7060
J. Phys. Chem. 1996, 100, 7060-7065
Hydrogen Atom Attack on 1,2-Dichlorotetrafluoroethane: Rates of Halogen Abstraction Jeffrey A. Manion* and Wing Tsang National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed: October 11, 1995; In Final Form: February 26, 1996X
Hydrogen atom attack on 1,2-dichlorotetrafluoroethane (DCFE) at temperatures between 970 and 1140 K and pressures of about 200-500 kPa (2-5 bar) has been studied with a single-pulse shock tube reactor. The observed products show that chlorine is readily abstracted, while fluorine is removed in at most trace quantities only. This is despite the greater exothermicity of the fluorine abstraction channel. Relative to the standard reaction of displacement of methyl from 1,3,5-trimethylbenzene, for which k[H + (CH3)3C6H3 f (CH3)2C6H4 + CH3] ) 6.7 × 1013 exp(-3255/T) cm3 mol-1 s-1, k(H + DCFE f CF2CF2Cl + HCl) ) 2.1 × 1014 exp(-5839/T) cm3 mol-1 s-1 and k(H + DCFE f CFClCF2Cl + HF, 1100 K) e 7 × 109 cm3 mol-1 s-1. Uncertainties are estimated to be factors of 1.1 and 1.4 on relative and absolute bases, respectively. Comparisons with relevant literature data are made. The slow rate of F abstraction implies that reaction of fluorinated compounds via this pathway will be unimportant except in compounds unable to react via other channels. Implications with regard to the destruction of fluorinated compounds in pyrolytic and combustion systems are discussed.
Introduction Halocarbons are effective flame inhibitors, but their use as fire extinguishers has been curtailed because of adverse effects on the ozone layer. The push to develop alternative compounds has resulted in substantial efforts to model and understand halocarbon combustion and flame inhibition on a fundamental level.1 This requires the input of high-quality kinetic data. Depending on conditions, combustion-like systems are expected to prominently feature the chemistry of atomic hydrogen, hydroxyl radicals, and oxygen atoms. Consequently, rates and mechanisms of the reaction of such species with halocarbons are of interest. Only some of these data are currently available, especially for reactions at high temperatures. Since halogenated hydrocarbons are a significant component of hazardous wastes, such information will also benefit our understanding of disposal methods such as incineration,2 hydrogenolysis,3 and reductive pyrolysis.4 One of the least understood aspects of the combustion of halons concerns the mechanisms by which fluorine is removed from organic structures. The extraordinary strength of C-F bonds means that many of the “usual” reactions do not apply. Abstraction of F by OH or O, for example, is endothermic by 220 kJ mol-1 or more. On the other hand, the very strong HF bond (570 kJ mol-1) makes abstraction of F by atomic hydrogen a feasible reaction. In the present paper, we report the results of a single-pulse shock tube study of the reaction of atomic hydrogen with 1,2dichlorotetrafluoroethane (DCFE). The two expected reaction channels are abstraction of either chlorine (reaction 1) or fluorine (reaction 2). H + ClF2CCClF2
a
F2CCClF2 + HCl b
H + ClF2CCClF2
a
X
C2F4 + Cl
FClCCClF2 + HF b
(1)
C2F3Cl + Cl
Abstract published in AdVance ACS Abstracts, April 1, 1996.
This article not subject to U.S. Copyright.
