3372
J. Phys. Chem. 1900, 84, 3372-3377
A Reexamination of the Photodissociation of CH2CICH2CIat 147 nm. Test for Chlorine Atom Reactions T. Yano and E. Tschuikow-Roux' Department of Chemistry, UniversnY of Calgaty, Calgary, Alberta, Canada T2N IN4 (Received: April 30, 1960)
1,2-Dichloroethanehas been photolyzed at 147 nm and -23 "C as a function of reactant pressure, the additives NO and CF4,and the conversion (ItlN) which is a significant variable of the system. Also investigated was the 147-nm photolysis of a mixture of 1,2-C2H4ClZ and c2H6 which provided unequivocal evidence for the presence of chlorine atoms in the system. The major primary process is the molecular production of ethylene = 0.67) by two C1-atom elimination. Other observed reaction products are C2H3Cland smaller quantities of C2H2and CH2C1CHC12 Approximately half of the vinyl chloride yield = 0.16) derives from the primary molecular elimination of HC1, while the balance can be attributed to radical reactions. The reaction kinetics are interpreted in terms of competitive processes of chlorine atoms which include H abstraction from the parent, C1 addition to the product olefins, and NO-catalyzed chlorine atom recombination, with H abstraction being the dominant secondary process. The results are discussed in relation to a recent investigation of this system, and contrasted to the 147-nm photolyses of more highly substituted chlorofluoroethanes for which the "two C1-atom" mechanism has been demonstrated previously. The extinction coefficient for 1,2-C2H4C12at 147 nm (296 K) has been determined as e = (l/PL) In (&/It) = 525 f 90 atm-' cm-'.
Introduction The 147-nm photodecomposition of a,@-disubstituted chloro- and chlorofluoroethanes1b2has shown that the formation of the major product olefins is associated primarily with dehalogenation processes. HC1 elimination also occurs but not to the extent as found in the case of some a-substituted chloro- and chlorofluoroethanes, where it is the principal reaction ~hannel."~In the photolyses of CH2FCH2Cl' and CH2C1CH2Cl2the yield of C2H4 was virtually unaffected by the addition of nitric oxide. This fact, coupled with the ostensible absence, or only low yield of C4 products from radical precursors, has led us to postulate that dehalogenation occurs primarily via molecular elimation. While such a process may indeed occur from an electronically excited state(s) upon photon absorption, the subsequent stability of molecular halogen (C12 or FC1) has remained an area of concern, in view of the large excess energies involved. In our recent studies of the 147-nm photodecomposition of CF2C1CH2C1,BCF2C1CHCl2? and CF3CHC1t we have obtained evidence that the principal mode of decomposition to the corresponding olefins involves the production of chlorine atoms, either by very rapid sequential (near simoultaneous) C-C1 bond scission reactions, or by the dissociation of an excited C12* molecule produced by molecular elimination in the primary process. In these studies the presence of chlorine atoms was diagnosed, in part, from the reactions by which the chlorine atoms are removed, specifically,C1 addition to the product olefins and hydrogen abstraction from the parent compound. Despite the very low conversions and consequently very low olefin concentrations the addition reactions assume a competitive role in these systems owing to the increasingly higher activation energy for hydrogen abstractionDwith increasing halogen, and, in particular, fluorine substitution.1° In good agreement with experiment, it has been s h o ~ nthat ~ . ~at constant total pressure the reciprocal of the observed olefin quantum yield (4) is a linear function of the conversion (ItlN),for small values of I t / N 4-1 = 40-1 + rn(It/N) (1) where $o is the true quantum yield and m is the rate constant ratio for the addition/abstraction reactions. 0022-3654/80/2084-3372$01.0010
Since C2H4 is the observed principal primary product in the photolysis of CHzC1CH2C12which showed one of the highest quantum yields for dehalogenation, the two chlorine atom elimination mechanism is also expected to occur. However, in this instance, the rate of hydrogen abstraction should be predominant owing to the lower activation energy for this reaction. Consequently, the second term in eq 1is expected to be small and the quantum yield of C2H4 less dependent on conversion. In this light, a reexamination of the CH2C1CH2C1system with the objective to explore the dependence of the C2H4 quantum yield on I t / N would provide further confirmation and extension of the two C1-atom mechanism. Although, such a process was included in the earlier investigation? the correlation of the quantum yield with conversion was not considered, and the C1-atom mechanism was restricted to experiments at lower pressures.
