J. Phys. Chem. 1995, 99, 13156-13162
13156
Rate Constants for the Reactions of Chlorine Atoms with Some Simple Alkanes at 298 K: Measurement of a Self-Consistent Set Using Both Absolute and Relative Rate Methods Peter Beichert, Lisa Wingen, Jason Lee, Rainer Vogt, Michael J. Ezell, Mark Ragains, Ruaidhri Neavyn, and Barbara J. Finlayson-Pitts* Department of Chemistry, University of Califomia, Irvine, Irvine, California 92717-2025Received: February 13, 1995; In Final Form: June 21, 1995@
In this study, both relative rate and absolute kinetic measurements were made at 298 K for the reaction of chlorine atoms with the C 1-C4 alkanes. The relative rate studies were carried out using two different reaction chambers and GC to follow the loss of the organics upon reaction with atomic chlorine generated from the photolysis of Clz or ClNO. Relative rate studies were also carried out for the chloro-substituted methanes. Absolute rate constant studies employed a fast flow discharge system with resonance fluorescence detection of atomic chlorine. Our relative rate constants for the ethane/propane and ethaneln-butane pairs are approximately 15 and 20% lower respectively than recent measurements carried out on a similar series of alkanes. The absolute rate constant for propane is 20% lower than previous absolute rate measurements and the currently recommended value. However, the ratios of the absolute rate constants measured in this study agree to within 10% with our independently measured relative rate constants. Thus, these studies provide for the first time a consistent set of reaction rate constants for the reaction of C1 with the simple alkanes. While the rate constants derived for C H X l and CH2C12 are in reasonably good agreement with the literature, that for CHC1, is approximately 50% higher (but within the wide error bars) of current recommendations. The trends in the rate constants are discussed in the context of recently derived structure-activity relationships.
Introduction Ozone plays a critical role in both the chemistry and radiation balance of the troposphere. As a result, understanding the factors controlling tropospheric ozone levels is critical to our understanding of a variety of issues in global chemistry and climate change. Chlorine atoms, which may be produced in the tropospherefrom the reactions of NaCl in sea salt particles,',2 have the potential to contribute significantly to the ozone balance in both the remote and polluted troposphere. Chlorine atoms can reduce tropospheric ozone concentrations by direct reaction with ozone:
c1+ 0,
-
c10
+ 0,
(1)
Alternatively, chlorine atoms can react directly with organics in the troposphere, for example by hydrogen abstraction from alkanes: C1+ RH-
HC1
+R
(2)
In the presence of sufficient NO.,, reaction 2 followed by reactions of the alkyl radical (R) with 0 2 and NO ultimately leads to ozone formation in the tr~posphere.~ Hence, having accurate rate constants for both reactions 1 and 2 is important. Accurate rate constants for the chlorine atom-alkane reactions are also needed to probe data on the alkane composition of the atmosphere where the contribution of chlorine atoms to the decay of these organics can be t e ~ t e d . ~ . ~ The kinetics of the reactions of chlorine atoms with alkanes have been studied by a number of researchers using both absolute and relative rate technique^.^.' However, while the relative rate constant ratios for chlorine atom reactions with Author to whom correspondence should be addressed. Phone: (714) 824-7670, -5485. FAX: (7 14) 824-3168. e-mail: BJFINLAY @UCI.EDU. Abstract published in Advance ACS Ahsrructs, August 1, 1995. @
0022-365419512099- 13156$09.0010
simple alkanes reported by different laboratories are generally in excellent agreement, there are discrepancies between these and the ratios derived from the absolute rate studies. For example, relative rate constant ratios using n-butane as the reference compound have been measured for a series of simple alkanes by Atkinson and c o - w o r k e r ~and ~ ~ by ~ Wallington and co-workers.I0 The ratio of the rate constants for ethane to the n-butane reaction was measured to be 0.324 f 0.009 and 0.344 f 0.026 (errors quoted are f 2 a ) , respectively.8.'0 However, the ratio of the absolute rate constants measured by Lewis et al." is 0.244 f 0.034 (2a). In the case of chloro-substituted alkanes such as the chloromethanes, the discrepancies are even greater. These discrepancies between the ratios derived from the relative and absolute methods may in part be due to the fact that only for methane and ethane have a number of absolute rate constant measurements been made by different laboratories and using different techniques. The need for additional studies of the n-butane-chlorine atom reaction is particularly acute, since a substantial fraction of the recent relative rate studies have used n-butane as the reference compound and then converted the rate constant ratios to absolute values based on the absolute rate constant for the n-butane reaction. The single absolute rate constant measurement for the n-butane reaction was reported by Lewis et al." who used a fast flow discharge system (FFDS). They reported evidence of wall reactions on the phosphoric acid coated walls of the flow tube which they minimized by careful conditioning of the surfaces with atomic oxygen prior to each run. Given these potential complications and the need to have accurate room temperature values, particularly for the n-butane reaction, additional absolute rate measurements are clearly needed. The present study applied a combination of both absolute and relative rate techniques to the reactions of chlorine atoms with simple alkanes. Absolute rate constants were obtained 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 35, 1995 13157
Reactions of Chlorine Atoms with Some Simple Alkanes using an FFDS with a halocarbon wax coated flow tube, combined with resonance fluorescence detection of chlorine atoms. Relative rate constants were obtained using a conventional relative rate technique with two different types of reaction chambers. We show that these different approaches give a consistent set of rate constants for the reactions of chlorine atoms with the Cl-C4 alkanes. In addition, the rate constants for a series of chloromethanes measured relative to methane are presented.
