 ppm hydrocarbons) was used as the diluent gas.

Irradiations were carried out in a -75-L FEP Teflon cylindrical reaction bag surrounded by 24 GE F15T8-BL 15-W blacklights. By switching off sets of lamps, light intensities of one third and two thirds of the maximum could be obtained. With all the lamps on, the light intensity, determined as the photolysis rate of NO2 in N2, was 0.27 min-l. In order to avoid any significant temperature ( 5 3 K) rise on irradiation, a stream of air was blown between the lamp assemblies and the Teflon reaction bag, and for all irra- diations the air temperature immediately outside the reaction bag was 299 f 2 K. Prior to irradiation, the reaction bag/lamp assembly was covered with an opaque cover to avoid any photolysis of the reactants, especially

Methyl nitrite was prepared [12] by the dropwise addition of 50% H2S04 to methanol saturated with sodium nitrite. The CH30NO produced was swept out of the reaction flask by a stream of ultra-high-purity nitrogen, passed through a trap containing saturated NaOH solution to remove any

of CH30NO.

OH RADICAL REACTIONS WITH ALKANES AND ALKENES 509

HzS04, dried by passage through a trap containing anhydrous CaC12, and collected in a trap at 196 K. The CH30NO was then degassed and vacuum distilled on a greaseless high-vacuum system and stored under vacuum at 77 K in the dark. Known amounts of the CH,ONO, NO, and the reference and reactant organics were flushed from Pyrex bulbs by a stream of ultra- zero air into the Teflon reaction bag, which was then filled with ultra-zero air.

The organic reactants were quantitatively monitored by gas chroma- tography with flame ionization detection, using a 5-ft X I/n-in. stainless steel (SS) column packed with Porapak N 8Ol100 mesh, operated at 333 K, for ethene, and a 20-ft X I/s-in. SS column with 5% DC703lC20M on 1001120 mesh AW, DMCS Chromosorb G, operated at 333 K, for CH30N0, the alkanes, and other alkenes. Cyclohexane and CH30NO were also moni- tored for the CH30NO-NO-cyclohexane-ethene irradiations on a 10-ft X

in. SS column of 10% Carbowax 600 on C-22 firebrick, operated at 348 K. NO, NO2, and NO, were monitored by chemiluminescence, and O3 was also monitored by a chemiluminescence instrument.

Results

A series of irradiations of CH30NO-NO-organic-air mixtures were car- ried out at 299 f 2 K. During these irradiations, CH30NO and the organics were observed to disappear, accompanied by NO to NO2 conversion and the appearance of expected products (e.g., methylnitrate).

In the presence of added organics, the OH radicals generated from the photolysis of CH3ONO in air react with these organics:

(1) OH + reactant - products

(2) OH + reference organic - products

and provided that the organics are consumed only by reaction with OH radicals (see later) and since dilution due to sampling is avoided by use of a collapsible Teflon reaction bag, then

(1) -d ln[reactant]ldt = kl[OH]

(11) -d ln[reference organiclldt = k2[0H]

where k1 and k2 are the OH radical rate constants for reactants (1) and (2), respectively. Hence

(111) d ln[reactant]ldt = kJk2 d ln[reference organiclldt

and

(IV)

510 ATKINSON ET AL.

where [reactantJt,, [reference organicIt, are the concentrations of the reactant and the reference organic, respectively, at time to , and [reactantJt, [reference organicIt are the corresponding concentrations at time t. Thus plots of ln([rea~tant]~,/[reactant]~) against In([reference organicIt,/[ref- erence organicJt) should yield straight lines with a zero intercept and a slope

Because of gas-chromatographic interferences, differing reference or- ganics were used in the various experiments. The following series of or- ganics were used in the CH30NO-NO-organic-air irradiations: n-hexane + neopentane; n-hexane + n-butane; n-hexane + cyclohexane; cyclohexane + n-butane; cyclohexane + cyclopentane; cyclohexane + 2,3-dimethyl- butane; cyclohexane + ethene; and propene + 2-methyl-2-butene + iso- prene + 2,3-dimethyl-2-butene. For all but the cyclohexane + n-butane system, duplicate irradiations were carried out.