(2)
The fact that two channels are available and that the alternative pathways result in significantly different products and consequences illustrates a general difficulty in studying the reaction of hydrogen atoms with halons. Modeling and understanding of the chemistry requires not only that the overall reaction rate is known but also that the relative importance of the channels be determined. Real time methods which follow only the temporal behavior of a single component are unable to do this. In this respect, our single-pulse shock tube technique offers an important advantage. Under our experimental conditions, the chlorofluoroethyl radical products of (1a) and (2a) rapidly and quantitatively split off chlorine atoms to yield tetrafluoroethene and chlorotrifluoroethene, respectively. Production of these two haloethenes can therefore be monitored and associated with the elementary abstraction reactions, thereby giving a quantitative measure of the importance of the two channels. It is difficult to draw any a priori conclusions about the relative importance of the two pathways. Although C-F bond energies (460-540 kJ mol-1) are significantly greater than those of C-Cl bonds (290-400 kJ mol-1), this is offset by the much greater bond energy of HF (570 kJ mol-1) in comparison with HCl (431 kJ mol-1). Both reaction channels are exothermic and might reasonably be expected to occur. Relatively little is currently known about the rates of halogen abstraction by atomic hydrogen. The most studied compounds are the monohalomethanes, and the data are not always in good agreement. The reactions of H + CH3Cl and H + CH3F have been studied by Aders et al.5 and Westenberg and de Haas6 using flow discharge systems. Absolute hydrogen atom concentrations were determined with ESR and the reactions followed with mass spectrometric detection of either products5 or reactants.6 Both groups assumed the reaction mechanism to be abstraction of halogen, H + CH3X f CH3 + HX. The only direct evidence for the mechanism is from the product study of Aders et al. However, rate constants from that investigation are factors of 15 and 120 larger than those of Westenberg and de Haas for CH3Cl and CH3F, respectively. This discrepancy, together with critical analyses of the papers, suggests that the study of Aders et al. may have been perturbed by surface
Published 1996 by the American Chemical Society
H Atom Attack on DCFE reactions. The mechanism or mechanisms of the H + CH3X reaction thus remain uncertain. The rate data of Westenberg and de Haas on chloromethane show upward curvature, and they have suggested that the primary reaction is halogen abstraction but that a contribution from H + CH3Cl f CH2Cl + H2 occurs at higher temperatures. However, because they followed only the decay in the CH3Cl signal, it is unclear if this is correct. They have presumably measured the sum of the H and Cl abstraction channels, but it could be that the data at low temperatures pertain to hydrogen abstraction, while it is only at higher temperatures that chlorine abstraction becomes important. The only data on chloromethane clearly pertaining to chlorine abstraction are from a hydrogenolysis study,7 where product analysis allowed the relative dechlorination rates of chloromethane and chlorobenzene to be determined at a single temperature, 1004 K. Using the known8 rate expression for chlorobenzene allows us to deduce an absolute value for the H + CH3Cl f CH3 + HCl reaction. This rate constant is about a factor of 2 smaller than that reported by Westenberg and de Haas. Although the available rate data are thus consistent with abstraction of both hydrogen and chlorine playing a role in the H + CH3Cl reaction, the combined uncertainties preclude any firm conclusions as to which is the dominent path or which mechanism is more important in a particular temperature regime. Other studies of abstraction of Cl by atomic hydrogen include that of Combourieu et al.9 on CH2Cl2 and that of Tsang and Walker10 on tetrachloroethene. Combourieu et al. used a discharge-flow system with mass spectrometric detection. Because they monitored only the disappearance of starting material, these results do not allow abstraction of Cl to be distinguished from abstraction of hydrogen. It is thus