3312
J . Phys. Chem. 1992, 96, 3312-3316
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Kinetic Study of the Reactions OH Mono-, Di-, and Trichloroacetaldehyde and Acetaldehyde by Laser Photolysis-Resonance Fluorescence at 298 K C. Balestra-Garcia, G . Le Bras, and H. Mac h o d * Laboratoire de Combustion et SystPmes RCactifs, CNRS, 45071 Orllans Cedex 2, France (Received: August 29, 1991; In Final Form: December 31, 1991)
Absolute rate constants for the gas-phase reactions of OH radicals with acetaldehyde and the three chloroacetaldehydes have been measured at 298 K using the laser photolysis-resonance fluorescence technique. The rate coefficients measured cm3 molecule-l s-l): k,(OH + CH3CHO) = 17 h 3; k2(OH + CH2C1CHO) = 3.0 & 0.6; at 298 K are (in units of k3(OH + CHCI2CHO) = 2.4 f 0.5; kd(0H + CCI3CHO) = 0.86 f 0.17. The results obtained for k , and k , represent the first absolute measurements of these rate constants. Rate constants kl-k3 are in good agreement with previous literature values but k, is a factor of 2 lower than three preceding measurements, two of them having been obtained by relative rate techniques. The possible explanations for this discrepancy are examined. The results are discussed in term of reactivity trend among chlorine substituted carbonyl compounds. The role of chloroacetaldehydes in transporting chlorine into the stratosphere is examined.
1. Introduction The consideration of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) as alternative replacement compounds for the fully halogenated chlorofluorocarbons (CFCs) currently in use has focused attention on the atmospheric fate of these compounds.Is2 The optimum HCFC and HFC candidates to replace the CFCs appear to be partially halogenated (with F or C1) ethanes.2 These compounds contain a t least one hydrogen atom; therefore, their main tropospheric oxidation pathway is the reaction with O H radicals3 The kinetics of the reactions O H + HCFCs and HFCs has been the subject of several recent studies“’ but the subsequent formation and fate of the products of these reactions are not well understood at the present time.2 It is important to identify these products and to determine their tropospheric lifetime since they can lead to halogen-containing compounds diffusing into the stratosphere. Despite uncertainties concerning the reactions of the HFCs and HCFCs under tropospheric conditiom2it appears very likely that several of these compounds, including methylchloroform (CH3CC13),will lead to the formation of halogenated acetaldehydes. For example, compounds of structure CX3CXH2will lead to CX3CH0 among other products (with X = H, F or C1).2 This work is the first step of a project aiming at understanding the tropospheric oxidation by O H radicals of halogenated aldehydes and carbonyl halides compounds produced in the primary oxidation of halogenated hydrocarbons. Chloroacetaldehydes (CH2C1CH0,CHCI,CHO, and CC13CHO) belong to this class of products. They are formed in the oxidation of some chloroalkanes and chloroalkenes commonly used in industry such as CH3CC13or CHCI=CHCH2C1.8-’o To date, there are only four kinetic studies of the reactions of O H radicals with chloroacetaldehydes in the literature: the rate constant of the reaction O H CC13CH0 has been measured in three laboratories. DdbE et al.IOhave measured it between 298 and 520 K in a fast flow reactor, at low pressure, with resonance fluorescence detection of OH; Zabel and Libuda,” Nelson et a].,* and Scollard et a1.I2have measured this rate constant by a relative method in a simulation smog chamber, at 298 K and atmospheric pressure, using gas chromatography and FT-IR detection of reactants and products. Starcke et al.I3 and Scollard et al.12have also measured the rate constants of the reactions O H + CH2ClCH0 and CHC1,CHO at 298 K . This is the first absolute measurement of the rate constants for the reactions O H + CH,ClCHO and CHClzCHOand the second one for the reaction O H + CC13CH0. We have also measured
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Author to whom correspondence should be addressed. Present address: Service d’ACronomie, CNRS, UniversitC Paris VI, Bohe 102, F-75252 Paris Cedex 05, France.
the rate constant of O H + CH3CH0 which is well-defined in the literatureIel6 in order to obtain a complete series of rate constants which will provide information on the corresponding reaction mechanism.