For the alkanes, irradiations were carried out at the maximum light in- tensity, and two or three data points were taken during each of the 30-min irradiations. As expected from the rapid photolysis rate of CH30NO [la], the OH radical concentrations during these irradiations (derived from the reference organic disappearance rates) decreased from initial values of 2 2 X 108 molec/cm3 to r r 3 X lo7 molec/cm3 after 30-min irradiation [13]. Ozone concentrations at the end of the irradiations were generally 50.1 ppm, though in two of these irradiations 0 3 concentrations attained 1.0 ppm. However, since the alkanes react exceedingly slowly with 0 3 [l-41, at these OH radical concentrations reaction with O3 can be calculated to be negligible under all conditions. Similarly, from the NO2 concentrations observed and the light intensity, reaction of the alkanes with O(3P) atoms [ 141 can be shown to contribute 5 1% of the alkane loss rates due to reaction with the OH radical. Hence the observed disappearance of the alkanes during the irradiations is solely due to reaction with the OH radical. Plots of eq. (IV) are shown in Figures 1 and 2, and the rate constant ratios k l lk2 obtained by least-squares analyses of these plots are given in Table I.

Since the alkenes react with 0 3 at significant rates [1-41, O3 formation was minimized by carrying out the irradiations with one-third light in- tensity and with reduced (-4 ppm) initial concentrations of CH30NO. For the cyclohexane-ethene system, observed 0 3 yields were 50.01 ppm, and at the OH radical concentrations observed, reaction with O3 and O(3P) atoms each contributed <1% of the ethene reaction rate with the OH rad- ical. The data are plotted in Figure 2, and the rate constant ratio klIk2 is given in Table I. Use of the cyclohexane data from the two gas chroma- tographic columns yielded rate constant ratios klIk2 which agreed to within 1%.

For the more reactive irradiated CH30NO-NO-propene-2-methyl-2- butene-isoprene-2,3-dimethyl-2-butene system, 0 3 formation was ob- served. Hence data were taken at irradiation times for which 0 3 formation

of kllk2.

OH RADICAL REACTIONS WITH ALKANES AND ALKENES 511

16

- e r-7 t 12 z t- 0

w a

a

a

u \ 0 0 8 f

r-Y

t- z t- o W 5 0.4

a

a

v C -

0

n - BUTANE

NEOPENTANE

0 0.4 0.8 1.2 I .6

In ( [n-HEXANE]+,/[n-HEXAN€]+ )

Figure 1. ane],) for neopentane, n-butane and cyclohexane.

Plots of In ([reactant]t,/[reactant]t) against In ([n-hexaneIt,/[n-hex-

was not observed by the chemiluminescence monitor and before the [NOz]/[NO] ratio exceeded 2 (corresponding to <7 ppb O3 as calculated from the photostationary state equation [ 151, especially since CH30NO and organic nitrates are analyzed quantitatively as NOz by chemilumi-

1.2 - - n t- 2

t- o w CK

a a 0.8 -

u \ 0 -

n t- z 2 0.4 - V

W a a

u v c -

2.3-DIMETHYLBUTANE

v- 0 0.4 0.8 1.2 I .6

In( [CYCLOHEXANEIt, / [CYCLOHEXANE] 1)

Plots of In ([reactant]t,/[reactant],) against In ([cyclohexane]t,/[cy- Figure 2. ~lohexane]~) for n-butane, cyclopentane, 2,3-dimethylbutane, and ethene.

512 ATKINSON ET AL.

TABLE I. Relative rate constant ratios k1/k2 at 299 f 2 K.

Organic kl/kza

Neopentane n-Butane Cyclopentane n-Hexane Cyclohexane 2,3-Dimethylbutane Ethene Propene 2-Methyl-2-butene Isoprene 2.3-Dimethvl-2-butene

0.135 f 0.007 0.453 f 0.007

1.00 1.32 f 0.04 1 .oo

0.341 f 0.002 0.704 f 0.007

0.827 f 0.004 1.12 f 0.05

1.00 3.43 f 0.13 3.81 f 0.17 4.28 f 0.21

a Indicated errors are two standard deviations of the slopes of the plots shown in Figures 1-3.

nescent NO-NOz-NO, analyzers [16]). Under these conditions, reaction with 03 can be calculated to contribute <1% of the OH radical reaction rate for propene and isoprene, <5% for 2-methyl-2-butene, and <11% for 2,3- dimethyl-2-butene [17-191, while reactions with O(3P) atoms [20-221 contribute 55% to the observed alkene disappearance rates.