11
VUV Resonance Lomp
-1%C121He
ReIwe
Gauge
Mg
F2 Window
A ,
Experimental Section Relative Rate Method. Chlorine atoms were generated directly from the photolysis of molecular chlorine: C1,
+ hv - 2C1
Monochromator
(3)
In several runs for ethane/propane in the Teflon reaction chamber, photolysis of ClNO was used as the source of atomic chlorine. ClNO
+ hv - C1+ NO
(4)
The loss of the organics by reaction with the atomic chlorine was followed by gas chromatography using a Carle gas sampling valve and a Hewlett Packard Model 5890 gas chromatograph equipped with a 2 m x 3 mm stainless steel analytical column packed with 3% SP-1500 on Carbopack-B (80-120 mesh, Supelco) and a flame ionization detector. Two types of reaction chambers and photolysis sources were employed. The first method employed a cylindrical quartz cell with a fixed volume of 200 cm3 as the reaction chamber, into which measured pressures of the two organics and molecular chlorine were introduced. The organic concentrations covered the range (1 -300) x loi4molecules ~ m - and ~ , C12 concentrations were (3-400) x lOI4 molecules ~ m - ~The . cell was then pressurized to 1 atm using ultra-high-purity air or He. Inherent in this method is a decrease in the cell pressure each time the Carle gas sampling loop is filled with a sample from the cell. The gas chromatograph peak areas were corrected for this pressure decrease by use of the ideal gas law. The light source for this reaction chamber was a Hg-Xe arc lamp (PTI Model LPS-250) equipped with a 10 cm water filter and quartz windows. In the second method, kinetic experiments were carried out at atmospheric pressure in an approximately 50 L FEP Teflon collapsible reaction chamber (Alltech Associates Inc.) The chamber was surrounded by 14 fluorescent blacklamps (Sylvania TL 20/08) which provided UV radiation in the region 300450 nm with an intensity maximum around 360 nm. The light intensity was varied by switching off various sets of lamps. Measured amounts of the reagents were flushed from Pyrex bulbs into the Teflon reaction chamber by a stream of zerograde nitrogen, air, or argon, and allowed to mix for about 30 min prior to photolysis. Because of the collapsible nature of the Teflon bag, pressure corrections were not necessary and peak areas could be used directly. The concentrations of the organics were (5-1000) x lOI4 molecules cm-3 and C12 (5-500) x lOI4 molecules cm-3. In both cases, in order to prevent prephotolysis of reactants, the reaction chamber was covered with an opaque material prior to commencing irradiation. A conventional mercury-free greaseless vacuum system was used to prepare the gaseous reactants for all experiments. Pressure measurements were carried out with a Datametrics Barocel Model 600 pressure sensor coupled to a Datametrics Model 1500 readout unit.
Figure 1. Schematic diagram of the fast flow discharge system. The organic compounds were introduced through port A; the second port, B, was not used in this study.
