4092
J . Phys. Chem. 1989, 93, 4092-4094
Pulse Radiolysis Study of the Gas-Phase Reaction of OH Radfcals with Vinyl Chloride at 1 atm and over the Temperature Range 313-1 173 Kt Andong Liu,t W. A. Mulac, and C. D. Jonah* Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: September 26, 1988; In Final Form: December 8, 1988)
The gas-phase reaction of OH radicals with vinyl chloride was studied at I-atm pressure of argon and over the temperature range 313-1 173 K by pulse radiolysis. The temperature dependence of the rate constants showed behavior similar to that of ethylene in that the predominant reaction changed from an addition-initiated reaction below 588 K to a hydrogen atom abstraction reaction above 723 K. The Arrhenius expression for the high-pressure second-order rate constants of the addition reaction was (2.14 X 10-12)e(7"20)/R7cm3 moleculed s-l. The second-order rate constant for the H atom abstraction reaction or (2.98 X 10-l')e(-4020*700)/RT cm3 molecule-I s-', The indicated error limit on the Arrhenius was (1.4 x 10-'7)T2e(-'2")/T activation energy is 1 standard deviation of the least-squares analysis.
Introduction The gas-phase reactions of the O H radical play an important role in both atmospheric and combustion chemistry and have been studied intensively by many techniques.' The temperature dependence of the rate constants of O H radicals with several unsaturated hydrocarbons2-I0 has shown that the predominant reaction switches from an addition reaction a t low temperatures to a hydrogen atom abstraction reaction a t high temperatures. For haloalkenes, no previous experimental studies spanned a sufficiently wide temperature range to observe a change in the predominant reactions. Perry et al. measured the rate constants of O H radicals with vinyl fluoride, vinyl chloride, and vinyl bromide over the temperatures from 299 to 426 K over the pressure range 50-100 Torr." Their results showed that the additioninitiated reaction is the predominant reaction in this temperature region. Those authors discussed the mechanisms and pointed out that there exist complications in the details of the reactions of OH with this class of organic compounds (see text below). W e felt that experiments covering a wider range of temperatures would be helpful in determining the mechanisms of these reactions. For this reason, a pulse radiolysis study of the reaction of O H radicals with vinyl chloride was carried out a t a total pressure of 1 atm and over the temperature range 3 13-1 173 K.
TABLE I: Measured Pseudo-First-Order Rate Constants for the Reaction of OH Radicals with Vinvl Chloride at 873 K concn, i o i 5 concn, i o i 5 molecule/cm3 k , I O 3 s-' molecule/cm3 k , I O3 s'I 11 39 3 I5 10 0 16 52 43 120 2 I5 65 5 35 15 0 32
90
The resonance line was isolated with an interference filter, and its intensity was monitored with a photomultiplier (Hamamatsu R928 using 5 dynodes). The output of the photomultiplier was digitized with a Biomation 8100 transient recorder and accumulated and averaged in a computer (DEC 11/23). The argon used in this study was U H P grade, 99.999%, from Matheson Gas Products, Inc. A mixture of 4.84% vinyl chloride in U H P argon was made by the same company. The concentration of vinyl chloride in the reaction cell was on the order of 10'5-10'8 molecules which was at least 2 orders of magnitude greater than the concentration of OH. This was necessary for a pseudo-first-order reaction. As discussed p r e v i o u ~ l y ~the ~ ~concentration ~'~~'~ was determined from the optical density. The concentration data were analyzed Experimental Section with a nonlinear least-squares fit to a single exponential. This The experimental details have been discussed previou~ly,4,~-'~,'~,'~ type of fitting gave