The rate constant data obtained under these conditions are shown in Figure 3, and the rate constant ratios are given in Table I. Data obtained

I n ( [PROPENE]to/[PROPENE]t)

Figure 3. peneIt) for 2-methyl-2-butene, isoprene, and 2,3-dimethyl-2-butene.

Plots of In ([rea~tant]t,/[reactant]~) against In ([pr~pene]~,/[pro-

OH RADICAL REACTIONS WITH ALKANES AND ALKENES 513

when the [NOz]/[NO] ratio was >5 yielded rate constant ratios for 2,3- dimethyl-2-butene and 2-methyl-2-butene which were 12% and 7% higher than the values obtained when [NO2]/[NO] <2 (Table I), confirming, as expected, that the effects of O(3P) atom and O3 reactions were minor under these conditions (<lo% of the OH radical reaction rates).

Discussion

It is evident from Figures 1-3, which show excellent agreement between data from duplicate irradiations, that the present experimental technique is extremely reproducible. Furthermore the excellent agreement between analyses on differing gas-chromatographic columns and the excellent agreement between k(OH + n-butane)/k(OH + cyclohexane) = 0.341 f 0.002 determined directly and the value of 0.343 f 0.012 derived from separate measurements of k(OH + n-butane)/k(OH + n-hexane) and k(OH + cyclohexane)/k(OH + n-hexane) indicate that this technique is also capable of considerable accuracy.

The rate constant ratios k l lk2 given in Table I can be placed on an ab- solute basis by using literature rate constants k2 which are accurately known. Of the alkanes and alkenes studied here, only for n-butane and propene have absolute rate constants been determined by more than two studies which appear to be free of complicating effects [4,7,8,23-271. Using the mean of these absolute rate constants at room temperature for n-butane [7,23-251 and propene [8,26,27], the rate constant ratios k l / k 2 given in Table I have been placed on an absolute basis, as shown in Table 11. Also given in Table I1 are previous literature rate constants obtained from both ab- solute and relative rate measurements.

It can be seen that, apart from the significantly low rate constant for 2,3-dimethyl-2-butene obtained by Ravishankara et al. [27] and to a much lesser extent, the somewhat high value for 2,3-dimethylbutane [23] (both of which have been discussed previously [4,28]), the present data (and the rate constant ratio of k(OH + propene)/k(OH + n-butane) = 10.4 f 1.0 [13] obtained with the present technique) are in excellent agreement with the absolute literature values. Furthermore the present data are in rea- sonable agreement with the relative rate constant measurements reported in the literature, within the generally wider error limits. In particular, the present rate constants for 2-methyl-2-butene and 2,3-dimethyl-2-butene are in agreement, within the experimental errors, with the previous data of Atkinson et al. [40], obtained from irradiations of NOX-cis-2-butene- isobutene-2-methyl-2-butene-2,3-dimethyl-2-butene-air mixtures.

From the present data and the previous absolute (and, to a lesser extent, relative) rate constants given in the literature, it is obvious that a self- consistent set of OH radical rate constants at room temperature is available. Thus at 298 K it is now evident that the rate constants for the reactions of

514 ATKINSON ET AL.

TABLE 11. alkenes at room temperature.

Rate constants k for the reaction of OH radicals with a series of alkanes and

lo1* X k (cm3/molec.s) ~~~~~~ ~

Literature Absolute Relative Rate

Organic This worka Measurements Measurementsb

Neopentane 0.77 f 0.05 0.825 [23], 0.98 f 0.16 (281 0.91 f 0.10 [7]

n-Butane (2.58Ic Cyclopentane 5.33 f 0.07 6.1 [29], 4.46 f 0.27 [28] n-Hexane 5.70 f 0.09 5.6 f 1.1 [30], 5.8 [31],

Cyclohexane 7.52 f 0.26 7.95 [23] 6.7 f 1.5 [33], 6.2 [32]