Absolute Rate Method. The fast flow discharge system (FFDS) with resonance fluorescence detection of chlorine atoms which was used in this study is shown in Figure 1. The flow tube of diameter 2.54 cm was coated with Halocarbon wax (Halocarbon Products, Series 1500) and was pumped continuously by a rotary pump (Leybold D25B). The temperature was controlled using an ethylene glycol-water mixture circulated through a jacket surrounding the flow tube by a Fisher Scientific Isotherm Digital Circulator (Model 91 10). The pressure inside the flow tube was measured using a Datametrics Barocel Model 600 pressure sensor (0-10 Torr) coupled to a Datametrics Model 1570 readout unit. The flow of the He carrier gas was controlled by a mass flow controller (MKS Model 1259C, calibrated 0-500 sccm He), whereas the flows of the mixtures of He with HCl and organics respectively were determined by the pressure drop in 5 L bulbs over the course of the experiment. The pressure gauge used for this purpose was a 0- 1000 Torr capacitance manometer (MKS Model 120AA). Typical operating conditions included total gas flows of approximately 340 pmol s-I at a pressure of 1 Torr and linear flow velocities of approximately 13 m s-l. The presence of molecular chlorine in the flow tube cannot be avoided if molecular chlorine is discharged directly. The discharge of molecular chlorine is irreproducible and quite inefficient; as a result, the unreacted molecular chlorine was partially photolyzed by the lamp used to generate the resonance fluorescence, leading to a significant increase in the background signal. In addition, the presence of C12 led to regeneration of atomic chlorine via the fast reaction (k5 = (2-6) x lo-" cm3 molecule-' s-I for R = C2H5 to C4H9)I2 of C12 with alkyl radicals produced in the initial reaction: C1+ RH R
+ Cl,
-
+
+R RCl + C1 HCl
(2) (5)
These problems could be minimized using concentrations of molecular chlorine below 1Olo molecules ~ m - ~ . However, in order to avoid them completely, in most experiments we used the reaction of fluorine atoms with HCl as a source of atomic chlorine:
F
+ HCl -H F + C1
(6)
Molecular fluorine could be dissociated with essentially 100% efficiency, and reaction 6 is sufficiently fast (k6 = 1.1 x lo-' I cm3 molecule-' s-I)I3 that atomic fluorine could be completely titrated to chlorine atoms.
Beichert et al.
13158 J. Phys. Chem., Vol. 99, No. 35, 1995
The chlorine atoms were detected at the end of the cell using I / 1resonance fluorescence. A microwave discharge lamp using a flow of 1% Cl2 in He at 0.5-0.6 Torr pressure with a MgF2 window was used as the excitation source. The fluorescence signal was detected at 90" to both the direction of the gas flow and to the collimated excitation beam. The C1(3p44s) ?P3/2/ (3p5) ? P v transition ~ at 135 nm was monitored with a Minuteman 0.2 m vacuum UV monochromator (Model 302 VM), an EMR / solar blind photomultiplier tube (Model 541G09-17) and a Princeton Applied Research Corp. Quantum Photometer (Model 0.2 I I/, II 1140). The photomultiplier tube was powered by an HP-65 16 DC power supply at 2700 V. The resulting data were digitized by a Keithley A/D-converter (Model DAS-802) and stored in a 16 PC-AT. In order to increase the intensity of the signal, the In( [XID1x1 l fluorescent light was focused through a MgF? lens cf= 64 mm) Figure 2. Typical kinetic plots for the relative rate experiments for onto the entrance slit of the monochromator. The fluorescence the simple alkanes in the Teflon reaction chamber. Squares (open, signal was typically of the order of 4000 Hz for a chlorine filled): [ethane] = 39, 91 ppm, [propane] = 39, 20 ppm, [Clz] = 78, 104 ppm respectively. Circles: [isobutane] = 15, 15 ppm, [n-butane] concentration of 1 x loioatoms ~ m - with ~ , a background signal = 5, 15 ppm, [Cl?] = 20, 15 ppm. Triangles: [ethane] = 15, 5 ppm, of 140 Hz. This gives a detection limit for atomic chlorine of [n-butane] = 15, I O ppm, [Cl?] = 15, 15 ppm. %5 x lo8 atoms ~ m - ~The . signal was averaged over a period of one minute for each data point. Typical initial concentrations of C1 were (0.5-5) x 1Olo atoms cm-3 and of the organic (1-10) x 10" molecules ~ m - ~ . C H K h CHI Absolute kinetic studies of the reaction of CHC13 with atomic CHClr CHI chlorine were also attempted. However, the signal dropped precipitously on introduction of the chloroform and did not recover on stopping the flow of the organic. It is likely that the chloroform effectively dissolved in the halocarbon wax coating, leading to efficient removal of the chlorine atoms at the walls. As a result, studies of chlorinated methanes were only carried out using the relative rate technique. Materials. The materials used and their stated purities and suppliers are as follows: ln([Xl~'[Xl,) Relative Rate Method. Chlorine (99.5%), methane (99.99%), Figure 3. Typical kinetic plots for the relative rate experiments for ethane (99.0%), propane (99.0%), butane (99.5%), and isobutane the chloromethanes in the Teflon reaction chamber. Squares (open, (99.5%) were supplied by Matheson Gas Products and methyl filled): [CH,CI] = 269, 17 ppm, [CH?] = 280, 17 ppm, [Cl?] = 305, chloride (99.5%), dichloromethane (99.9%), and tric hlo20 ppm respectively. Circles: [CH?CI?]