a comparable result to the fitting of a concurrent first- ( O H + reactant) and second-order ( O H + OH) and only the basic principles and changes to the system will be decay where the rate constant for the second-order decay was described here. A flow system was used for this experiment, and determined from the decay of O H in the absence of r e a ~ t a n t , ~ ~ ~ ' ~ the composition of the sample was controlled by varying the flow except the rate constant is larger by a constant. This leads to the rates of the reactants [flowing a t 0.005-0.01 slm (standard liters nonzero intercept as shown in Figure 1 and does not affect the per minute)] and the buffer gas (argon a t 1 slm and containing second-order rate constant determined from the slope. The quality about 6 Torr of water). The reaction cell was heated by an oven, of fits is analogous to those shown in ref 4, 8-10, 12, and 13, where and the temperature was measured a t the outlet of the cell and agreement within the experimental noise is found over the entire controlled within i 2 K. The gas mixture was irradiated in the cell with an electron beam ( 1 5 MeV, 0.25-3 bs, peak current 1.5 A) that ionized the argon. The ion recombination and further reactions give Ar* and Ar2*, ( 1 ) Atkinson, R. Chem. Rev. 1986, 86, 69. ( 2 ) Smith, G. P.; Fairchild, P. W.; Crosley, D. R. J . Chem. Phys. 1984, which can transfer energy to the water molecules and decompose 81, 2667. them to form H and O H . These reactions take place in less than ( 3 ) Tully, F. P. Chem. Phys. Lett. 1983, 56, 148. 1 ps. From the maximum absorption of the OH radicals and from (4) Liu, A.; Mulac, W. A,; Jonah, C. D. Int. J . Chem. Kinet. 1987, 19, 25. the rate of disappearance of O H through a second-order reaction (5) Smith, G. P. Inr. J . Chem. Kinet. 1987, 15, 269. with itself, the initial concentration of O H was estimated to be (6) Tully, F. P.; Goldsmith, J. E. M. Chem. Phys. Lett. 1985, 116, 345. (7) Smith, G. P.; Fairchild, P. W.; Jeffries, J . B.; Crosley, D. R . J . Phys. on the other of 1012-10'3 molecules ~ m - ~ . Chem. 1985, 85, 1269. Resonance absorption was used to measure the O H concen(8) Liu. A.; Mulac, W . A,; Jonah, C. J. J . Phys. Chem. 1988, 92, 131. tration. A microwave discharge in a low-pressure water-helium (9) Liu, A.; Mulac, W. A,; Jonah, C. D. J . Phys. Chem. 1988, 92, 3828. mixture was used to generate the OH resonance line (308 nm). ( I O ) Liu, A.; Mulac, W. A,; Jonah, C. D. J . Phys. Chem. 1988, 92, 5942. 'Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, U.S. DOE under Contract No. W-31109-ENG-38. 'On leave from the Institute of Low Energy Nuclear Physics, Beijing Normal University, Beijing, China.
( I I ) Perry, R. A.; Atkinson, R.; Pitts, J. N.,Jr. J . Chem. Phys. 1977, 67, 458. (12) Jonah. C. D.; Mulac, W. A,; Zeglinski, P. J . Phys. Chem. 1984, 88, 4 100. (13) Beno, M. F.; Jonah. C. D.; Mulac, W . A . Inr. J . Chem. Kinet. 1985, 17, 1091.
This article not subject to U.S. Copyright. Published 1989 by the American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989 4093
Gas-Phase Reaction of O H Radicals with Vinyl Chloride
, 0
1
2 Conc x
3
ioi5
* 1.o
1
4 5 mo~ecicm~
0.5
6
Figure 1. Plot of pseudo-first-order rate constants versus concentrations of vinyl chloride at 873 K. TABLE 11: Measured Absolute Rate Constants of the Reaction of OH Radicals with Vinyl Chloride at 1 atm in Argon Buffer Gas and over the Temperature Range from 313 to 1173 K k," cm3 k," cm3 T, K molecule-' s-I T, K molecule-1s-I 7.55 5.70 4.46 4.40 4.14 4.10 4.16 4.18
313 353 418 458 413 513 563 588
723 773 813 923 1033 1073 1 I73
1.90 2.43 2.48 3.20 3.61 5.10 7.62
"The estimated overall error limit for the measurement is *IO%. decay, and therefore, the results are not shown here.
