719
J. Phys. Chem. 1982, 86, 719-723
Thermal, Unsensltlzed Infrared Laser, and Laser SIF, Sensitized Decomposition of 1,P-Dichloropropane W. Tsang;
J. A. Walker, and W. Braun
National Bureau of Standards, Chemical Kinetics Division, Washington, D.C. 20234 (Received: July 27, 198 1; I n Final Form: October 5, 1981)
1,2-Dichloropropanedecomposes via four reaction channels forming 3-chloropropene, cis-1-chloropropene, trans-1-chloropropene,and 2-chloropropene. All pathways have been observed in thermal and laser-induced processes. Rate parameters for the thermal processes have been derived from comparative rate, single-pulse shock-tubestudies. They are as follows: k(3-chloropropene)= 1013.48 exp(-27187/T) s-l; k(cis-1-chloropropene) - 1012.98exp(-26853/ 7') s-l; k(truns-1-chloropropene)= 1013.19exp(-27855/ T ) s-l; k(2-chloropropene)= 1013.05 exp(-29782/T) s-l; k(cis,trans-1-chloropropene)= 1013.34 exp(-27235/T) s-l; k(tota1) = 1013.70 exp(-27250/T) d;T = 940-1060 K. The focused-laser experiments, both unsensitized as well as those sensitized with SiF4, yield product ratios which are very similar and suggest that the sensitized experiments also inolve a photolytic process. With the thermal results as a base, we find lifetimes for laser-induced decomposition of s and energy content prior to decompositionequivalent to 41 photons per molecule. In the nonfocused SiF, sensitized experiments the results indicate a temperature of 1100 K and reaction time of -1 ps. The cis-1-chloropropene to tram-1-chloropropeneratios from the laser experimentssuggest that this represents a final product distribution.
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Introduction This paper is concerned with the stability of 1,2-dichloropropane under a variety of excitation mechanisms: viz., thermal (via comparative-rate, single-pulse shock-tube studies), and unsensitized or sensitized [with SiF4 as the sensitizer] infrared-multiphoton photolysis with a COP TEA-laser under focused and nonfocused conditions. 1,2-Dichloropropane has previously been thermally decomposed in flow and static e~periments.'-~The decomposition mechanism involves the four-center elimination of HC1 through a variety of reactive channels. The specific processes can be seen in Figure 1. Experimental rate parameters from the earlier studies are summarized in Table I. Our primary interest is in the rate constants and Arrhenius parameters for the individual reaction channels. It can be seen that the published data show wide variations. Nevertheless, it is clear that elimination of a chlorine atom from a secondary carbon is considerably faster than for that from a primary carbon. Due to the fact that the latter constitutes such a minor channel, the published rate parameters are undoubtedly in error, and it has been suggested that surface reactions are the primary cause. Comparative-rate, single-pulse shock-tube experiments offer a clearcut means of determining high-accuracy rate parameters for these processes.4 Such experiments are free from surface effects, and, by suitable choice of conditions, radical processes can be suppressed. Through the presence of an internal standard the usual uncertainties in the rate parameters are drastically reduced. Such results are of intrinsic interest inasmuch as they are the fundamental quantities which determine the stability of polyatomic molecules. In the present context, the thermal data form a basis for the interpretation of the laser-induced reactions. Of particular importance is the multiplicity of reaction channels, since (1) D. H. R. Barton and A. J. Head, Tram. Faraday SOC.,46, 114 (1950). (2) G. J. Martens, M. Godfroid, and L. Ramoisy, Int. J. Chem. Kinet., 2, 123 (1970). (3) K. A. Holbrook and J. S. Palmer, Trans. Faraday SOC.,67, 80 (1971). (4) W. Tsang, J . Chem. Phys., 41, 2487 (1964).