= 40, 120 ppm, [CH?] = 40, romethane (99.0%) by Aldrich. All were used as received. Only 40 ppm, [CL?] = 38, 38 ppm. Triangles: [CHClJ = 244, 38 ppm, [CHJ] = 270, 38 ppm, [Cl?] = 292, 19 ppm. the methyl chloride had significant concentrations of impurities detectable by GC-FID; in this case propane (identified by GCProvided that reaction with C1 atoms is the only significant loss MS) was present at approximately 6%, well above the manuprocess for both reactants, it can be shown that facturer's stated total impurity levels. The diluent gases nitrogen, air, and argon had stated purities of greater than 99.999% and were supplied by Spectra Gases. The helium (ultra high purity, 99.999%) was supplied by Union Carbide. The subscripts 0 and t indicate concentrations at the beginning Absolute Rate Method. Helium (99.9999%) was supplied by of the experiment and at time t, respectively. Thus, a plot of Air Liquide and HC1(99.99%) by Scott-Marin Specialty Gases; ln([X]d[X],) vs ln([YJd[Yl,) should be linear with a slope equal methane, ethane (containing 34 ppm C2H4 according to the to the relative rate constant ratio kxlky. supplier), and isobutane (all 99.99%) by Matheson Gas Products; Typical plots in the form of eq I are shown in Figures 2 and propane and n-butane (99.99%) by AGA Specialty Gases. The 3 for experiments involving the simple alkanes and the gas mixtures for the microwave discharge were supplied by chloromethanes respectively. The rate constant ratios, kxlky, Spectra Gases (1% Clz in He) and Matheson Gas Products (1% obtained from such plots are given in Table 1, along with the F2 in He) and were used as received. number of runs carried out in each case. There was no systematic variation in the relative rate constants with the Results concentration of the reactants used, the ratio of the reactants, the light intensity, or the photolysis time. In addition, the two Relative Rate Method. The relative rate technique is based runs for ethane/propane in which ClNO was used as the source on the competition between two reactants for reaction with one of the chlorine atoms were in excellent agreement with those reactive species. For two organics, X and Y, reacting with using Cl2. chlorine atoms, their respective reactions can be written as The standard deviations associated with the experiments in follows: the Teflon chamber were smaller than those for the experiments in the quartz cell; this may be due to the need to measure kX X C1- products (7) accurately and correct for the pressure drop during the run in the fixed volume quartz cell. In any case, because of the lower k, Y C1- products (8) standard deviations in the Teflon reaction chamber, these are I )
+ +
J. Phys. Chem., Vol. 99,No. 35, 1995 13159
Reactions of Chlorine Atoms with Some Simple Alkanes TABLE 1: Summary of Relative Rate Constant Measurements in This Study relative rate constantsa reactant pair
quartz cell
Teflon reaction chamber
weighted average ( 2 ~k,lk, ) ~
ethane/propane ethaneh-butane isobutaneh-butane CHdl/CH4 CH2Cl*/CHd CHC13ICHj
0.442 f 0.035 (20)' 0.279 k 0.057 (21) 0.660 f 0.064 (7) n.d.d n.d. n.d.
0.395 f 0.012 (7) 0.259 f 0.02 1 (6) 0.693 f 0.041 (7) 4.65 =k 0.57 3.45 f 0.18 1.13 f 0.07
0.409 f 0.043 0.261 f 0.013 0.683 f 0.030
fl Errors shown are 2 standard deviations of the least-squares slope. Weighted average of runs in quartz cell and Teflon reaction chamber using weighting factors w = 1 h z . Numbers in parentheses are the number of runs. n.d. = not determined; see text.
TABLE 2: Average Relative Rate Constant Ratios Obtained in This Work and Comparison to Literature Values from Other Relative Rate Studiee reactant pair ethane/propane
ethaneh-butane isobutane/n-butane CH3CVmethane
CHzClz/methane CHClJmethane
rate constant ratio, kxlky
reference
0.390 f 0.14 0.476 f 0.022b 0.484 0.039b 0.411 f 0.015 0.409 f 0.043" 0.324 f 0.009 0.344 f 0.026 0.261 f 0.013" 0.695 f 0.009 0.671 f 0.042 0.683 f 0.030" 5.1 =k 1.3 5.14 f 0.72 4.9 & 0.3 4.79 f 0.39 4.65 f 0.57 4.38 f 0.52 3.51 f 0.32 3.45 f 0.18 0.68 f 0.07 1.20 =k 0.10 1.13 f 0.07
Pritchard et aI.l4 Atkinson et aI.* Wallington et a1.I" Donaghy et aI.I5 this work Atkinson et a1.* Wallington et a1.I" this work Atkinson et a1.* Wallington et a1.I" this work Pritchard et al.I4 Tschuikow-Roux et al.I6 Hewitt et a1.I' Wallington et a1.18 this work Knox!g.?I Tschuikow-Roux et al.?" this work Knox19.21 Hewitt et al." this work
Weighted averages from Table 1; errors shown are 2 standard deviations. Calculated from the reported ratios of ethaneln-butane and propaneh-butane.