uncertain to which reaction or reactions the data refer. The shock tube study of Tsang and Walker on C2Cl4 involved a relative rate technique similar to that of the present work. They were able to associate a unique product with the chlorine abstraction channel, so the mechanism and rate constant should be reliable. The data on defluorination are even more sparse than those on dechlorination. The results of Aders et al. appear unreliable as discussed above, so the only available data on fluoromethane are those of Westenberg and de Haas. They state that hydrogen abstraction should be negligible and propose the reaction to proceed via H + CH3F f CH3 + HF. There is no direct evidence of this, however. Their data could refer to abstraction of H, F, or both hydrogen and fluorine. The only other data on abstraction of F from a fluoroalkane of which we are aware are from Kochubei and Moin,11 who reported a study of the reaction of H + CF4 f CF3 + HF. They used gas chromatography to follow the disappearance of CF4 and CO2 in a hydrogen-oxygen flame and determined the rate of fluorine abstraction relative to H + CO2 f CO + OH. At 1100 K, the rate constant reported for CF4 is more than 5 orders of magnitude slower than that for CH3F. Such a large rate difference is unreasonable if the reaction mechanisms both involve fluorine abstraction as suggested. The inference is thus that there is a problem with either the rate data or the mechanistic assumptions of one study or the other. One of the major aims of the present work was to resolve this issue. This is of critical importance, since the rate of the H + RF f R + HF reaction will determine which mechanistic pathways are available in the destruction of fluorinecontaining molecules. Experimental Section Experiments were carried out in a heated single-pulse shock tube. Details of the system have been published previously.12,14
J. Phys. Chem., Vol. 100, No. 17, 1996 7061 Briefly, a gas mixture is shocked, and H atoms are generated from the thermal decomposition of a precursor molecule. The hydrogen atoms react competitively with a reaction standard and the compound to be studied. Compounds are carefully chosen so that each reaction channel leads to a unique product. Following the shock, the gas mixture is immediately analyzed utilizing a series of valves and sample loops to admit controlled amounts into two gas chromatographs equipped with flame ionization detectors. Our source of hydrogen atoms is hexamethylethane (HME), which decomposes thermally via
HME f 2(tert-butyl) f 2(i-C4H8) + 2H
(3)
Decomposition of HME is monitored by isobutene formation, which also gives a measure of the number of H atoms initially released into the system. Since the rate expression k[HME f 2-(tert-butyl)] ) 3 × 1016 exp(-34500/T) s-1 has been previously determined,13 the extent of HME reaction can be used to determine the reaction temperature, a frequent source of uncertainty in single-pulse shock tube experiments. In a typical experiment, a small quantity (50-200 ppm) of HME is reacted in the presence of much larger quantities (0.22.5% v/v) of 1,3,5-trimethylbenzene (135TMB) and 1,2dichlorotetrafluoroethene (DCFE). The 135TMB, in one role, serves as the reaction standard, undergoing the displacement reaction
H + 135TMB f 1,3-(CH3)2C6H4 + CH3
(4)
This is the only route to m-xylene (1,3-dimethylbenzene) in our system. Relative rate experiments over the temperature range 950-1100 K and near 250 kPa have previously determined that k4 ) 6.7 × 1013 exp(-3255/T) cm3 mol-1 s-1.14 This rate expression was measured relative to the well-established15 rate constant k(H + CH4 f CH3 + H2, 950-1200 K) ) 2.4 × 1014 exp(-7000/T) cm3 mol-1 s-1. Comparison of the amounts of the reaction products m-xylene, tetrafluoroethene, and chlorotrifluoroethene gives a direct measure of the relative reaction rates. Thus, k1/k4 ) [C2F4]‚ [135TMB]/[m-xylene][DCFE] and k2/k4 ) [C2ClF3][135TMB]/ [m-xylene][DCFE], where the square brackets refer to concentration. Because of their large excess, the concentrations of 135TMB and DCFE are essentially unchanged over the course of the reaction. Two gas chromatographs were used in the analysis of the reaction mixture. Light organics were separated on a 2.5-m HayeSep N (80-100-mesh) column16 operated