Experimental Section The apparatus, light source, and experimental procedure have been described in previous publications?*' Photolyses were carried out at room temperature (23 "C) in a spherical Pyrex glass vessel of 215 cm3volume. Quantum yields were determined by chemical actinometry with the photolysis of C2H4 as a standard'l (4c?H2 = 1.0 at 147 nm). Since CzH4 is a reaction product, actinometric measurements were carried out after each photolysis of the test gas to avoid possible cell contamination. One photolysis was carried out at relatively high total pressure with excess CF4 as an unreactive bath gas. In the associated actinometric measurement an equal amount of CF4was also added to compensate for any effect on the quantum yields resulting from residual trace impurities (air) in CF4. Product analysis was carried out by flame ionization gas chromatography (Hewlett-Packard, Model 5830A, with dual FI detectors). Ethylene, C2H2, and C2H3C1were separated by using a 3.6-m Porapak N column at 125 'C with He as carrier at a flow rate of 30 cm3/min. In the case of actinometry, the Porapak N column was used at 80 "C to achieve better resolution between the C2H4 and the product C2H2. In a limited number of runs, a 1.8-m Durapak N column (n-octaneon Porasil C) at 100 "C was used to determine a minor product, CH2C1CHC1z. Some dif0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 25, 1980 3373
Photodissociation of CH2CICH2CI
TABLE I : Product Quantum Yields in the 147-nm Photolysis of CH,ClCH,CI
--
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
a
photons
~
additive
3.0 3.0 3.0 9.0
2!.9 21.9 2.9 10.3 10.5 91.8 11.1 11.1 10’.2 10’.1 42,.6 42.5 42.3 42.4 38.0 3.0
10-531,
quantum yield, 6
press., torr
-run no. CH,CICH,Cl --
0.3 (NO) 0.3 (NO) 0.3 (NO)
0.6 (NO) 0.6 (NO)
1.0 (NO) 209 (CF,)
C,H,
0.67 0.69 0.68 0.65 0.66 0.66 0.64 0.67 0.67 0.70 0.73 0.70 0.66 0.67 0.64 0.65 0.71 0.70 0.64 0.54
C,H,Cl
C,H,
0.33 0.36 0.41 0.37 0.16 0.16 0.16 0.28 0.31 0.31 0.34 0.32 0.16 0.16 0.29 0.29 0.41 0.34 0.17 0.16
CH,ClCHCl, -~
0.080 0.073 0.078 0.073 0.069 0.073 0.074 0.077 0.077 0.076 0.085 0.083 0.074 0.074 0.065 0.069 0.085 0.077 0.069 0.057
s-l
0 0 0
2.2 2.7 1.8 2.4 3.6 3.4 5.1
a
1.3
a
2.2 2.0 2.7 2.5 4.1 4.1 3.0 3.8 4.7 5.6 2.6 2.5
a a
0.10 a
0.05 0.07 0.03 0 0 a
a a
-0 0 a
104(zt/~) 3.8 7.9 10.7 14.3 7.4 10.0 15.0 1.2 1.8 2.1 3.1 5.9 1.7 3.5 0.60 0.77 1.4 2.3 0.88 10.8
Not determined.
ficulties in quantification were encountered in this analysis since CH2ClCHC12‘wasfound to be a trace (0.002%) impurity in the reactant (correction for which was applied a t low conversions) and, more troublesome, it eluted on the shoulder of the large parent peak. In all cases, identification and quantitative determination was carried out by comparison of retention times and peak areas with those of authentic samples for which the relatives sensitivities had been previously determined. As a test for the presence of chlorine atoms several diagnostic experimentii were performed in which a mixture of CH2ClCHzCland C2H6was photolyzed. In the absence of NO, the additional products in this system were CH4, n-C4H,o and one other product, with a retention time just between the retention times of 2,3-C4H8C12and 1,3C4H8C12,samples of which were available. This reaction product was deduced to be 1,2-dichlorobutane, based on the correspondence of retention times and boiling points for these three dichlorobutanes. The relative sensitivity of 1,2-C4H8C12was taken as equal to that of 1,3-c4H&l2. The reactant, CI12C1CH2C1, was obtained from the Fisher Scientific Co. and was distilled under ethanol slush cooling. Its final purity was better than 99.9%. The only residual impurity of any significance was -0.1% of CH3CHC12which had, however, no consequence on the photolysis product amalysis. Nitric oxide and CF4 (both from Matheson) were purified by trap-to-trap distillation a t liquid nitrogen temperature, prior to their use. In separate experiments the extinction coefficient of CHzClCH2Clvvas determined as t = (1/PL) In (Io/&)= 525 f 90 atm-’ cm-l with1 a double-cell method.12 Thus, even at the lowest pressurelei of our experiments all incident light was absorbed.