weighted more heavily in the final results shown in Table 1, and runs for the chlorinated methanes were only carried out in this apparatus. Table 2 summarizes the results of our studies, along with literature ratios obtained from previous relative rate studies. The relative rate technique relies on the assumption that both reactants are removed solely by reaction with C1 atoms. Prior to any kinetic experiments being carried out, tests were performed to verify this assumption. First, mixtures of the organic species and molecular chlorine were prepared and allowed to stand in the dark for several hours. In all cases, the reaction of the organic species with molecular chlorine was negligible in the absence of photolysis over the typical time periods used in this work. Second, in order to test for a possible (but unexpected) photolysis of the organic reactants in the absence of molecular chlorine and for other unrecognized reactions, e.g., on the walls, mixtures of each organic were irradiated in the absence of Clz for periods of greater than 30 min. No photolysis reaction of any of the reactants was observed. Finally, a further potential complication following irradiation of the gas mixtures was interference in the analysis of the reactants by products formed in the oxidation of either of the organics used. Products having retention times which are indistinguishable from those of the organics under study will obviously interfere with the gas chromatographic analysis. To ascertain if there were any such interferences in our systems,
separate experiments were carried out in which mixtures of molecular chlorine or ClNO and each organic were irradiated, and a search carried out for products with the same retention time as the other organic. No interferences were observed for the irradiation times and gas chromatographic conditions employed in this work. Absolute Rate Method. The kinetics were determined by following the decay of atomic chlorine by resonance fluorescence in the presence of a great excess of the organic. In the configuration used in these studies, the chlorine atoms were formed at a fixed point at the upstream end of the flow tube. If the loss of atomic chlorine is first order with rate constant k, in the absence of the organic, and if this changes to k,' upon addition of the organic (but is independent of the organic concentration), the chlorine atom decay rates can be described by eq I1 ln([C1]/[C1]O)= -(k,[RH]
+ Ak,,,)t= -k't
(11)
where [Cl] is the atomic chlorine concentration measured at time t, [Cllo is that measured in the absence of the organic, k2 is the bimolecular rate constant for the reaction 2 of C1 with the organic RH, and Ak, = (k,'- k,) is the change in the rate constant for the loss of C1 atoms at the walls of the flow tube upon the addition of RH. The resonance fluorescence intensity, I , is proportional to the concentration of atomic chlorine; this was c o n f i i e d in separate experiments by measuring the resonance fluorescence intensity at various concentrations of atomic chlorine from 1 x 1O'O to 70 x 1O'O atoms cm-3 which were determined independently by titration with ClNO. A plot of In IN0 vs reaction time should be linear with slope given by k' = (k2[RH] Ak,). Note that [RH] = [RHIOis constant, since the organic is present in great excess. Thus k' plotted against the initial concentration of the organic should be a straight line with slope equal to the rate constant of interest, k2, and intercept, Ak,. Figure 4 shows typical first order decay plots for chlorine atoms in the presence of n-CdHlo. As expected, the decay of chlorine atoms follows eq 11; the small nonzero intercept is believed to be due to incomplete mixing around the addition port for the organic. Figure 5 shows a plot of the pseudo-firstorder rate constants (k') as a function of the concentrations of the organics. All are linear with intercepts within experimental error of zero, indicating that the change in the wall loss of C1 atoms upon addition of the organic, Le., Ak,, is negligible. The first-order rate constants obtained from plots such as shown in Figure 4 were corrected for axial diffusion usingz2
+
where k' is the observed first order rate constant, D is the diffusion coefficient for chlorine atoms in He,'' and v is the linear flow velocity. The corrections were less than 2.5% and
Beichert et al.
13160 J. Phys. Chem., Vol. 99, No. 35,1995 0, .
. . . .
.
.
.
,
.
.
.
.
,
.
.
.
.
.
.7
-0.5
-1
-a 1 -
-
-1.5
-2
-2.5
-3
20
10
0
30
40
time [ms]
Figure 4. Typical first-order decay of chlorine atoms in the fast flow discharge experiments. [Cllo = 1.5 x 1O'O cm-3 at 298 K and a total pressure of 0.96 Torr in He carrier gas.