Results and Discussion The measurements, which were performed from 31 3 to 1173
K at 15 different temperatures, gave an exponential decay for O H at all temperatures. Concentrations of vinyl chloride were selected so that the time was less than 1 ms. A typical plot of the measured pseudo-first-order rate constant k'vs vinyl chloride concentration is shown in Figure I , and the data are listed in Table I. The bimolecular rate constant was obtained from the slope. The intercept of this plot reflects the rate of O H disappearance by reactions with itself and impurities in the sample and decreases with a decreasing of the dose of the electron beam. When the dose of the electron beam was varied by a factor of 10, the slope of the plot was constant within the experimental accuracy, showing that there is no significant interference from second-order reactions under our conditions. All rate constants are given in Table I1 and plotted in Figure 2. The change in the predominant reaction as a function of temperature is obvious in Figure 2. The temperature dependence of the rate constants is very similar to the case of ethylene4 and propylene.6 We assume that they have similar mechanisms. The previous values of Perry et al." are also plotted in Figure 2 for comparison. We see that our measurements at low temperatures are slightly higher than the data from Perry et al., which were measured at a total pressure of 50 Torr. The mechanism of the reactions of O H radicals with vinyl halides has been discussed in detail by Perry et al." and by Atkinson.' Their mechanism for the initial stages of the reaction can be described by the following reactions"
OH
+ CH,=CHX
HOC2H3X*
+M
+
s HOC2H3X* HOCIH3X
+M
(1, -1) (2)
where X = F, CI, or Br. That is, the reaction involves the initial addition of O H radicals followed by the stabilization of the OHvinyl halide adduct. The elimination of a halogen atom is another possible pathway that can compete with the back reaction (-1): HOC2H3X*
-
CH,=CHOH
+X
1.0
1.5
2.0
1ooom
2.5
3.0
+
Figure 2. Arrhenius plot of rate constants of OH vinyl chloride. Key: circle, this work; square, ref 11; -, (2.14 X 10-12)e350/T; --, (1.4 X 10-17)j"%12W/7;- -, (2.98 X 10-'1)e-20101T cm3 molecule-' s-I.
exothermic.'' If (3) were fast enough to compete with the reaction (-I), pressure dependence of the addition-initiated reactions would be very small and different behavior would be expected between the reaction of O H with vinyl chloride and the reaction of O H with ethylene (for C2H4,Le., X = H, reaction 3 is about 7 kcal/mol endothermic"). However, our data show that the Arrhenius plot of vinyl chloride is very similar to ethylene4s9and propylene,6 as mentioned above, and the sharp falloff in the rate constant from 588 to 723 K in Figure 2 is apparent. This implies that a t high temperatures reaction -1 becomes very fast and the addition reaction is no longer at the high-pressure second-order limit. This is consistent with the work of Howardi4who reported the pressure dependence for reaction of O H with vinyl chloride at 296 K. Those results, which were done in 1-7 Torr of helium show (1) there is a substantial pressure dependence on these rate constants at room temperature that is in agreement with the rate at room temperature depending on collisional deactivation; (2) the rate constants are still increasing at 7 Torr of helium, which explains why their rate constants are slower than ours; and (3) the rate constant extrapolated to zero pressure is not large, which suggests that the non-collision-dependent rate is small. Thus, our results and those of HowardI4 imply that reactions 1, -1, and 2 are dominant at room temperature and moderate pressures and that reaction 3 is but a minor component. Two possible mechanisms for explaining this behavior were suggested by Perry et al." and Atkinson.' The first is that the OH radical addition only occurs a t the 0-carbon and the 1,2migration of O H is rate determining. The second is that the addition occurs a t both the a- and @-position,but mainly at the @-position, and