isothermally at temperatures ranging from 80 to 110 °C. Larger species were analyzed using a 30-m, 0.53-mm-i.d. 5% phenylmethylsiloxane column operated in the temperature program mode between 35 and 260 °C at 8 °C/min. Retention times and response factors of the species HME, isobutene, 135TMB, m-xylene, DCFE, tetrafluoroethene, and chlorotrifluoroethene were determined from pure samples of the compounds. Our principal experimental problems were trace impurities in DCFE and 135TMB. In the case of DCFE, GC/MS analysis showed a small amount of perfluorocyclobutane to be present in the sample. Perfluorocyclobutane decomposes via c-C4F8 f 2C2F417-19 and could lead to a secondary source of C2F4 in our experiments. However, the reported rate parameters are in good agreement and indicate insignificant decomposition even at the upper temperature range of our study. In agreement with this, the amount of c-C4F8 did not change measurably in the experiments. This also shows that the reverse dimerization is unimportant under our conditions. For our standard reaction,
7062 J. Phys. Chem., Vol. 100, No. 17, 1996
Manion and Tsang
the trace background of m-xylene (0.1-0.3 ppm, depending on the mixture) in 135TMB had to be subtracted from our measurements. This correction is negligible, except at the very lowest degrees of conversion where the background signal is up to 20% of the total. To generate more hydrogen atoms at low temperatures and hence increase the conversion of DCFE and 135TMB to more easily measurable values, we added 20% H2 to our gas mixtures. This has the effect of creating a moderate chain reaction. In the presence of H2, radicals such as Cl and CH3 can regenerate hydrogen atoms via
X + H2 f XH + H
(5)
Because these reactions convert reactive radicals to atomic H, they help eliminate the possible sources of secondary chemistry. As an alternative to reaction with H2, radicals can also abstract a benzylic hydrogen from 135TMB,
(CH3)3C6H3 f (CH3)2C6H3CH2 + RH
(6)
Radical species that could perturb our results are thus removed by both H2 and 135TMB. The 3,5-dimethylbenzyl radical formed in reaction 6 is quite stable and does not decompose under our conditions. Because of its large resonance energy, it reacts only very slowly with closed-shell species in our system. Its primary sink is recombination, either with itself or with other radicals in the system. These recombinations are undoubtedly the primary chain termination steps in this system. Consistent with this, small amounts of 1-ethyl-3,5-dimethylbenzene (the product of methyl + 3,5-dimethylbenzyl) were observed at higher degrees of reaction. Reactions 5 and 6 are particularly significant for atomic chlorine, since it reacts rapidly with hydrocarbons and could possibly induce decomposition of HME. This will be unimportant in the present system, however, because the reactions of Cl with H2 and 135TMB are very fast. For H2, the rate parameters are well established,20,21 with k(Cl + H2 f HCl + H) ) 2.2 × 1013 exp(-2300/T) cm3 mol-1 s-1. Data on the reactivity of Cl with hydrocarbons22-24 demonstrates that the rate of the Cl + 135TMB reaction should be nearly gas kinetic, with a barrier of a few kilojoules or less. Given the above rates and the large excess of H2 and 135TMB, the Cl + HME reaction should be negligible. Reaction 6 with R ) H is one way of removing H atoms from our system without generating a product related to the primary reactions of interest. The ratio of hydrogen abstraction to methyl displacement has been previously determined as kabs/ kdisp ) 5.6 exp(-1086/T),14 so that kabs/kdisp = 2 in the temperature range of our study. Although varying with the mixture, abstraction of H from 135TMB is expected to be a sink for about 30% of the H atoms generated in our system. This decreases the absolute amounts of our observed products but otherwise does not affect the results. This decrease in sensitivity is also offset by the chain reaction induced by the addition of H2 to our mixtures. Because each isobutene is equivalent to one H atom released into the system, we can calculate the chain length from the amount of isobutene in comparison with the summed products from H. After a calculated correction for the unobserved products from the H + (CH3)3C6H3 f (CH3)2C6H3CH2 + H2 reaction, we find a chain length of 2-4 for the mixtures studied here. The value for a particular mixture varied only slightly with the temperature of reaction. The key assumptions that we make in our analysis of data are as follows: (A) that there are no other significant sources