TABLE I: Previous Results on 1,2-Dichloropropane Decompositiona ref temp, K rate parameters, s-l 1 689-725 2
653-863
exp(-27644/T) k(tota1) = k(tota1) = exp(-26722/T) k ( 3-chloropropene):k(1-chloropropene): k ( 2-chloropropene) = 1 9 : 1 1 . 5 : 1 k (cis-1-chloropropene) : k(trans-1 chloropropene) = 1 0 . 0 9 e x p ( l l S O / R T ) k(tota1) = exp(-27573/T) k ( 3 chloropropene) = exp(- 2 7 1 9 0 i T ) k(cis-1-chloropropene) = lo".' exp(- 2 7 4 4 2 / T ) h(truns-1 chloropropene) = exp(-28248/T) k ( 2-chloropropene) = lo8.' e x p ( - 2 1 5 9 1 / T ) ~
3 666-743
a Other less reliable data are summarized in ref 2 .
the product distribution can be used to deduce information on the state of the molecule immediately prior to decomposition. This is of considerable significancefor the proper interpretation of the laser-induced process. Work of a similar nature has been carried out by Colussi, Benson, Hwang, and Tiee,5 Rosenfield, Brauman, Barker, and Golden! and Richardson and Setser.' The processes of interest were CH2DCH2Cl C2H4+ DCl C2H3D + HCI
---+
CzH50CzH3
FCH2CHzBr
CH3CHO + CzH4 CzH5. + .0C2H3
+
C2H3F HBr C2H3Br+ HF
If we assume energy randomization, lifetimes were of the (5) A. J. Colussi, S.W. Benson, R. J. Hwang, and J . J . Tiee, Chem. Phys. Lett., 52, 349 (1977). (6) R. N . Rosenfeld, J. L. Brauman, J. R. Barker, and D. M. Golden, J. Am. Chem. SOC.,99, 8063 (1977). (7) T. H. Richardson, and D. W. Setser, J . Phys. Chem., 81, 2301 (1977).
This article not subject to U S . Copyright. Published 1982 by the American Chemical Society
720
The Journal of Physical Chemistry, Vol. 86, No. 5, 1982
Tsang et ai.
REACTION MECHANISM 25
CHT-CH-CH2CI
H
1,2-DICHLOROPROPANE
-CI 3-CHLOROPROPENE + HCI
20
CIS-1-CHLOROPROPENE
15
CHTCH=CHCI
'=, +--CI --H
TRANS -I-CHLOROPROPENE
~\
+ HCI '~\ \
CHrCCI--CH2 H ---ti
'\ 2-CHLOROPROPENE + HCI
Flgure 1. Mechanism for the thermal decomposition of 1,2-dichloropropane.
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order of lo*, and s, respectively. The exact quantitative significance of these results is difficult to assess because high-accuracy thermal data were not available, and the conclusions are necessarily uncertain due to the need for estimating the thermal rate parameters. For ethyl vinyl ether, there are also uncertainties in the reaction mechanism since unstable products are formed. In this paper we utilize a high-accuracy method to determine these numbers, and with the availability of four reaction channels, any conclusions can be substantiated. Finally, we wish to investigate in a similar manner the SiF,-sensitized decomposition. The information to be obtained from the laser studies are excited-state lifetimes, energies, time scales, and temperatures.
Experimental Section The experimental setup and procedure for the shocktube and laser-hotolysis experiments have been reported in detailgl0 previously. Gas chromatography with isohexyl sebacate columns and flame-ionization detection are used for analysis. The internal standard for the comparativerate, single-pulse shock-tube experiments was the reverse Diels-Alder decyclization of 4-methylcyclohexene with the rate expression
The unsensitized photolytic decomposition of 1,2-dichloropropane was carried out with the 9.28-pm line, while the sensitized decomposition studies used infrared radiation at 9.74 pm. It should be noted that although there may be some degree of absorption, without sensitization, reaction yields at the 9.74-pm line were insignificant. The COz TEA laser used had an energy of approximately 0.3 J/pulse and a pulse rate of 1-2 pulses/s. Photolysis was carried out until sufficient products have been accumulated. No attempt was made to measure energy absorbed per pulse. 1,2-Dichloropropane was purchased from Aldrich Chemical.ls SiF, was from Matheson, while the 4methylcyclohexene and toluene were from Chemical Sam(8)W.Tsang, J . Chem. Phys., 40,1498 (1964). (9)W.Braun and W. Tsang, Chem. Phys. Lett., 44,354 (1976);D. Gutman, W.Braun, and W. Tsang, J. Chem. Phys., 67,4291 (1977). (IO) W.Tsang, J . Chem. Phys., 42, 1805 (1964).