2,3-Dimethyl- 6.26 f 0.06 7.45 [23] 4.8 f 1.0 [34], 4.3 f 1.4 [35],

Ethene 8.48 f 0.39 7.85 f 0.7ge [36] 7.7 f 1.6 [30]

Propene (25.2)f 2-Methyl- 86.4 f 3.3 78 f 8 [38], 119 [39], 87 f 6 [40]

Isoprene 96.0 f 4.3 93 f 9 [41] 799 [42] 2,3-Dimethyl-2- 108 f 6 110 f 22 [43], 153 [39], 122 f 8 [40]

5.7 [32]

7.57 f 0.05d

butane 5.36 f 0.28 [28]

10.0 f 1.7e [37]

2-butene 87.3 f 8.8 [22]

butene 56.9 f 1.3 [27]

a A t 299 f 2 K. Indicated errors are two standard deviations. Taken from [4], which should be consulted for details. The data of [28,30,31], which are

relative to n-butane, have been adjusted to be consistent with the presently used OH + n- butane rate constant.

c Mean of (X10l2 cm3/molec-s): 2.57 (231, 2.35 [24], 2.72 [25], and 2.67 [7]. The two values arise from the differing reference organics; see Table I.

e High-pressure limiting values. Mean of (X10l2 cm3/molec-s): 25.1 [26], 26.0 (271, and 24.6 [8] .

g Relative to ethene, using k(OH + ethene) = 8.5 X cm3/molec s, as obtained in this work.

OH radicals with several alkanes and alkenes are well defined to be: n- butane, 2.6 X cm3/molec-s; ethene, 8.5 X cm3/molec-s; propene, (2.5-2.6) X cm3/molec-s; and 2-methyl-2-butene, 8.7 X lo-" cm3/ molec-s, and that these rate constants will serve as a basis for the deter- mination of further sets of self-consistent and accurate data.

Acknowledgment

The authors wish to thank Dr. W. P. L. Carter for helpful discussions. This work was supported by the U.S. Environmental Protection Agency under EPA Grant R-806661-01.

OH RADICAL REACTIONS WITH ALKANES AND ALKENES 515

Bibliography

[l] B. J . Finlayson-Pitts and J . N. Pitts, Jr., Adu. Enuiron. Sci. Technol., 7,75 (1977). [2] J . T. Herron, R. E. Huie, and J. A. Hodgeson, Eds. “Chemical Kinetic Data Needs for

Modeling the Lower Troposphere,” National Bureau of Standards, Spec. Publ. 557, Aug. 1979.

[3] D. G. Hendry and R. A. Kenley, “Atmospheric Reaction Products of Organic Com- pounds,” EPA-560/12-79-001, June 1979.

[4] R. Atkinson, K. R. Darnall, A. C. Lloyd, A.M. Winer, and J. N. Pitts, Jr., Adu. Photo- chem., 11,375 (1979).

[5] K-M. Jeong and F. Kaufman, Geophys. Res. Lett . , 6,757 (1979). [6] M. J . Kurylo, P. C. Anderson, and 0. Klais, Geophys. Res. Lett., 6,760 (1979). [7] G. Paraskevopoulos and W. S. Nip, Can. J . Chem., 58,2146 (1980). [8] W. S. Nip and G. Paraskevopoulos, J . Chem. Phys., 71,2170 (1979). [9] F. P. Tully, A. R. Ravishankara, R. L. Thompson, 3. M. Nicovich, R. C. Shah, N. M.

Kreutter, and P. H. Wine, J. Phys. Chem., 85,2262 (1981). [lo] J. M. Nicovich, R. L. Thompson, and A. R. Ravishankara, J. Phys. Chem., 85, 2913

(1981). [ 111 R. Atkinson and A. C. Lloyd, “Evaluation of Kinetic and Mechanistic Data for Modeling

of Photochemical Smog,” Final Rep. to EPA Contract 68-02- 3280,1980; ERT Doc. P- A040, Westlake Village, CA, J. Phys. Chem. Ref. Data, in press (1982).

[I21 W. D. Taylor, T. D. Allston, M. J . Moscato, G. B. Fazekas, R. Kozlowski, and G. A. Takacs, Int. J . Chem. Kinet., 12,231 (1980).