70
t
/
60 50 40 30
20
10
0 0
1
2
3
4
5
6
7
8
9
10
[Alkane] (x 10" molecule
Figure 5. Typical variation of the pseudo-first-order rate constant (k') with the concentration of the simple alkanes (X = 13 for CHI, X = 11 for the other alkanes) for the fast flow discharge experiments.
were more typically 1%. No corrections were made for radial diffusion since these were insignificant. Table 3 summarizes the experimental conditions used and the rate constants obtained for the C1 -C4 alkanes, along with previously reported values from studies in which absolute rate constants were determined.
Discussion Table 4 compares the relative rate constants measured in this study to the ratios of our absolute rate constants for the simple alkanes. The agreement between the two is excellent, within 10% for ethane/propane and within 3% for the other two pairs. Thus a major result of these studies is a consistent set of room temperature rate constants for the reaction of atomic chlorine with the simple alkanes which can be used with some confidence to convert relative rate constants from studies in other laboratories into absolute values. Our relative rate measurements are in excellent agreement with the recent measurements of Atkinson and co-workerss and Wallington et al.'O for isobutaneln-butane, Our ratio for the ethane/propane pair is approximately 15% lower than reported in those studies, although the error bounds just overlap. However, our lower value for this pair is in excellent agreement with the recent work of Donaghy et al.I5 For the ethaneln-butane pair, our value is approximately 20% lower than the earlier studiess.l0and outside the stated experimental errors, even though these earlier studies are in excellent agreement with each other. The reasons for the discrepancies between the present and the earlier work are not clear. The results in our two different reaction chambers are within
experimental error of each other, ruling out a systematic error in our systems due to different concentration ranges or to unrecognized interferences from wall reactions. Furthermore, our relative rate constants are in excellent agreement with the ratio of the absolute rate constants determined in this study as well as in the work of Lewis et a1.l1 Numerous studies have been carried out to determine the absolute rate constants at room temperature for the reactions of atomic chlorine with methane and ethane. These are reviewed in the JPL6 and IUPAC evaluations,' and the recommendations of these two panels, along with more recent studies, are shown in Table 3 for comparison to the present work. Our results are within experimental error of both sets of recommendations, and for ethane, in good agreement with the recent measurements of Stickel et al.23 The upper error bounds of our value for ethane just overlap the lower bound of the value of Kaiser et al.,24 who measured the rate constant by following the production of HC1; in these latter studies, high (Torr) concentrations of reactants were used and it was necessary to measure small concentrations of HC1 which, as Kaiser et al. discuss, is difficult to do with high accuracy. Only one absolute kinetic study of the reaction of chlorine atoms with larger alkanes has been carried out. Lewis et al.lI used a fast flow discharge system with resonance fluorescence detection of atomic chlorine similar to ours. The most likely cause of systematic errors in such systems is the occurrence of unrecognized secondary reactions at the walls of the flow tube. Indeed, to study the reactions of atoms and free radicals in such systems, it is well-known that it is necessary to coat the walls to render them as inert as possible. Lewis et a1.I' used a phosphoric acid coating on both the flow tube walls and the discharge region, which was then baked to remove water; even with this coating, they found it was necessary to condition with atomic oxygen from a discharge of 0 2 in He prior to the kinetic runs. Without these precautions, higher effective rate constants were observed, indicating a contribution from a reaction of the organic with atomic chlorine on the walls of the flow tube. In our studies, the flow tube wall coating was halocarbon wax which, from studies in a variety of laboratories, is known to be inert to highly reactive species such as OH and atomic chlorine. The fact that our results for methane and ethane are in good agreement with a variety of studies using different techniques where wall reactions are not expected to be significant indicates that such heterogeneous processes are not important in our system. In addition, the zero intercept in Figure 5 indicates that addition of the organic did not change the rate of loss