05
0
LOGlo
KT
10
I4.METHVLCYCLOHEXENE]
15
[i'i
Flgure 2. Comparative-rate plots for the processes involved in the thermal decomposition of 1,2dichloropropane. 4-Methyicyclohexene decomposition is the internal standard. Shock-tube studies are with 0.05% 1,2-C3H&I, and 0.02% 4-methylcyclohexene in 1% toluene and argon at 1.5-5.0 atm pressure. Temperature = 940-1060 K.
ples Co. Except for the usual vacuum degassing all substances were used without further purification.
Results The main products in all the decomposition studies are the four isomeric chloropropenes. In the case of the shock-induced decomposition, no other products were detectable. From the laser-pyrolysis studies, smaller quantities of lighter hydrocarbons, principally methane and Cz compounds, are also formed. We feel that they are not present in sufficient quantities to affect our conclusions. The results of comparative-rate, single-pulse shock-tube studies are summarized in Figure 2. The rate expressions for the formation of the four isomeric chloropropenes are ki=--1n t xl i
(
1-
[ c h l o r ~ p r o p e n e ]Xi ~~~/ [ 1,2-dichloropropane]inital
)
where
xi = c 4
j=1
[chl~ropropene]~,,+ [chl~ropropene]~~~~
and the 4 refers to the products. For 4-methylcyclohexene decomposition, the rate expression is k(4-methylcyclohexene) =
The rate relations are log k(3-chloropropene) = (0.814 f 0.004) log k[CH3C6H9]+ (1.024 f 0.004) log k(cis-1-chloropropene) = (0.804 f 0.004) log k[CH3C6Hg]+ (0.677 f 0.004)
The Journal of Physical Chemistry, Vol. 86, No. 5, 1982
Decomposition of 1,P-Dichioropropane
log k(trans-1-chloropropene)= (0.834 f 0.004) log k[CH&eHg]
721
+ (0.430 f 0.004)
log k(2-chloropropene) = (0.892 f 0.01) log k[CH&Hg] - (0.594 f 0.01) and lead to k(3-chloropropene) = 1013.48exp(-27187/T) s-l k(cis-1-chloropropene) = 1012.98exp(-26853/T) s-l k(trans-1-chloropropene)= 1013.19exp(-27855/7') s-l k(2-chloropropene) = 1013.06exp(-29782/ T ) s-l in the temperature range 950-1060 K. Furthermore k(cis-1-chloropropene + trans-1-chloropropene) = 1013.34 exp(-27235/T) s-l k(3-chloropropene + 1-chloropropene + 2-chloropropene) = 1013.'0 exp(-27250/ T ) s-l The following ratios are of particular interest: k(3-chloropropene)/{k(cis-l-chloropropene+ trans-1-chloropropene)] = 1.38 exp(48/T) k(3-chloropropene)/k(2-chloropropene)= 2.69 exp(2595/T)
-
-
1.46 320
31-42
k(cis-1-chloropropene)/k(trans-1-chloropropene) = 1.6-1.8 exp(1002/T)
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The results of the laser-induced experiments are summarized in Table 11. They are displayed in the form of the three ratios given above so as to facilitate comparisons. In the case of the unfocused photolysis we also include data on yield per pulse. Although the data show some scatter the general pattern is clear. Under all conditions the ratios of cis-1-chloropropene/trans-1-chloropropene and 3chloropropene/l-chloropropeneare constant at 2 and 1.5, respectively. At pressures under 32 torr (1torr = 133.3 Pa) the results of the focused and unfocused photolytic studies show wide differences. The ratio of 3-chloropropene/2-~hloropropeneranges from 10 for the former to -27 for the latter. Of particular interest is the relative similarity of the results for the SiF4-sensitized decomposition and the unsensitized