of the products that we identify with channels 1, 2, and 3; (B) that the products do not react further; and (C) that all the chlorofluoroethyl radicals formed in reactions 1 and 2 decompose via expulsion of a chlorine atom. Regarding assumption A, a possible source of haloethenes other than reactions 1 and 2 is through radical reactions of the types R + DCFE f F2CCClF2 + RCl and R + DCFE f FClCCClF2 + RF. The primary R species that we might expect in our system are halogen atoms, benzyl-type radicals, and methyl radicals. In the cases of halogen abstraction by halogens, the reactions are highly endothermic and cannot compete with the fast reactions 5 and 6. Similarly, if R is benzylic, the reaction is = 50 kJ mol-1 endothermic and, based on the halogen-transfer reactions of methyl,22-24 has a minimum intrinsic barrier of 45 kJ mol-1. This leads to an estimated pseudo-first-order half-life of > 1 s, far too slow to occur on the 500-µs time scale of the experiment. Methyl radicals are more reactive than the benzylic species but are effectively scavenged by reactions 5 and 6. Also, the chloromethane product, although not fully separated from c-C4F8 impurity on our column, would have been observed if it were formed in amounts of more than a few percent relative to C2F4. As our results will demonstrate, abstraction of F is much slower than abstraction of Cl and is even less important. The most likely source of C2F4 other than from abstraction of Cl is from simple bond fission:
DCFE f C2F4Cl + Cl
(7)
C2F4Cl f C2F4 + Cl
(8)
Experimental data on C-Cl bond fission are scarce. From shock tube experiments with tetrachloroethene and trichloroethene, Zabel25 has reported the high-pressure rate expressions for C-Cl bond fission to be k∞ ) 4.5 × 1014 exp(-40280/T) s-1 and k∞ ) 7.0 × 1015exp(-42300/T) s-1, respectively. The C-Cl bond energy in DCFE has not been measured, but we estimate it to be about 350 ( 12 kJ mol-1. By using this value, we estimate k7,∞ ≈ 3 × 1015exp(-41100/T) s-1. Even without allowing for falloff, these parameters indicate that only a few percent of the total C2F4 would result from the C-Cl bond fission channel at the highest temperatures. Consistent with this, no systematic differences in the rate constant for C2F4 formation were observed despite a nearly 7-fold variation in the starting concentration of DCFE. This demonstrates that bond fission reactions can be making at most a very small contribution to our data. Assumption B, that our products do not react further, should also be valid. The C-F bonds are very strong, and dissociation will not occur. The primary unimolecular reaction that needs to be considered is dissociation of C2F4
C2F4 f 2CF2 (singlet)
(9)
In a shock tube study,26 Schug and Wagner found k9,∞ ) 2.8 × 1015 exp(-34278/T) s-1 over the temperature range 11501600 K. Based on the pressure dependence observed in that study, under our conditions, k9 will be a factor of 4-5 smaller than the value calculated from the above expression. At the upper end of our temperature range, where the reaction would be most important, this would imply a correction of less than 1% to the C2F4 yields. The reversibility of the reaction further reduces the need for this correction. The analogous dissociation of C2F3Cl is even less important since its carbon-carbon bond is estimated to be at least 40 kJ mol-1 stronger than that of C2F4. Bimolecular reactions of closed-shell species, such as
H Atom Attack on DCFE
J. Phys. Chem., Vol. 100, No. 17, 1996 7063
TABLE 1: List of the Mixtures Used in the Present Investigations mixture
componentsa
A
100 ppm HME; 0.20% v/v 135TMB, 20% v/v DCFE; 20% v/v H2, in Ar 125 ppm HME; 0.20% v/v 135TMB, 3.1% v/v DCFE; 20% v/v H2, in Ar 150 ppm HME; 0.11% v/v 135TMB, 0.46% v/v DCFE; 20% v/v H2, in Ar
B C
a HME ) hexamethylethane; 135TMB ) 1,3,5-trimethylbenzene; DCFE ) 1,2-dichlorotetrafluoroethane.