Results In agreement with the previous investigation2 the observed reaction products in the photolysis of pure CHzClCkZC1 are C2H4,C2H3C1,and smaller quantities of C2H2 and CH2C1CIIC12. Table I summarizes the product quantum yields as a function of reactant pressure, the additives NO and CFd,and the conversion I t / N . As noted tlhe latter quantity has been found to be an important variable of the system, where t is the photolysis time, I is the total light intensity absorbed per unit time (photons/s), and N is the total number of reactant mole-
cules in the reaction cell. The addition of nitric oxide does not affect the quantum yield of the principal product, C2H4,which confirms that it is formed molecularly in a primary process, Also, the yield of C2H2is not affected by NO, while the yield of C2H3C1is significantly reduced to a limiting value = 0.161, and CHZC1CHCl2is not observed in the presence of NO. It is therefore clear that the 1,1,2-trichloroethaneand part of the vinyl chloride are produda of free radical origin, while C2H2and the residual C2H3Clderive from molecular processes. Further, from Table I it can be seen that at any constant reactant pressure the quantum yield of C2H4 shows no marked trend with the conversion, and, within experimental error, the yield is constant with increasing values of ItlN. This observation, though anticipated, is in stark contrast to the quantum yields of the principal olefins in the photolyses of a,P-polysubstituted chlorofluoroethanesaWOn the other hand, there apperlrs to be a just perceptible increase in the yield of vinyl chloride with increasing It/ N at constant pressure. Comparison of the data a t 3 torr reactant pressure shows that the addition of a large excess of CF4 as “inert gas” decreases the quantum yields of both olefiis and the acetylene, in agreement with previous findingsq2 However, the very strong decrease in the yield of C2H3C1 to the limiting value as found in the presence of NO most likely reflects (at least in part) the scavenging of the radical presursors of C2HaC1by the oxygen impurity in CF,, rather than solely collisional stabilization of its molecular precursor. The yield of CH2C1CHCl2,which was previously believed to be linked with the yield of C2H3Clof free radical origin, was lower in the present study and an adequate explanation for this could not be found, though some analytical dlifficulties with respect to this product have been pointed out earlier. The photolysis of the mixture of CH2ClCHzCland CzHs in the absence and pregence of nitric oxide provides a diagnostic test for the presence of chlorine atoms and the results are listed in Table 11. Also listed are the results of the photolyses of the pure compounds for direct comparison. These experiments were carried out to relatively high conversions (- 1%) without actinometric measurements. Accordingly, the yields reported are relative to ethylene (C2H4 = 100). To be’noted is the large increase in the yield of n-C4Hlo in the photolysis of the mixture compared to the photolysis of C2Hs alone, and the ap-
3374
The Journal of Physical Chemistry, Vol. 84, No. 25, 1980
Yano and Tschuikow-Roux
TABLE 11: Relative Product Abundancea in the 147-nm Photolysis of a CH,ClCH,Cl/C,H, Mixture press., torr run no. 1 2 3 4
CH,ClCH,Cl 4.35 4.23 0 4.15
C,H, 2.85 2.03 2.71 0
relative yields
NO 0 0.16 0 0
CH, 3.5 0 13.6 -0
C,H2 13.0 10.9 169.3 12.8
C,H, 100 100 100 100
C,Hgb 4.0 0 20.5 0
n-C,H,, 64.7 0 40.0 0
1,2C,H,CI, 10.0 0 0 0
C,H,Cl 24.2 20.1 0 42.7
a Except as otherwise indicated, relative sensitivities have been determined experimentally. S(CH,/C,H,) = 0.75, S(C,H,/C,H,)= 1.35 from 0. E. Schupp 111, "Technique of Organic Chemistry", Vol. XIII, Interscience, New York, 1968.