photolytic data with focused radiation. At the highest pressures (32 torr) the unsensitized, focused photolytic data and the unfocused, SiF4-sensitizedresults appear to converge to a ratio of 17 for 3-chloropropene/2-chloropropene. The high-pressure rate parameters parameters permit the calculation of the specific rates of decomposition of vibrationally excited 1,2-dichloropropane molecules at all levels of excitation. We have used the procedure suggested by F0rst.l' The fundamental relation is k(E) = A,N(E - E , , ) / N ( E ) E > E,, =0 E C E,,
-
-
where N ( E ) is the density of states of the reactant molecules at energy E , and A , and E,, are the high-pressure, unimolecular rate parameters. The substantial equivalence of the specific rates obtained in this fashion with that from RRKM calculations have been demonstrated.12 It should be especially satisfactory in this application since, unlike the situation involved in bond-breaking processes, the temperature dependence of the Arrhenius parameters (11)W.Forst, J. Phys. Chem., 76,342 (1972). (12)W.Tsang, Znt. J. Chem. Kinet., 8,193 (1976).
400
480
560
ENERGY, KJ/MOLECULE
Figure 3. Calculated specific rates of reaction channels in 1,2dichloropropane decomposition as a function of energy per mol. Compounds are the decomposition products.
should be much smaller. Furthermore, inherent with RRKM calculations are assumptions regarding transition-state structure and the variation of the entropy of activation with temperature. Frost's approach deals with observables and, in view of his latest work,13 also permits an assessment of errors. The results of such calculations are summarized in Figure 3 and are based on the following pattern of vibrational frequencies (cm-') for 1,2-dichloropropane: 2900 (6), 1400 (3), 1300 (2), 1200 (2), 1100 (3), 900,800, 700, 600, 400, 350 (4), and 2 free rotors. These have been estimated from the published vibrational analysis of isopropyl chloride and n-propyl chloride.14J5 The density of states is calculated on the basis of the Whitten-Rabinovitch approximatiode and should be quite accurate at the present levels of excitation.
Discussion The thermal results derived from the shock-tube experiments are in satisfactory agreement with the general pattern of literature results summarized in Table I. In terms of the total rate constant (sum of all the individual specific rates) the extrapolated shock-tube results are factors of 1.4,2.5, and 1.3 larger than the results of Barton and Head,' Martens, Godfroid, and Ramoisy,2 and Holbrook and Palmer? respectively. Since the data now cover 8 orders of magnitude in rate constants, better agreement than that obtained in the first' and last3 cases probably cannot be expected. The disagreement with respect to the second case2 may be due to the fact that the earlier ex(13)W.Forst, J . Phys. Chem., 83,100 (1979). (14)J. H.S. Green and D. J. Holden, J. Chem. SOC.,1794 (1962). (15)N. Sheppard, J. Chem. SOC.,533 (1950). (16)W.Forst, "Theory of Unimolecular Reactions", Academic Press, New York, 1973,p 105. (17)W.Tsang, Znt. J. Chem. Kinet., 8, 173 (1976). (18)Certain commercial materials and equipment are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the purpose.
722
The Journal of Physical Chemistry, Vol. 86,No. 5, 7982
Tsang et al.