the known dimerization of chlorotrifluoroethene,27 are far too slow to play a role under our conditions. Bimolecular reactions of the haloethenes with radicals are ruled out by the very large excess of other reactive compounds, H2, DCFE, and 135TMB. Finally, regarding assumption C and the decomposition route of the intermediate radical products, the only unimolecular reaction that might be expected to compete with loss of Cl from chlorofluoroethyl radicals is the alternative expulsion of atomic fluorine. This is energetically unfavorable, however. From the thermodynamic data,28 we calculate that at 1000 K, fission of a C-F bond in CF2CClF2 is 155 kJ mol-1 more endothermic than fission of C-Cl. Since activation energies for the reverse halogen additions are low, the endothermicity difference is approximately equal to the difference in activation energies for the β-fissions. By assuming equal preexponential factors, we calculate that at 1050 K, expulsion of Cl will be more than 7 orders of magnitude faster than F. Decomposition of the radical intermediates must also be fast on the 500-µs time scale of our experiment. The thermodynamic properties of chlorofluoroethyl radicals are unfortunately not well established. In β-chloroethyl radicals, where the thermodynamics are better known, the C-Cl bond strengths are all = 90 kJ mol-1 and the analogous decompositions rapid. For the 1,1,2,2-tetrachloroethyl radical, Benson and O’Neal29 have recommended the rate expression 5 × 1013 exp(-10300/T) s-1. At the lowest temperatures of our study, this would indicate a lifetime of nanoseconds. In CF2CClF2, the situation is less clear. On the basis of C-Cl bond energies in C2F5Cl, C2H5Cl, and 1,2-C2H4Cl2, derived from the thermodynamic data,28 we estimate ∆Hf(CF2CClF2, 1000 K) ) -667 ( 20 kJ mol-1. This leads to a C-Cl bond strength in CF2CClF2 of 135 ( 25 kJ mol-1, much stronger than in related unfluorinated species. By assuming that the reverse chlorine addition has no barrier, as is usual,22-24 and that the A factor is the same as that given earlier, we arrive at k(CF2CClF2 f C2F4 + Cl) ) 5 × 1013 exp(16000/T) s-1. After taking into account falloff, we estimate the half-life for decomposition to be about 10 µs. Similar parameters and rates can be obtained for decomposition of the product of channel 2a, CFClCF2Cl. These rates are fast enough so that the radicals will be unable to undergo any reactions other than decomposition. In our product analyses, we have also looked carefully for other compounds that could stem from the chlorofluoroethyl radicals. Even at the highest levels of conversion, we found no unidentified peaks amounting to more than a few percent of the sum of those from C2F4 and C2F3Cl. All indications are thus that reactions 1b and 2b are complete and quantitative. Results The mixtures used in the experiments are listed in Table 1. Experimental kinetic results are summarized in Figure 1. The concentrations of HME, 135TMB, and DCFE were varied in order to test the correctness of our postulated mechanism. In addition, reactions were carried out at pressures ranging from
100 to 300 kPa. Within experimental error ((10%), none of these changes affected the relative rates. The major products observed were m-xylene, tetrafluoroethene, and methane. Traces of chlorotrifluoroethene were detected in mixtures reacted at higher temperatures. Also found in the experiments in the upper temperature range were traces of three compounds other than C2F4 eluting in the time regime corresponding to fluoroethenes or fluoroethanes. These species were not identified, but summed, they amounted to only between 0.2% and 3% of the C2F4 in experiments at temperatures ranging from 1065 to 1150 K. Methane must be formed from methyl radicals abstracting H from either H2 or 135TMB. The primary source of methyl radicals is reaction 4, H + 135TMB f 1,3-(CH3)2C6H4 + CH3. Almost no C2H6, the recombination product of CH3, was detected. About 90-95% of the methyl radicals formed from 135TMB could be accounted for as methane. Most of the remainder are present in the form of 1-ethyl-3,5-dimethylbenzene (the recombination product of methyl and 3,5-dimethylbenzyl radicals). We were first able to detect chlorotrifluoroethene, the product nominally corresponding to abstraction of fluorine from DCFE, in mixtures reacted at about 1100 K. At this temperature, the ratio C2F4/C2F3Cl ) 180. This number corresponds to the lower limit for the ratio of chlorine to fluorine abstraction and shows that abstraction of Cl is overwhelmingly favored at these temperatures. Figure 1 shows an Arrhenius-type plot of k4/k1, the relative rate constant for displacement of methyl from 135TMB and abstraction of Cl from DCFE with atomic hydrogen as the attacking agent. The following rate expression is obtained from experiments over the temperature range 970-1140 K