2. Experimental Section The kinetic experiments reported in this paper were performed with a laser photolysis-resonance fluorescence (LP-RF) system described in detail in a recent publication.” Hence the apparatus is only briefly described below. The reaction cell was a Pyrex cross-shaped reactor with a volume of -750 cm3. The photolytic light source was a pulsed excimer laser (Lambda Physik LPX 100) operated on the KrF line at 248 nm. The hydroxyl radicals were produced by photodissociation of nitric acid diluted in argon at 248 nm. The initial O H concentration was typically (2-4) X 10” molecules ~ m - The ~. ( I ) World Meteorological Organisation Global Ozone Research and Monitoring Project Report N o 20, Scientific Assessment of Stratospheric Ozone, 1989; WMO: Geneva, 1990 Vol. 1, pp 401-410. (2) World Meteorological Organisation Global Ozone Research and Monitoring Project Report No 20, Scientific Assessment of Stratospheric Ozone, 1989; WMO: Geneva, 1990; Vol. 2, pp 161-266. (3) Derwent, R. G.; Volz-Thomas, A. in World Meteorological Organisation Global Ozone Research and Monitoring Project Report N o 20, Scientific Assessment of Swatospheric Ozone, 1989; WMO: Geneva, 1990; Vol. 2, pp 125-146. (4) Liu, R.; Huie, R. E.; Kurylo, M. J. J. Phys. Chem. 1990. 94, 3247. (5) Gierczak, T.; Talukdar, R.; Vaghjiani, G. L.; Lovejoy, E. R.; Ravishankara, A. R. J. Geophys. Res. 1991, 96 (D3). 5001. (6) Talukdar, R.; Mellouki, A.; Gierczak, T.; Burkholder, J. B.; McKeen, S. A.; Ravishankara, A. R. J. Phys. Chem. 1991, 95, 5815. (7) Brown, A. C.; Canosa-Mas, C. E.; Parr, A. D.; Wayne, R. P. J. Atmos. Chem. 1990, 24A, 2499. (8) Nelson, L.; Shanahan, 1.; Sidebottom, H. W.; Treacy, J.; Nielsen, 0. J. Inr. J. Chem. Kiner. 1990, 22, 577. (9) Tuazon, E. C.; Atkinson, R.; Winer. A. M.; Pitts, J. N . Jr. Arch. Ennviron. Contam. Toxicol. 1984, 13, 691. (IO) DbE, S . ; Khachatryan, L. A.; Berck, T. Ber. Bumenges. Phys. Chem. 1989, 93, 847. ( I I ) Zabel, F.; Libuda, H. G., private communication, 1991. (12) Scollard, D.; Corrigan, M.; Sidebottom, H. W. STEP/AFEAS Workshop, Dublin (Ireland), 14-16 May 1991. (13) Starcke, J.; Zabel, F.; Elsen, L.; Nelsen, W.; Barnes, 1.; Becker, K. H. In Physico-Chemical Behaoiour of Atmospheric Pollutants; Restelli, G., Angeletti, G., Eds.; Kluwer Academic Publishers: Dordrecht, 1990; p 172. (14) Atkinson, R.; Baulch, D. L.; Cox, R. A,; Hampson, A,; Kerr, J. A,; Troe, J. J. Phys. Chem. Ref. Data 1989. 18, 881. (15) Atkinson. R. J. Phys. Chem. ReJ Data 1989, Monograph I . (16) NASA Panel for Data Evaluation, Chemical and Kinetic Data for use in Stratospheric Modeling; JPL Publication No. 90-1; Jet Propulsion Laboratory: Pasadena, CA, 1990; Evaluation No. 9. (17) Mac Leod, H.; Balestra, C.; Jourdain, J. L.; Laverdet, G.; Le Bras, G.I n ! . J. Chem. Kiner. 1990, 22, 1 167.