of atomic chlorine at the walls of the flow tube. Finally, our results for the higher alkanes are generally in agreement with those of Lewis et al.," whereas interference from wall reactions is expected to lead to higher rate constants. Lewis and co-workers" also used a discharge of C12 as their source of atomic chlorine. As discussed above, at higher Cl2 concentrations we encountered difficulties with this source due to inefficient discharge of the Cl2, resulting in interferences from the undissociated molecular chlorine. However, some studies of the n-butane reaction were carried out using small concentrations of C12 as the source of atomic chlorine; at [Clz] < 10" molecules ~ m - these ~ , interferences were minimal. For example, the rate constant measured with C12 as the source was (2.18 f 0.34) x compared to (2.11 f 0.18) x 1O-Io cm3 molecule-' s-' using F HC1 as the source. The only reactant for which our results lie outside the experimental error of Lewis et a1.I' is propane, where our rate constant is 20% lower. Because of this, our calculated ratio of the ethane/propane rate constants, 0.450 f 0.040, is about 20%
+
J. Phys. Chem., Vol. 99, No. 35, 1995 13161
Reactions of Chlorine Atoms with Some Simple Alkanes
TABLE 3: Summary of Absolute Rate Constants for the Reaction of C1 with Alkanes Determined in This Study and Comparison to Recommendations and Recent Literature Values alkane no. of runs [Cl] ( x 10'' ~ m - ~ ) [RH] ( x 10l2cm-3) k' (SKI) ( k f 20) cm3 molecule-' s - I reference CHJ 18 0.8-70 9-337 1-33 (9.40 f 0.40) x this work (1.0 i 0.1) x 10-13 JPL6
~
C2H6
12
0.7-6
0.3- 1
14-50
C3H8
10
0.6-3
0.2-0.5
26-52
n-CJHio
10
0.6-3
0.2-0.5
53-85
i-C4HIO
12
0.6-3
0.08-0.5
9-62
TABLE 4: Comparison of Rate Constant Ratios Obtained in This Study Using Relative Rate and Absolute Methods method relative rate method absolute rate method
ethanelpropane ethaneln-butane isobutaneln-butane 0.409 i 0.043
0.261 i0.013
0.683 i 0.030
0.450 i 0.040
0.262 i 0.025
0.664 f 0.068
higher than the 0.363 f 0.050 calculated from their data. However, agreement of our ratio with that from our relative rate measurements gives some confidence in the lower absolute rate constant for propane. Our rate constant for the n-butane reaction is in excellent agreement with that of Lewis et al.'] As discussed earlier, this rate constant is of particular interest since it has been used as the reference compound in a number of relative rate studies. It is noteworthy that our absolute measurement is within experimental error of the values of 1.97 x and 1.94 x cm3 molecule-' s-I derived by A t k i n s ~ nand ~ . ~co-workers by carrying out a least-squares fit of absolute rate constants for ethane, propane, n-butane, and isobutane to their measured relative rate constants. As discussed earlier, the interaction of the halocarbon wax coating on the flow tube with the chloromethanes precluded carrying out absolute kinetic studies. However, our absolute rate constant for methane, (9.40 f 0.40) x cm3 molecule-' S K I , can be used to convert the relative rate constants in Table 2 into the absolute values in Table 5 . Our value for the methyl chloride reaction is in reasonably good agreement with both the JPL and W A C recommendations and the recent relative rate measurements of Hewitt et al.I7 For dichloromethane, our results are in excellent agreement with the JPL6 recommendation. They lie approximately 20%below the IUPAC recommendation, but within the wide error bars cited. In addition, the ratio of the relative rate constants k(CH3Cl)Ik(CH2C12) = 1.34 f 0.09 derived from the data in Table 2 is in excellent agreement with the Niki et aL2' measurement of 1.31 f 0.14. Our chloroform rate constant is about 50% higher, although within the wide assigned error bars, of the current JPL6 and IUPAC' recommendations, which are based to a large extent on the Knox ~ t u d y . ~However, ~ . ~ ~ it is in excellent agreement with the recent relative rate measurements of Hewitt et al." Several approaches have been used in the past to interpret trends in reactivity. For example, Aschmann and Atkinsong showed that structure-activity relationships could be derived for the abstraction of hydrogen atoms in the simple alkanes by
(1.0 i 0.2) x 10-13 (5.53 & 0.21) x IO-'' (5.48 i 0.60) x IO-'' (5.7+06-05) x 10-1' (5.9+'9-a8) x IO-" (5.9 i 0.6) x IO-" (7.05 f 1.4) x lo-" (1.23 i 0.10) x IO-'" (1.51 f 0.12) x IO-'' (1.6+08-05)x IO-" (1.4+'4-03) x IO-'" (2.11 i 0.18) x IO-" (2.25 i0.20) x IO-'" (1.40 i 0.08) x lo-'" (1.46 i 0.12) x lo-'"
IUPAC7 this work Lewis et al." JPL6 IUPAC' Stickel et ai.?' Kaiser et aLZ4 this work Lewis et al." JPL6 IUPAC7 this work Lewis et al.l' this work Lewis et al."