log k4/k1 ) (-0.496 ( 0.130) + (-1122 ( 141)/T The above uncertainties are 2σ and are a measure of precision only. It is well-known that experimental uncertainties are often systematic. Overall, we estimate that the relative rate constants should be accurate to within (10%, while the estimated uncertainty in the relative activation energies is about 5 kJ mol-1. By using k4 ) 6.7 × 1013 exp(-3255/T) cm3 mol-1 s-1,14 we obtain
k1 ) 2.1 × 1014 exp(-5839/T) cm3 mol-1 s-1 Taking into account the uncertainties in the standard reaction, the measurement, and possible systematic errors, the absolute uncertainties are estimated to be about a factor of 1.4 in rate, 10 kJ mol-1 in the activation energy, and a factor of 3 in the A factor. In order to prevent interference from secondary reactions, we generally prefer to limit ourselves to experiments where the total conversion of HME is e 50%. This corresponds to temperatures e 1140 K. In the case of fluorine abstraction, however, this would give us too short a temperature range to obtain Arrhenius parameters for the reaction. We therefore did experiments at temperatures as high as 1195 K, corresponding to about 75% HME conversion. Data obtained on the rate of chlorine abstraction, although not used in the determination of k1, were in excellent agreement with the data from lower temperature experiments. For formation of C2F3Cl, the product nominally pertaining to fluorine abstraction, we obtained the parameters k2 ) 4 × 1022 exp(-32500/T) cm3 mol-1 s-1 from the experiments with mixture A between 1100 and 1195 K. The apparent A factor is obviously too large. This is a strong indication that at least some of our C2F3Cl stems from reactions
7064 J. Phys. Chem., Vol. 100, No. 17, 1996
Manion and Tsang
TABLE 2: Literature Rate Constants for Abstraction of Chlorine and Fluorine from Alkanes and Alkenes by Atomic Hydrogen compound (ref) CH3Cl (5)a CH3Cl (6)b CH3Cl (6)b CH3Cl (7,8)c CH2Cl2 (9)b C2Cl4 (10) CF2ClCF2Cl (this work)
CH3F (6)d
rate expression, cm3 mol-1 s-1 [temp/K]
k, cm3 mol-1 s-1 (temp/K)
A. Reported Rate Constants for Abstraction of Cl by H 1.1 × 1013 exp(-2320/T) [298-652] 3.7 × 1013 exp(-4683/T) [500-800] 6.4 × 1014 exp(-6752/T) [775-998]
2.3 × 1011 (600) 1.5 × 1010 (600) 7.7 × 1011 (1004) 3.5 × 1011 (1004) 6.6 × 1010 (600) 6.9 × 1011 (1004) 4.6 × 109 (600) 6.6 × 1011 (1004) 1.2 × 1010 (600)
1.1 × 1013 exp(-3070/T) [298-460] 1.2 × 1015 exp(-7484/T) [950-1100] 2.1 × 1014 exp(-5839/T) [970-1140] B. Reported Rate Constants for Abstraction of F by H 1.8 × 1013 exp(-4730/T) [605-871]
CH3F (5)a
6.3 × 1013 exp(-2617/T) [298-652]
CF4 (11) CF2ClCF2Cl (this work)
1.1 × 1015 exp(-22300/T) [1173-1573]
6.8 × 109 (600) 2.4 × 1011 (1100) 8.0 × 1011 (600) 5.8 × 1012 (1100) 1.7 × 106 (1100) e7 × 109 (1100)
a Rates from this study appear too high when compared with other measurements. b The authors proposed the main reaction to be abstraction of Cl, but no attempt was made to distinguish that reaction from abstraction of hydrogen. c Based on relative rates of dechlorination of CH3Cl and C6H5Cl. d We suggest these results refer to abstraction of H, H + CH3F f H2 + CH2F (see text).
TABLE 3: Chlorine and Fluorine Bond Strengths of Selected R-X Species at 298 Ka R X F Cl
H 570 431
CH3 465 350
CFClCF2Cl b
(502)
C2F5
CF3
C2Cl3
CF2CF2Cl
531 351
543 358
351
(350)b
a
Unless noted, bond energies are derived from the data of ref 28. Units are kJ mol-1. b Estimated by present authors.
Figure 1. Plot of the rate constant for the chlorine abstraction H + DCFE f F2CCF2Cl + HCl relative to the standard reaction H + 135TMB f m-xylene + CH3. Symbols: open circles, mixture A, 2 bar; open triangles, mixture B, 3 bar; filled triangles, mixture B, 5 bar; open squares, mixture C, 2 bar. Least-squares fit (970-1140 K): log k4/k1 ) -0.496 + 1122/T.