the 1,2-migration is sufficiently slow as to be negligible. Because the elimination can proceed only when the O H is at the a-position, the elimination would play only a minor role in the addition-initiated reactions in O H vinyl chloride. Because these measurements were performed at only one pressure, they are insufficient to establish whether the addition reaction is at the high-pressure second-order limit; the measurements a t different pressures are needed to unambiguously establish the high-pressure limit. However, Perry et al. have found that there was no pressure effect in the O H reaction with vinyl chloride at room temperature and pressures above 50 Torr within their experimental accuracy." An Arrhenius expression of (1.14 x 10-12)e(1040*300)~RT cm3 molecule-' s-l was suggested as the second-order high-pressure limit from 299 to 426 K." Because our measurements were performed at a total pressure of 760 Torr, the rate would be expected to remain close to the high-pressure limit at even higher temperatures. If we compare ethylene and vinyl chloride, the substitution of a chlorine atom for an H atom in ethylene will slow the unimolecular decomposition of the energy-rich adduct of vinyl chloride relative to ethylene. Thus, we suggest that our measured rate constants are at (or very close to) the high-pressure limit over the temperature range 3 13-599 K. A least-squares fit of these data gives the Arrhenius expression k = (2.14 X 10-'2)e(7M)*120)lRT cm3 molecule-l s-I (the quoted error in activation energy is 1 standard deviation of the least-squares
+
(3)
For X = F, (3) is endothermic, but for X = CI and Br, it is
3.5
(14) Howard, C. J. J . Chem. Phys. 1976,65, 4771.
J . Phys. Chem. 1989, 93, 4094-4098
4094
analysis) for the addition reaction. Above 723 K , the measured rate constants increase with increasing temperatures. This is very similar to what occurs in ethylene, suggesting that the predominant reaction a t higher temperatures is the H atom abstraction reaction. The data at high temperatures show a distinct curvature, similar to the event of O H haloalkane. For this reason, the Arrhenius expression would be expected to be of the form AT2e-'iRT. The least-squares analysis of the measured rate constants from 723 to 1173 K gives the r< as k = (1.4 X 10-17)T%-i2M))/RT cm3 molecule-' s-'.-The uncertainties in these parameters are very large (*50%) because of the limited temperature range. It can also be fit to the form k = (2.98 x 10-ii)e(-4020*7w)~RT c m j molecule-] s-' (the quoted error in activation energy -_is 1 standard deviation of the leastsquares analysis).
Conclusion
+
The absolute rate constants for the reaction of O H vinyl chloride were measured from 313 to 1173 K a t 1 atm in Ar by pulse radiolysis resonant absorption. The temperature dependence of this reaction is similar to the reaction of O H + ethylene and implies similar reaction mechanisms. The predominant reactions are suggested from the addition-initiated reactions a t low temperatures to the H atom abstraction reaction at high temperatures.
+
Acknowledgment. W e gratefully acknowledge the assistance of the Argonne accelerator operators, George Cox, Don Ficht, and Ed Kemereit, The assistance of ~~~i Engelkemeier in the analysis of our samples is gratefully acknowledged, Registry No. OH, 3352-57-6; vinyl chloride, 75-01-4
A Study of the Kinetics of the Reactions of Ethyl Radicals with O,, CI,, and CI E. W. Kaiser,* L. Rimai, and T. J. Wallington Chemistry Department, Ford Motor Company, Dearborn, Michigan 481 21 -2053 (Received: September 26, 1988: In Final Form: December 20, 1988)
The yield of C2H4formed during the reaction of C2H5 with O2 has been determined at 298 K for total pressures from 6.5 to 650 Torr. Ethyl radicals were formed by pulsed laser photolysis of C12 in mixtures of C2H6,Clz, 02,and, in selected cases, either He or N2 diluent. Gas chromatographic analysis of the reactant and product concentrations showed that the C2H4 yield depends inversely on total pressure over the range measured, and at 650 Torr,