pearance of a new product, 1,2-dichlorobutane,which was not observed in the photolyses of either pure CHzCICHzCl or pure CZH6. The addition of NO completely supresses the formation of n-C4Hlo and 1,2-C4H8Cl2which again is indicative that these products have radical precursors. The relative product abundance in the photolysis of pure CHzCICHzCIis in good agreement with the data in Table I, even at this high conversion. Also, the relative abundance of the products in the C2H6 photolysis is in qualitatively reasonable agreement with the literature data of Hampson et al.13 Discussion Photolysis of the C H Z C ~ C H ~ C ~ Mixture. / C ~ H ~ The photolysis of the CHzC1CH2C1/CzH6 mixture provides an easy and effective test for the presence of chlorine atoms in the system owing to the fact that these compounds have extinction coefficients which ,differ by more than one order of magnitude, and the further fact that the photolysis of ethane is very well established.13J4 Thus the n-butane which is not a product in the photolysis of pure CHzCICHzCl but is observed as a major product in the photolysis of the mixture cannot solely result from the photolysis of C2H6 itself. This can be readily shown, for example, from a consideration of the extinction coefficients and partial pressures in this experiment. Using the value determined in the present study el = 525 atm-l cm-l for CHzCICHzC1, and ea = 40 atm-l cm-l for CzH6 from the literature,14J5we obtain for the partial pressures in run 1,Table 11,clPl/(qPl + e2P2)N 0.95. According to this estimate only -5% of the observed CzH4 in the mixture can be assigned to the photolysis of CzHG.Further, from Table 11,for the mixture we have the product ratio (n-C4Hlo/CZH4)hN 0.65. Since there is no reason not to assume that the n-butane/ ethylene ratio arising from the photolysis of CzH6 in the mixture is the same as in the photolysis of pure CzH6 (n-C4Hlo/C2H4N 0.4, run no. 3) only ca. 3% [= 5% X (0.4)/0.65] of the n-C4Hlo formed in the mixture is contributed by the CzH6 photolysis. An even smaller contribution is predicted from a consideration of the detailed photolysis mechanism of CzHG.Based on isotopic product analysis in the 147-nm photolysis of CzH6 + CzD6, Hampson et alJ3 have established the following primary processes: CzH6 C2H4" + H2 (85%) (R1) CzH4 + 2H (15%) (R2) where the percentages indicated are relative probabilities. Butane formation was shown to arise from the association of ethyl radicals formed by the addition of H atoms to the product ethylene. Thus, using the above statistical factors the maximum contribution to the n-butane yield is estimated as -0.8% (=5% X 0.15). It is clear, therefore, that the cause for most of the I Z - C ~ production H~~ in the photolysis of the CHzClCH2CI/C2H6mixture must be sought in the photolysis of the CHzClCH2Clcomponent. Furthermore, the appearance of 1,2-C4HsC12in the photolysis
-
of the mixture must be linked with the formation of nC4Hlo. Since the addition of nitric oxide completely suppressed the formation of n-C4Hlo and 1,2-C4H8Cl2(run 2, Table 11),the most plausible precursors are inferred to be the C2H5 and CH2ClCHClradicals. These radicals will be undoubtedly produced in the presence of chlorine atoms by the abstraction of hydrogen from the reactant molecules even at room temperature.16 We can therefore explain the principal observations in the photolysis of the CH2CICHzC1/CzH6mixture by the following reaction scheme: CH&lCHZCl+ hv
+
CHZClCH2Cl'
CHzClCHzClt -+ CzH4 + 2C1 (or Clz* C1+ CzH6
---
C1+ CHzCICHzCl
-
2C1)
HC1+ CzH6
(R3) 034)
(R5)
HC1+ CH2ClCHCl (R6)
2Cz~5 n - ~ 4 ~ 1 o CzH5 + CHzCICHCl 1,2-C4HsCl2
(R7) (R8)
where it is understood that the photolysis of and hence its contributions to the CzH4 and n-C4Hlo yields, plays only a minor, subsitiary role as discussed above. In reaction R4 the production of chlorine atoms without the concurrent formation of a chloroethyl radical is implied. This is supported by both product analysis (no 1,4-C4H8C12 observed) and thermochemical considerations. The more than sixfold larger yield of n-C4Hlo in comparison to 1,2C4H8ClZreflects the higher abundance of the C2H5 radical due to the lower activation energy for the abstraction reaction R517 in comparison to reaction R6.18 It is for this reason also that, in this system, the recombination of CHzCICHClradicals is unimportant in relation to reaction R8. The sum of n-C4H,o and 1,2-C4H&l2is 74.7% of the observed C2H4 (Table 11). In addition to the recombination reactions R7 and R8, ethyl and CH2ClCHCl radicals are also expected to undergo disproportionation reactions which produce mainly CzH4 and the parent compounds by hydrogen transfer CzH5 + CzH6 CzH4 + CzH6 (R9) C2H5 + CH2ClCHCl- C2H4 + CH2ClCHzCl (R10) +
CHClCHCl + CzHG (RW where reaction R11 must apparently be unimportant since no measurable CHClCHCl was observed. The disproportionation/combination ratio for thermal C2H5radicals is knownls ( k d / k , = 0.14). If, for the purpose of this discussion, we assume the same k d / k , value for the pair C2H5/CHzC1CHC1and consider the material balance in the reaction scheme R4-RlO we obtain the ratio ([n-C&i,I + [ ~ , ~ - C ~ H ~ C ~ ~ I ) / + ( [ [CZH~I) C Z H ~ IeO0.75 -+
where [C2H4],is the ethylene produced in reaction R4.