other than (2). We are thus unable to recommend rate parameters for this reaction. From our data, we can only determine a maximum rate, obtaining
k2(1100 K) e 7 × 109 cm3 mol-1 s-1 If the preexponential factor is assumed to be 8 × 1014 cm3 mol-1 s-1, we calculate E2 g 110 kJ mol-1. Discussion The results presented above indicate that the rates of formation of m-xylene and C2F4 can be related to the rates of the elementary reactions of hydrogen with 135TMB and DCFE. Literature rate expressions for abstraction of chlorine by atomic hydrogen are listed in Table 2. Compared with previous results on C2Cl4,10 we find that Cl is abstracted from DCFE about twice as fast on a per-chlorine basis. The measured activation energy and A factor are smaller for the alkane, although on a per-site basis, the preexponential factors are the same within the combined experimental uncertainties. Compared with chloromethane, on a per-chlorine basis, DCFE reacts either half as fast or with about equal rate, depending on which literature
results5-8 are selected. In any case, all of the examined compounds have similar rates of Cl abstraction by atomic hydrogen. Table 3 shows that these species all have C-Cl bond energies of about 350 kJ mol-1, although we note that the values for DCFE and C2Cl4 are relatively uncertain. It is thus unclear to what extent the rate may scale with the strength of the C-Cl bond. It may be that larger rate differences will be observed for species with significantly weaker or stronger C-Cl bonds. As discussed in the Results section, C2F3Cl is formed in much smaller quantities than C2F4. It is present at about the same level as trace secondary products. Moreover, the abnormally high Arrhenius parameters indicate that at least some of this product does not stem from abstraction of fluorine by atomic hydrogen. Our data therefore pertain to the maximum rate and show only that abstraction of fluorine from DCFE is very slow. Figure 2 compares the present results with literature reports on the rate of fluorine abstraction from CH3F and CF4. Our results are not consistent with the data of either Aders et al.5 or of Westenberg and de Haas6 on fluorine abstraction from CH3F. Extrapolating the rate constant of Aders et al. to 1100 K yields a rate that is 3300 times faster on a per-fluorine basis than the maximum rate found here for DCFE. This seems to indicate that product formation in their system was not due to gas-phase abstraction of F. At 1100 K, the rate constant of Westenberg and de Haas is 140 times faster than our maximum value, again on a per-fluorine basis. Their activation energy is 70 kJ mol-1 lower than the minimum value that we estimate from our data. While a C-F bond in DCFE is about 35 kJ mol-1 stronger than that of fluoromethane (Table 3), abstraction of fluorine by atomic hydrogen is highly exothermic in both cases. The rate difference cannot therefore be attributed to the difference in thermochemistry. One explanation is that because Westenberg and de Haas monitored only the disappearance of their mass spectral signal for CH3F, their results could be perturbed by
H Atom Attack on DCFE
J. Phys. Chem., Vol. 100, No. 17, 1996 7065 defluorination reactions involving the carbon-centered radical fragments. Such routes are still slow, however, so it is not surprising that perfluorinated compounds are among the most thermally stable of all organic compounds. Acknowledgment. This work was sponsored by the Strategic Environmental Research and Development Program (SERDP). References and Notes
Figure 2. Comparison of reported rates of fluorine abstraction by atomic hydrogen. Solid lines: H + CH3F f CH3 + HF, refs 5 and 6; dashed line, H + CF4 f CF3 + HF, ref 11; square, maximum value for H + DCFE f C2F3Cl2 + HF from this work. The data on H + CH3F are suggested to be incorrect or pertain to abstraction of hydrogen, H + CH3F f CH2F + H2 (see text).
alternative reaction pathways. We suggest that their data are probably correct but refer to abstraction of hydrogen and not fluorine. The present results are in much better agreement with those of Kochubei and Moin on CF4. Their data suggest a rate constant for fluorine abstraction of some 3 orders of magnitude smaller than our maximum value at 1100 K. This is not inconsistent, however, since we were only able to obtain a maximum value. If their data are correct, it would indicate that the traces of C2F3Cl observed in our experiments are due entirely to secondary reactions or impurities. Given that we found obviously incorrect Arrhenius parameters for the C2F3Cl formation channel, this is not an unlikely scenario. Thus, although we are unable to directly confirm the rate expression of Kochubei and Moin for fluorine abstraction by atomic hydrogen, it is the only experimental value that is compatible with the present data. The above results have several important implications with regard to the behavior of fluorinated organics in combustion and other systems at high temperatures. First, it is clear that direct attack of atomic hydrogen on the C-F bond is not a facile channel for the removal of fluorine from organics, despite the exothermicity of the reaction. Because attack by atomic hydrogen is by far the most favorable of the metathesis reactions involving abstraction of F, it is unlikely that any radical typically present in combustion systems will readily abstract fluorine from a closed-shell species in the gas phase. Removal of fluorine from organics is therefore expected to proceed mostly by indirect routes. In partially fluorinated molecules, this is likely to include unimolecular elimination of HF and radical attack on the more labile portions of the molecule. Species which lack these channels, such as perfluoroalkanes, have no obvious low-energy reaction pathways. The extreme case is CF4, which is anticipated to react only by fission of the extremely strong C-F bonds or by the very slow abstraction of fluorine. In larger fluorinated organics, carbon-carbon bond fissions will eventually lead to
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