TABLE 11: Results of Laser Photolysis Experiments
sample press., torr
cis-lchloro3-chloro- 3-chloro- propene/ trans-lpropene/ propene/ 2-chloro- 1-chloro- chloropropene propene propene
unsensitized focused experiments with neat 1,2-dichloropropane at 9.28 pm
8 16 32
10.5 9.0 15.1
1.41 1.41 1.5
2.24 2.11 2.07
SiF, sensitized decomposition of 1,2-dichloropropane with focused 9.74 p m radiation, SiF4/l,2-dichloropropane = 2
2 4 8 16 32
9.3 10.1 8.9 9.1 10.1
1.51 1.51 1.38 1.62
2.35 2.02 2.02 2.05 2.1
3 4 6 8 16 32
27.5 29.5 32 30 25 17.7
1.43 1.55 1.62 1.53 1.48 1.36
2.21 2.03 2.18 1.99 1.85 1.87
SiF,-sensitized decomposition of 1,2-dichloropropane with unfocused 9.74 p m , SiF4/l,2-dichloropropane =2
periments are derived from flow determinations and thus the conditions are subject to larger errors. The distribution of the products [3-chloropropene, cis-1-chloropropene, and trans-1-chloropropene] found in this study are in substantial agreement with the results of Holbrook and Palmer: and Martins, Godfroid, and Ramoisy.2 The rate parameters for the production of these three products in the former work track our results very well. The one significant area of disagreement is in the rate constants and parameters for 2-chloropropene production. The present results are to be preferred. 2-Chloropropene production represents an extremely minor decompositional channel. In the older experiments, using classical techniques, surface effects are known to significantly affect 2-chloropropene yields. Indeed, Holbrook and Palmer3 attribute the peculiar rate parameters for this process to contributions from surface effects. Obviously, such phenomena cannot occur in shock-tube experiments. The rate parameters from the present study follow the general patterns that have been observed in earlier shock-tube studies involving four-center molecular elimination processes. Particularly striking is the constancy of the Arrhenius A factor per available H atom. In the present case the numbers are 1013.0,1013.05,and s-l for 3-chloropropene,2-chloropropene,and 1-chloropropene, respectively. This can be compared with a value of 1012.93 s-l for all data involving dehydrohalogenation from simple alkyl halides. The 247.4 kJ/mol activation energy for 2-chloropropene formation is 15.1 kJ/mol higher than that found by Barton for n-propyl chloride and is in line with the difference of 12.1 kJ/mol between 2-chloropropene formation (225.9 kJ/mol) and isopropyl chloride decomposition (213.8 kJ/mol). The 3-chloropropene/l-chloropropene ratio of 1.46 is very close to the statistical value of 1.5. This is contrary to what is expected if C-H bond strengths are to play any role in these dehydrochlorination processes. The present results demonstrate that, for dehydrochlorination, the position of chlorine substitution has exactly the same effect on the 1 or 3 dehydrochlorination processes. We have noted analogous effects in dehydration. Combination of the data from the laser experiments with the thermal results permit deductions with respect to the nature of the laser-induced process. The two possibilities are (1) some new type of decomposition mechanism (bond-specificchemistry?) and (2) randomization of energy with decomposition occurring out of a variety of distribution functions. As could have been expected, our results
convn per pulse, %
0.2 0.5 0.39 0.45 3.3
give very little support for the first possibility. It is obvious that all four decomposition channels are operative, even though, in the case of the direct photolysis, energy is being poured in at a frequency associated with one particular normal mode. Of special interest is the equivalence of the two sets of results with focused radiation. In the case of the SiF4-sensitizeddecomposition some sort of collisional energy transfer must occur and it would be extraordinary in the context of nonradomization of energy to have the ultimate results turn out to be equivalent. Further quantitative support is furnished from the equality of the activation energies for 1-chloropropene and 3-chloropropene formation. For such a situation, the branching ratio for those two channels must be constant if energy is randomized irrespective of the nature of the distribution function. An examination of the 3-chloropropene/ 1chloropropene ratios tabulated earlier for all experiments shows that as required a constant value of -1.5 is obtained. Energy randomization also means that the branching ratios must yield the s&ne energy content for the decomposing molecule. In Figure 4 we plot calculated branching ratios as a function of energy content in the molecule, assumming a delta function with respect to the energy distribution function of reacting molecules. This assumptions has been used by Colussi, Benson, Hwang, and Tiee5 and for present purposes should be satisfactory.16 