The Journal of Physical Chemlstty, Vol. 84, No. 25, 1980 3375
Photodissociationof CH,CICH2CI
This ratio is in excellent agreement with the experimental value and provides further support that the production of CzH4 in the primary process R4 is associated with the concurrent formation of two C1 atoms. Photolysis of CHzClCH2C1. (a) Molecular Formation of CZH4, C2H3Cl,and C,H,. As noted earlier, the yields of CzH4, CzHz,and part of the vinyl chloride are not affected by nitric oxide which indicates that they are produced in molecular processes. The absence of any noticeable pressure dlependence of the quantum yields of the products in the priessure range 3-42 torr of CHzCICHzCl irrespective of conversion places an upper limit on the lifetimes of their respective precursors. At higher pressure, in the experiment with added CF4, the decrease in the quantum yield of C2H4 does suggest that some collisional deactivation of the! photolytically excited CHzCICHzCl occurs, but not to the highly vibrationally excited ground state, which, contrary to observation, would be reflected in an increme in the yield of C2H&l with pressure as a result of the preferential elimination of HC1 from the ground state.20 The! formation of acetylene must occur in a secondary molecdar elimination proceas, though its exact origin cannot be ascertained. Based on thermochemical and other considerations, the C2Hzproduced in the 147-nm photolysis of 1,2-fluorochIoroethane' was thought to originate primarily frolm the vibrationally excited halogenated product olefins. In the present system, this would require the yields of CzH3Cl to increase, and that of CzH2to decrease, with increasing pressure, which again is not supported by experiment. A ossible explanation of the observed pressure indlepende ce of the yields may be sought in the mode in which the e menta of (HC1) are eliminated from the initially excited tate. Thus, we can assign, somewhat arbitrarily, the &3 elimination of HC1 to lead directly to vinyl chloride 3yhich is stable with-respect to further decomposition, while the a,aelimination of HC1 may result in a vibrationally excited chloromethylcarbene (CHzCICH), which rapidly dissociates into HC1 and acetylene. A related proposal has been made by Callear and Cvetanovii.21~2" who invoked a vibrationally excited triplet ethylidene as a possible intermediate to explain the kinetics of the mercury-photosensitized decomposition of cis-1,2-dideuterioeithylene. The molecular processes discussed above may be summarized by the reaction scheme
1 ~
-
-
CHzC1CH2Clt-% CHzCICHzCltt C21H4+ 2C1 (or C12* 2C1)
-
a,B
-*
CzH3Cl
+ HC1* (HC1*
-
H
+ C1)
(R12) (R4)
(R13) CHzCICH* + EICl** (HCl** H C1) (R14) (R15) CHzCICH* CzHz + HC1 where the asterisks denote some residual excitation above the threshold for decomposition. In the ground state, reaction R4 is endothermica3by ca. 101 kcal mol-l, and since the photon energy is 194 kcal molb1, the residual excess energy is about 93 kcal mol-l. While this energy exceeds the activatiion energy (E, C= 80 kcal mol-') for H2 elimination from C2H4,=there is no reason to assume that almost all of the excess energy will reside in the C2H4. This argument may be taken as a supplementaryreason for not including the product ethylene as a source of the acetylene. In contrast, ithe enthalpy change in reaction R13 is only AH" C= 17 kea1 m ~ l - land, , ~ ~for C2H3Clto be stable, ita energy content must be less than 69 kcal mol-l, which is the activation energy for HC1 elimination, the latter being the preferred decomposition channel.% Thus the minimum excess energy which must be carried by the co-product in --+
+
reaction R13 is -108 kcal mol-' which exceeds the bond dissociation energy of HC1. It is therefore very likely that the HC1 produced in reaction R13 will dissociate, thus providing an additional, albeit minor, contribution of C1 atoms. The he,& of reaction of the sum of reactions R14 and R15 is -41 kcal mol-', and it is therefore possible that the HC1 produced in reaction R14 [but not that produced in reaction Rlli] may also undergo dissociation. (b) Reactiomr of Chlorine Atoms. Chlorine atoms produced in the syeltemwill abstract hydrogen from the parent 1,2-dichloroethane leading to the formation of thermal CH2ClCHClradicals.