0022-365419212096-3312%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, NO. 8, 1992 3313
Gas-Phase Reactions of O H Radicals 1
I
Preparation of the Aldehydes. Acetaldehyde ( F l u b >99.5%) was thoroughly degassed at 156 K before each use. Monochloroacetaldehyde was prepared from a 50% solution in water (Aldrich) by extraction with ether followed by fractionated distillation (bp = 85-85.5 "C). Dichloroacetaldehyde (bp = 90-91 "C) was prepared by reacting dichloroacetaldehyde diethyl acetal (Fluka >98%) with benzoic anhydride under acidic conditions (concentrated HSO,)." The solution obtained was distilled a t 90 "C to separate CHClzCHO from the byproduct C6HSC(0)OC2H5. Trichloroacetaldehyde (Fluka >98%) was thoroughly degased and purified by several freezepumpthaw cycles. The three chloroacetaldehydes were stored in the dark at 278 K. Under these conditions CH2ClCH0 and CHClzCHO polymerized within a few days to a few weeks. The compounds obtained had the following measured vapor pressures at 298 K: 18,39, and 48 Torr for CH,ClCHO, CHCl,CHO, and CCI,CHO, respectively. 3. Results
time (ms)
Figure 1. Typical OH temporal profile following laser photolysis of HNO,/CCI$HO/argon mixtures. Experimental conditions: T = 298 K; P = 27 Torr; [HNO,] = 6.0 X lot4molecules cm-); [CCI,CHO] = 1.66 X 10" molecules ~ m - ~The . insert is a plot of In [OH] versus reaction time. The solid line represents the least-squares linear fitting of the data between 0.2 and 6.5 ms; the slope is equal to 366 s-'.
photolysis fluence was varied between 160 and 300 mJ/(pulse an2) at the laser output (which corresponds to about 16-30 mJ/(pulse cm2) at the center of the cell) in order to check for any enhancement of the OH radical decay rates due to reactions with radicals formed from the photofragmentation of the aldehydes at 248 nm. The OH radicals were detected by resonance fluorescence at 308 nm ((A22+,0) (Xzn,0)). The fluorescence was excited by a continuous-wave OH resonance lamp. The laser beam and the lamp beam were collimated and directed along the two axes of the cell perpendicular to each other. The OH resonance fluorescence produced at the intersection of the light beams was detected perpendicular to both beams as a function of time after the laser pulse. The crossing volume was imaged on the photocathode of a photomultiplier tube through an appropriate set of lenses and filters in order to discriminate fluorescence photons against laser and resonance lamp scattered light. The signal from the photomultiplier was amplified and fed into a multichannel averager-computer system (ATNE-TANDON). The O H concentration temporal profiles were obtained by summing this signal during 512 laser shots with a repetition rate of 4 Hz. Under these conditions, the detection limit for the OH concentration was 1O1O molecules cm-3 (signal-to-noise ratio = 1; noise amplitude corresponds to the resonance lamp background light). The OH decay was checked to be exponential over at least three ( l / e ) lifetimes. The O H decay rate (Le., the pseudofirst-order rate constant k? was calculated from a least-squares analysis of plots of In [OH] against time. An example of the OH signal decay and the determination of the corresponding value of k' is given in Figure 1 for the reaction O H CC13CH0. Nitric acid and the reactant were premixed with the argon buffer gas (Alphagaz purity >99.995%) before entering the cell. The pressure in the reactor was measured by a capacitance manometer (MKS Baratron). Reactant flow rates were determined from the pressure drop in calibrated volumes and the argon flow was regulated and measured by a mass flow controller (Tylan FC260). Experiments were carried out under slow flow conditions (94 cm3 s-I at P = 27 Torr) to avoid accumulation of photolysis or reaction products. The total flow rate was varied between 40 and 140 cm3s-I and the overall pressure between 15 and 100 Torr. Anhydrous HNOl was prepared by reaction under vacuum of N a N 0 3 with excess H2SO4 and was collected at 77 K. HNO, samples contained less than 0.5% of NO2.'*
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(18) Zagogianni, H.;Mellouki, A,; Poulet, G.C. R. Acad. Sci. Paris Ser. 2 1987, 304, 5 1 3 .