TABLE 5: Absolute Rate Constants at 298 K for the Reactions of the Chloromethanes with Chlorine Atoms from This Study" Compared to Recommended Values and Recent Studies rate constant x i o i 3(cm3 molecule-' s - ' ) ~ JPL compd this study recommendation'
IUPAC recommendationd
Hewitt et
CH3C1 4.4 i 0.6 4.9 (+1.0,-0.8) CH?C12 3.2 i O . 2 3.3 ($3.3,-1.7) CHC13 1.1 zkO.1 0.76(+1.5,-0.5)
4.9 (+2.0,-1.4) 4.1 (+3.2,-1.8) 0.76(+0.76,-0.38)
4.6 i0.3 n.d! 1.1 kO.l
fl From the relative rate constants in Table 2 and the absolute value for CH4 determined in this study, (9.4 f 0.4) x cm3 molecule-' s-l (Table 3). Errors are 2 standard deviations. Reference 6, based on the absolute rate constants of Clyne and Walker25 and Manning and Kuryloz6 and the relative rate studies of Wallington et al.,IEKnox,I9 Tschuikow-Raux et al.,?' and Niki et aL2' Reference 7, based on the absolute rate constants of Davis et al.,**Clyne and Walker,25Manning and Kury10,~~ and the relative rate studies of Wallington et a1.I8 and e Converted from relative rate measurements using k(CH4) Niki et = (9.4 i 0.4) x cm3 molecule-' s-l. fn.d. = not determined.
TABLE 6: Comparison of Experimentally Determined Rate Constants at 298 K with Those Predicted Using Previously Developed Structure- Activity relationship^^>*^ kxpO organic
(cm3 molecule-' s-l) (9.40 i0.40) x (5.53 Ik 0.21) x lo-" (1.23 i0.10) x IO-'' (2.11 i O . 1 8 ) x lo-'' (1.40 i 0.08) x IO-'' (4.4 k 0.6) x 10-13 (3.2 i0.2) x 10-13 (1.1 i 0.1) x 10-13
!ismb (cm3 molecule-' s-I)
6.64 x 1.36 x 1.84 x 1.40 x 3.93 x 3.80 x 1.09 x
IO-" IO-'' IO-''
kSAR/kexp
1.20 1.10 0.87
.oo
10-I'
1
10-13
0.90 1.17 1.02
10-13 10-13
' I Rate constants from this study (Tables 3 and 5 ) . Predicted using the structure-activity relationship of Aschmann and Atkinsong for the simple alkanes and of Senkan and Quamz9for the chlorinated methanes.
atomic chlorine, which can then be used to predict room temperature rate constants to within 11%. Senkan and Quam29 showed that a similar structure-activity approach could be applied to a series of halogeqated alkanes and oxygenated hydrocarbons, which would allow prediction of rate constants to within a factor of 2, and on average, within 50% of experimentally determined values. Table 6 shows the ratio of our experimentally determined rate constants to those calculated from the structure-activity relationships derived by Aschmann and Atkinsong and Senkan and Quam.29 For the simple alkanes the agreement is generally
13162 J. Phys. Chem., Vol. 99, No. 35, 1995 within 10%. The agreement is not as good for ethane due to our lower relative and absolute rate constants. For chloroform and methyl chloride, our experimental values are in good agreement with Senkan and Quam’s estimated values. Indeed, the agreement is better than they reported at the time using the experimental values then available. Summary A consistent set of rate constants for the reaction of atomic chlorine with the C1-C4 alkanes using both absolute and relative rate methods has been measured at 298 K. The two approaches yield data in agreement to within 10% and resolve discrepancies in the literature between rate constant values obtained using relative vs absolute methods. The major differences with literature values are in the relative rates for the ethaneln-butane pair where our results are approximately 20% below published values, and in the absolute rate constant for propane, where our value is also about 20% below the previous determination. Relative rate constants for the chloromethanes were also measured; the largest discrepancy with currently recommended values is for chloroform, where our value is almost 50% higher than the recommendations, although within the wide error bars cited. The results are shown to be consistent with recent structure-activity relationship^^,^^ for the alkanes and chloroalkanes. Acknowledgment. The authors gratefully acknowledge the financial support of the Department of Energy (Grant No. DEFG03-94ER6 1899),the National Science Foundation (Grant No. ATM-9302475), a Bristol-Myers Squibb Company Award of Research Corporation, and the Joan Irvine Smith and Athalie R. Clarke Foundation. We thank Dr. Roger Atkinson for the initial loan of a Teflon reaction chamber, Dr. Atkinson, Dr. Scott Hewitt, Dr. Bill Keene, and Dr. Tom Graedel for making preprints available prior to publication, and Dr. James N. Pitts Jr. for helpful discussions. Thanks are also given to Jorg Meyer for his expert glass-blowing and machining assistance as well as to Dr. John Greaves for his GCMS analysis. References and Notes (1) Finlayson-Pitts, B. J. Res. Chem. Int. 1993, 19, 235.
(2) Graedel. T. E.; Keene, W. C. Global Biogeochem. Cycles 1995, 9, 47.
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