kabl
C1+ CHr2C1CH2C1
HC1+ CHzCICHCl (R6)
In general, with increasing conversion, chlorine atoms will also add to the product olefins giving rise to the corresponding chemically activated haloethyl radicals. In the case of the principal product, CzH4, this process may be represented by reaction R16, where k, and k d are, reC1+ (zzH4
kd
-
CzH4C1*
U P
CzH4Cl
(R16)
spectively, the irate constants for activation and dissociation, 2 is the gas-kinetic collision frequency per unit pressure, X is a collisional deactivation efficiency factor, and P is the (reactant) pressure. An analogous equation can also be written for the minor olefin, CzH3C1. In the presence of nitric oxide a third sink for the removal of chlorine atoms is provided by way of their NOcatalyzed recombination via nitrosyl chloride as an intermediateP
ci + NO + M
~ C ~ +NMO C1+ ClNO Clz + NO
-
(R17) (R18)
Since reaction lR18 is known to be very the ClNO concentration an be considered to be quasi-stationary and, under conditions of our experiment, it can be sh0wn7t8that the rate of removal of C1 atoms by recombination is given by R17 = 2ko,[M][C1][NO] where ko, is the low-pressure third-order ratat constant. It may be noted that it is the competition between reactions R6, R16, and R17 that determines the functional dependence of the olefin quantum yields. (c) Dependence of on I t / N , P, and [NO]. If we consider reaction R4 as the only source of chlorine atoms (Le., neglecting other minor contributions) and reactions R6, R16, and R17 as the only processes for C1-atom removal, then, subject to the usual steady-state approximation for [Cl], it can be shown7p8that for small values of the conversion, I t / N , the observed quantum yield (4) is given by
where 4o is the true (limiting) quantum yield of C2H4, kadd(P) = k,/(I -I- k d / U p ) is the pressure-dependent rate constant for the! addition reaction R16, and A' has been introduced to allow for a higher collisional deactivation efficiency of CH[&lCH2C1in comparison to the bath gas for which ko, ha@ been determined. In the absence of NO, or if the NO-catalyzed C1-atom recombination rate is unimportant, eq 2 reduces to 4-l ='
40-l
== 40-1
+ [kadd(P)/kabSl(It/n?
+ m(P)(It/N)
(3)
which predicb that at constant pressure the reciprocal of
3378 The Journal of Physical Chemistry, Vol. 84, No. 25, 1980
4
4 t 3
1
1
1
It-
0
Yano and Tschuikow-Roux
O
n
”
d
0
5
10
15
0 0
( I t / N ) X IO4
Flgure 1. Plot of the reciprocal quantum yield of CpH, vs. I f l N at different reactant pressures, in the absence and presence of NO: (Circle) 3 torr, (Square) 10 torr, (triangle) 42 torr; (open symbols) experiments without NO, (solid symbols) experiments with NO.