[13] R. Atkinson, W. P. L. Carter, A. M. Winer, and J . N. Pitts, Jr., J. Air Pollut. Contr. Assoc., 31,1090 (1981).

[14] J. T. Herron and R. E. Huie, J . Phys. Chem. Ref. Data, 2,467 (1973). (151 T. J. Kelley, D. H. Stedman, J. A. Ritter, and R. B. Harvey, J. Geophys. Res., 85,7417

[I61 A. M. Winer, J. W. Peters, J. P. Smith, and J. N. Pitts, Jr., Enuiron. Sci. Technol., 8,1118

1171 S. M. Japar, C. H. Wu, and H. Niki, J. Phys. Chem., 78,2318 (1974). [18] R. E. Huie and J. T. Herron, Int . J . Chem. Kinet., S1,165 (1975). [19] R. Atkinson, A. M. Winer, and J . N. Pitts, Jr., Atmos. Enuiron., in press (1982). [20] D. L. Singleton, S. Furuyama, R. J. Cvetanovic, and R. S. Irwin, J. Chem. Phys., 63,1003

[21] R. Atkinson and J . N. Pitts, Jr., J. Chem. Phys., 67,38 (1977). [22] R. Atkinson and J. N. Pitts, Jr., J. Chem. Phys., 68,2992 (1978). [23] N. R. Greiner, J . Chem. Phys., 53,1070 (1970). [24] F. Stuhl, 2. Naturforsch., 28A, 1383 (1973). [25] R. A. Perry, R. Atkinson, and J . N. Pitts, Jr., J. Chem. Phys., 64,5314 (1976). [26] R. Atkinson and J. N. Pitts, Jr., J . Chem. Phys., 63,3591 (1975). 1271 A. R. Ravishankara, S. Wagner, S. Fischer, G. Smith, R. Schiff, R. T. Watson, G. Tesi,

I281 K. R. Darnall, R. Atkinson, and J. N. Pitts, Jr., J. Phys. Chem., 82,1581 (1978). [29] D. H. Volman, Znt. J . Chem. Kinet., S1,358 (1975). [30] A. C. Lloyd, K. R. Darnall, A. M. Winer, and J . N. Pitts, Jr., J. Phys. Chem., 80, 789

[31] I. M. Campbell, D. F. McLaughlin, and B. J. Handy, Chem. Phys. Lett., 38, 362

[32] C. H. Wu, S. M. Japar, and H. Niki, J . Enuiron. Sci. Health, All , 191 (1976). [33] R. A. Gorse and D. H. Volman, J. Photochem., 3,115 (1974). [34] K. R. Darnall, A. M. Winer, A. C. Lloyd, and J. N. Pitts, Jr., Chem. Phys. Lett., 44,415

(1980).

(1974).

(1975).

and D. D. Davis, Int. J . Chem. Kinet., 10,783 (1978).

(1976).

(1976).

(1976).

516 ATKINSON ET AL.

[35] R. Atkinson, K. R. Darnall, A. M. Winer, and J . N. Pitts, Jr., Final Rep. to E. I. duPont

[36] R. Atkinson, R. A. Perry, and J. N. Pitts, Jr., J . Chem. Phys., 66,1197 (1977). [37] R. Overend and G. Paraskevopoulos, J . Chem. Phys., 67,674 (1977). [38] R. Atkinson, R. A. Perry, and J. N. Pitts, Jr., Chem. Phys. Lett . , 38,607 (1976). [39] E. D. Morris, Jr., and H. Niki, J. Phys. Chem., 75,3640 (1971). [40] R. Atkinson, K. R. Darnall, and J. N. Pitts, Jr., unpublished data, cited in [4]. [41] G. W. Harris, T. E. Kleindienst, and J. N. Pitts, Jr., unpublished (1982). [42] R. A. Cox, R. G. Derwent, and M. R. Williams, Enuiron. Sci. Technol., 14,57 (1980). [43] R. A. Perry, Ph.D. Thesis, University of California, Riverside, Aug. 1977, cited in [4].

de Nemours and Co., Inc., Feb. 1,1976.

Received July 30,1981 Accepted September 28,1981