All experiments were carried out under pseudo-first-order conditions with [aldehydelo > 100 [OH]@ The OH temporal profile is described by the following expression: In ([OH],/[OH],) = (k[aldehyde]
+ k'?t
k"= ~ H N ~ , [ H N+O kd ~]
= k't
(I) (11)
where k and kHNo are the bimolecular rate constants of the reactions OH aldehyde and OH H N 0 3 , respectively. kd is the first-order decay rate of OH not due to reaction (mainly diffusion out of the observation volume, kd was typically equal to 30 s-I). The initial concentration of H N 0 3 was kept constant and equal to (7-8) X lOI4 molecules ~ m - ~The . aldehyde concentration was varied over at least a 10-fold concentration range. The pseudo-first-order rate constant k' was measured for each aldehyde concentration from the OH fluorescence signal decay in the presence of the aldehyde and H N 0 3 . Values of k were calculated from least-squares linear fittings of plots of k'against [aldehyde] according to eq I, which yielded k as the slope and k"as the ordinate intercept. The decay rate k"was also measured directly before each set of experiments by monitoring the OH decay in the absence of aldehyde and was always equal to 130 f 30 s-l, in agreement with eq I1 since kHNO, = 1.3 X cm3 molecule-' s-' a t 27 Torr of argon." On the average, each value of a rate coefficient k was obtained from at least five individual runs in identical experimental conditions, each run resulting from 10 or more data points. 3.1. Reaction OH CHJCHO(1). The measurement of this reaction rate constant, which is already well-known1e16had two purposes. One was to obtain a consistent set of rate constants by studying under similar experimental conditions the reactions of OH with the unsubstituted and the three chloro-substituted aldehydes. The second purpose was to assess the potential role of acetaldehyde photofragments involved in eventual secondary reactions which would lead to enhanced O H decay rates. It should be noted that, while the photodissociation products and quantum yields of acetaldehyde have been studied at several wavelengths,14 the photoproducts of the chloroacetaldehydes are presently not known. The initial concentration of C H 3 C H 0 was varied from 6 X lo1*to 8 X 10I3molecules ~ m - ~The . rate constant was measured at two different initial H N 0 3 concentrations (at a constant laser fluence equal to =25 mJ/(pulse cm2)) in order to verify that the initial OH concentration had no influence on the kinetic measurements. The laser fluence was varied (between 16 and 30 mJ/(pulse cm2)) at the lower H N 0 3 concentration and consequently [OH], was varied proportionately. The corresponding plot in Figure 2 shows that it had negligible influence on the measured rate coefficient. The pressure of argon was maintained equal to 27 Torr. The plots of k'against [CH,CHO] are shown in Figure 2 for the 'typical" and the "high" concentrations of H N 0 3 . kl and k" are calculated from a least-squares linear fitting of these plots:
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3314
The Journal of Physical Chemistry, Vol. 96, No. 8, 1992
kl = (1.71 f 0.03) [HNOJo = 7.5
X IOl4
X
lo-" cm3 molecule-'
s-I
molecules cm-); k" = 140 f 20 s-I
k, = (1.71 f 0.08) [HN0310= 2.5
X
Balestra-Garcia et al.