It can be seen that the 3-chloropropene/2-chloropropeneand l-chloropropene/2-chloropropeneratios results yield an energy content of -490 kJ mol-' immediately prior to decomposition. This is equivalent to 41 photons mol-' and suggests a lifetime of 6 X 10-lo s. This presupposes a photolytic (non-Boltzmann)mechanism for decomposition. Results for a thermal distribution are illustrated in Figure 5. It can be seen that our data now project a temperature of approximately 2000 K with a corresponding lifetime of 2 X lo-* s. This is shorter than the laser pulse and indicates that the former is closer to the true situation. The equivalence of sensitized and unsensitized photolytic results with focussed radiation is also suggestive of this. It strikes one as extremely unlikely for the two cases, with the drastic differences in excitation, to somehow result in the same temperature. We suspect that in the sensitized situation sufficient energy is transferred from vibrationally excited SiF, to 1,2-dichloropropane so as to excite it into vibrationally excited states where it can now strongly absorb the infrared radiation and ultimately decompose. We cannot specify the levels at which this occurs but it must
The Journal of Physical Chemistty, Vol. 86, No. 5, 1982
Decomposition of 1,2-Dichloropropane
'
723
I
0 W 0 w W
i
t 2000K
N
334
320
0.5
400
480
\
IlOOK
560
1.0
1000/TIKI
ENERGY, KJ/MOLECULE Flgure 4. Calculated specific rate ratios for 3-chloropropene/2chloropropene and l-chloropropene/2-chloropropene, and k, (total) as function of energy. The experimental ratios and consequent energies and specific rates are given. The dashed lines connect ratios with total rates. An altenative treatment of data is given in Figure 5 (see text for further details).
Flgure 5. Calculated rate constant ratios for 3-chloropropene/2chloropropene and l-chloropropene/2-chloropropene, and k T(total) as a function of temperature. The experimental rates and consequent temperature with specific rates are given. The dashed lines connect ratios with total rates. An alternative treatment of data is given in Figure 4 (see text for further details).
be fairly low lying. With respect to the sensitized decomposition with unfocused radiation, a photolytic mechanism will imply -80 kJ mol-' excess energy prior to decomposition and a lifetime of about the duration of the laser pulse, 100 ns (see Figure 4). This would appear to be a rather long time in view of the pressures at which these experiments are carried out. At 16 torr this implies 60-70 collisions and would suggest deactivation plus heating of the system. In contrast, at lifetimes of 5 X 10-los, the collision number approaches one only at pressure in excess of 50 torr. This may well account for the observed divergence as one approaches this pressure. From Figure 5 it can be seen that a thermal mechanism will suggest a temperature near 1100 K with a rate constant of 1500 s-l, and, from our conversion per pulse, a minimum thermal reaction time of the order of 1-2 ps. [This allows for the possibility of a photolytic contribution.] Considering the size of the cell and the nature of the excitation the numbers are not unreasonable. Finally, we note that the seemingly higher state of excitation in the 32 torr runs is accompanied by much larger conversions per pulse. It may be that at these higher pressures sufficient energy is accumulated in SiF4so as to excite 192-dichloropropaneinto the absorption continuum
and thus add a photolytic component into the essentially thermal process. On the other hand, in the photolysis with focused radiation, the 32-torr results deviate from the lower-pressure results due to the introduction of a thermal component. The data on cis-l-chloropropeneltrans-l-chloropropene ratios are not in accordance with the explanations given above. In particular, for a photolysis experiment, the 490 kJ mol-l energy should have yielded a product ratio of close to 1 as opposed to a measured value of 2. Similarly, a thermal mechanism would have led to a reaction temperature of 800 K and a preposterous lifetime of 0.5 s. In view of the fact that the thermal ratio of 1.6-1.8 is very close to the measured number of 2 we suspect that the presently observed distribution of the cis- and trans-lchloropropene is a reflection of the final product distribution after passage of the activated molecule across the reaction barrier. That is, molecular elimination occurs previous to the formation of the double bond. The above is indicative of the complexity of the phenomena associated with infrared multiphoton induced decomposition. It would appear that a wide range of processes can occur and that only with definitive highpressure unimolecular rate parameters can unambiguous interpretations be made.
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