X
lo-" cm3 molecule-' s-'
10l5 molecules ~ m - ~k"; = 400 f 20 s-'
where the quoted statistical errors are 2 standard deviations. 3.2. Reaction OH + CH,CIcHO (2). The initial concentration of CH2CICH0was varied from 2.5 X 1013to 5.3 X lOI4 molecules ~ m - ~The . total pressure of argon was varied from 27 to 100 Torr. The laser fluence was varied (between 16 and 30 mJ/pulse cm2) and no change was observed in the measured rate coefficient. k2 was calculated from a least-squares linear analysis of the plot of k' against the CH,ClCHO concentration. This plot is shown in Figure 3 and the slope is equal to
k2 = (3.0 f 0.1) X
cm3 molecule-' s-I
where the quoted statistical error is 2 standard deviations. The ordinate intercept of the plot is equal to 140 f 30 s-l. 3.3. Reaction OH + CHCl,CHO (3). The initial concentration of CHC12CH0 was varied from 4 X lOI3 to 6 X 1014 molecules ~ m - The ~ . total pressure of argon was varied between 27 and 100 Torr. The laser fluence was varied (between 16 and 30 mJ/(pulse an2))and no change was observed in the measured rate coefficient. k3 was calculated from a least-squares linear fitting of the plot of k'against the CHC1,CHO concentration. This plot is shown in Figure 4 and the slope is equal to
k3 = (2.4 f 0.1)
X
0
3x1013
6~10~'
Figure 2. Plot of the decay rate of OH, k'versus CH3CH0 concentration. The experiments were performed at 298 K in the presence of 27 Torr of argon and two different initial HNO, concentrations: (+) [HN0310 7.5 X 1014 molecules cm-); (B) [HN0310 2.5 X l O I 5 molecules cm-3
-
-
cm3 molecule-' s-l
where the quoted statistical error is 2 standard deviations. The ordinate intercept of the plot is equal to 170 f 20 s-'. We conducted 10 different runs with four different batches of the compound (the synthesis procedure has been described in the Experimental Section). One of the preparations yielded a significantly higher value of the rate constant, k 3.5 X cm3 molecule-' s-I for an unknown reason, and the corresponding kinetic data were rejected. 3.4. Reaction OH CCI,CHO (4). The initial concentration of CC13CH0 was varied from 8 X lOI3 to 1 . 1 X 10l5 molecules ~ m - The ~ . total pressure of argon was varied between 15 and 27 Torr. The laser fluence was varied between 16 and 30 mJ/(pulse cm2). Four runs which were conducted at laser fluences of 22, 24,26, and 30 mJ/(pulse cm2), respectively, yielded significantly higher and inconsistent rate constants (12 f 2, 10 f 2, 14 f 2, and 1 1 f 2) X an3molecule-' s-I, respectively. We suspected that the amount of CC1,CHO photofragments reacting with O H became significant at these higher laser energies. We succeeded in obtaining a consistent reproducible set of data (presented in Figure 5 ) by running kinetic measurements at laser fluences comprised between 16 and 20 mJ/(pulse cm2). Below 16 mJ/ (pulse cm2), the initial O H concentration was too low to obtain a reasonable fluorescence signal sensitivity. k4 was calculated from a least-squares linear fitting of the plot of k'against the CC13CH0concentration. This plot is the result of over 20 individual runs of 12 data points each. We have multiplied the number of experiments in many different conditions of flow rate and pressure because of the possible photofragmentation of CC13CH0. This plot is shown in Figure 4 and the slope is equal to
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+
k4 = (8.6 f 0.2)
X
cm3 molecule-' s-I
where the quoted statistical error is 2 standard deviations. The ordinate intercept of the plot is equal to 160 f 10 s-I. 4. Discussion 4.1. CompnrisOn with Iiterature Data. Rate constants obtained
in this work for the reactions O H with acetaldehyde and chloroacetaldehydes are compared in Table I with the results previously reported in the literature. The major source of error in the determination of rate coefficients in pseudo-first-order conditions
0
2~inl4
4x1014
Figure 3. Plot of the decay rate of OH, k'versus CH,CICHO concentration. The experiments were performed at 298 K in the presence of 27 Torr (+) and 100 Torr (B) of argon.
Figure 4. Plot of the decay rate of OH, k'versus CHCI,CHO concentration. The experiments were performed at 298 K in the presence of 27 Torr (+) and 100 Torr (m) of argon.
was the aldehyde concentration. Given the fact that aldehydes are sticky compounds which tend to polymerize, we estimated that the systematic error in the aldehyde concentrations was equal to f 20% and was then much larger than the statistical error on our rate constants measurements (