the observed quantum yield is a linear function of conwith a positive slope determined version (It/N 5 2 X by the rate constant ratio for the addition/abstraction reactions, and in the limit ( I t / N ) 0 , 4 q+,. Pursuing this line further, it is instructive to consider the limiting cases of the slope m(P). Thus at high pressures m(P) ka/k&, at sufficiently low pressure m(P) kaXZP/kdkabs and in the limit as P 0, m(P) 0. Physically, the low but finite pressure case would correspond to the situation where the addition reaction is no longer competitive since stabilization of the chemically activated radicals (reaction R16) would be insufficient and most of these radicals would redissociate to the olefin plus C1. As indicated in the Introduction, in our recent studies of the photolyses of CFzCICHCIJ and CF3CHC12,8eq 2 and 3 provided a satisfactory interpretation of the functional dependence of the olefin quantum yields on pressure and conversion, and were cited in support of the proposed two C1-atom mechanism. For these systems the abstraction reactions are slow owing to the higher activation energies associated with these highly halogenated compounds, and the addition reactions in these become perceptible even at reactant pressures of 1torr and conversions as low as 1X In contrast, in the case of CHzCICHzClthe addition/abstraction rate ratio is much lower, and hence the pressure dependence of the slope is expected to be much smaller. Figure 1shows a plot of 1/4 vs. I t / N for the ethylene data in Table I. The result is rather striking; while a linear relation is obtained the quantum yield is almost invariant with conversion, nitric oxide, and/or reactant pressure. The conclusion which follows is that in the present system neither the addition reaction R16 nor the NO-catalyzed recombination of C1 atoms (R17) is competitive with the abstraction reaction R6. This conclusion finds further support by use of rate constant data from the literature. For C1 addition to CzH4,Chiltz et aLZ8 have reported the rate constant kadd = loi3.’ eoIRTCm3mol-’ s-l, while for H abstraction from l,l-CzH4Cl2by C1 atoms Cillien et al.18 give kab = 1013.8e-3100/RT cm3mol-l s-l, hence a t 298 K kadd/kabs 47. Since l/do 1.5, it follows that the even at our highest conversion of I t / N = 1.5 X second term in eq 3 contributes only -4.7% (i.e., l/$= 1.57) which is too small a change to be detected within the experimental scatter. Similarly we can compare the rate ratio of the abstraction to the recombination reaction: Rab/Rrw = kab/2X’korw[NO]. Hippler and Tree% reported the value kore, = 1.4 X 10l6 cm6 m o P s-l at 298 K for helium as bath gas. Assuming A’ = 10, we calculate Rsbs/Rrec= 22 a t our highest pressure of NO of 1 torr. Although there is considerable uncertainty with respect to the chosen value of A’, this calculation shows, never-
- - - -
0 Without NO
-
5
10
15
( I t / N ) X IO4
Figure 2. Plot of the reciprocial quantum yield of CpH,CI vs. I t l N a t different reactant pressures: (clrcle) 3 torr, (square) 10 torr, (triangle) 42 torr.
theless, that the NO-enhanced chlorine atom recombination is too slow to inhibit significantly the rate of hydrogen abstraction. Therefore, in the present system, nitric oxide only plays its traditional role, that of a radical scavenger, but has little effect on the haloethyl radical formation process. An analogous expression to eq 3 may also be deriveda for the quantum yield of that fraction of the minor olefin, CzH3CI,which is produced molecularly. Figure 2 shows a plot of l/&H&1 vs. conversion. In the presence of NO the data at a d reactant pressures again fall on a straight line with “zero” slope, in agreement with the preceding discussion. Also included for comparison is the data in the absence of NO, though it must be emphasized that in this case the plot is strictly empirical. ( d ) Formation of C2H3C1(of Radical Origin) and Other Products. In our previous investigation of this system2the quantum yields of C2H3Cland CH2C1CHC12appeared to be linked, both increasing with decreasing reactant pressure in the lower pressure range. While this pressure dependence could not be then satisfactorily explained, nor is it discernable in the present study, a reexamination of the earlier data has shown that the apparent increase in the yields correlates with increasing values of conversion, which must reflect contributions from secondary reactions. Table I also shows a perceptible trend in this direction in the case of vinyl chloride. This trend may be represented by the equation = 44 + A@, where 4‘ is the observed quantum yield of CzH3Cl and A# is an undetermined function of conversion and possibly pressure, as well. Taking the inverse and expanding in power series one obtains
Equation 4 is of the form which is suggested by the data in Figure 2 in tlie absence of nitric oxide. However, the reaction(s) which contribute to the vinyl chloride yield remain unclear, and the following discussion is, in part, rather speculative. On the basis of the foregoing considerations it is natural to turn to the abundant CHzCICHCl radicals produced in reaction R6 as the most significant precursor to secondary products. First, we note that the thermal decomposition of CHzCICHClradicals cannot be the source of vinyl chloride at the temperature in question.29 In our previous attempt2to explain the formation of CH2C1CHClZand the C2H3C1of radical origin we considered the disproportionation of CH&lCHCl radicals by chlorine atom transfer (R19)which could theoretically explain the simultaneous formation of these products:
The Journal of Physical Chemistry, Vol. 84, No. 25, 1980 3377
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Photodissociation of CH,CICH,CI
2CH2C1CHCl C2H3C1+ CH2ClCHC12 (R19) CHClCHCl -I. CHzClCHzCl (R20) 1,2,3,4-C4H&14 (R21) Some limitations o l reaction R19 have been pointed out2 and these must now be emphasized; it is the selectivity of chlorine rather tlhan H-atom transfer which must be questioned, particularly in the case of thermal radicals. Moreover, the correspondence between the yields of C2H3Cland CW2C1CIICl2could not be reproduced. The more likely process of hydrogen transfer (R20) apparently does not occur to a significant extent since no CHClCHCl product was observed.30 Nevertheless, some contribution to the vinyl chloride yield from reaction R19 cannot be entirely ruled out. Vinyl chloride may also be a product in the disproportionation of CzH4Cland CH2ClCHClradicals by hydrogen transfer CH2ClCHz + CHzClCHCl C2H&l+ CHzClCH2Cl (R22) ---* 1,2,4-C4H&13 (R23) where the selective transfer from the chloroethyl radical is indicated and other possible reactions, including the disproportionation between two C2H4C1radica1s:l are omitted on the basis of product analysis (e.g., no significant C2HBClwas observed) and the fact that C2H4Clradicals would be present in much lower abundance than the CH2ClCHCL radicals. If reaction R22 is operative the question which arises is as to the source of C2H4C1radicals? An unlikely prospect is the secondary abstraction of chlorine from the parent by thermal CHzCICHClradicals. Though this reaction is almost t h e r m o n e ~ t r a lit, ~is~ expected to have a significant activation e n e r d 2 and hence should be negligible a t room temperature. Were this not the case, one should observe a larger yield of the coproduct CH2C1CHC12.Returning to primary processes, one may consider the possibility that a small fraction of the initially photoexcited CH2C1CH2C1decomposes by C-C1 bond fission, perhaps after a cross over to the potential energy surface of another excited state, but one different from that indicated in1 reaction R12, since in the latter case the C2H3C1would be expected to increase with pressure. If no energy degradation occurred in the afore-mentioned process, then, for the chloroethyl radical to be stable with respect to further decomposition (C2H4C1* CzH4 + C1; AHo = 21 lrcal mol-’) would require that the initially formed C1 atom carr,ymore than 94 kcal mol-l of the excess energy, since the C-Cl bond dissociation energy in CHzClCH2Clis 79 kcal ~ n o l - Parenthetically, ~.~~ we may also note that the alternate route of decomposition yielding vinyl chloride directly (C2H4C1* C2H3C1 H; AH” = 40 kcal mol-l) would require that “hot” C2H4C1radicals be scavengeable by NO, in order to explain the nitric oxide data. In search of further possibilities we may cite the effects of inhomogeneity in the reaction vessel, particularly in the vicinity of the cell window. While the addition of C1 to C2H4 (R16) is slow in relation to abstraction reaction R6, this will apply to the reactor volume as a whole. However, owing to the! high extinction coefficient of 1,2-C2H4C12, there will exist a diffusion-controlled concentration gradient of the primary products. Thus the C2H4 concentration will be higher in the region near the cell window where C1 addition arid collisional stabilization of the adduct could occur. In as much as the local C2H4 concentration would be (semi) quasi-stationary, the consumption of C2H4 from this region would not be directly reflected in its
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quantum yield with conversion. Finally, one must consider the abstraction of chlorine from the parent CH2ClCH2Clby hydrogen atoms produced in reactions R113 and R14: H CH2ClCH2Cl HCl + CH2ClCH2 (R24) Though, in general, this type of methathetical reaction is expected to have a significant activation energy,%it cannot be excluded since the hydrogen atoms generated in this system will have residual excess kinetic energy, as is apparent from the earlier thermochemical discussion. From the alternatives considered, it is this latter process which now appears to be the more attractive to explain the source of C2H4Cland hence the yield of C2H3C1from radical precursors. In concluding we note that the lack of observable recombination products (1,2,3-C4H7C13and 1,2,3,4-C4H6C14) is not unexpected in view of their high boiling points, the overall very lour conversion, and their possible absorption on surfaces. In our previous study the presence of 1,2,3,4-C4H6C14 was, in fact, confirmed in selected experiments. Acknowledgment. The continued financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
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References arid Notes T. Ichimura, A. W. Kirk, and E. Tschuikow-Roux, Int. J . Chem. Klnet., 9, 743 (1977). D. Salomon, A. W. Kirk, and E. Tschuikow-Roux, J. Phys. Chem., 83, 2569 (1979). D. Salomon, A. W. Kirk, and E. Tschuikow-Roux, Int. J . Chem. Khet., 9, 61!3 (1977). D. Salomon, A. W. Kirk, and E. Tschuikow-Roux, J . Photochem., 7, 345 (1977). T. Ichimura, A. W. Kirk, and E. Tschuikow-Roux, Int. J . Chem. Klnet., 9, 697 (1977). T. Yano and E. TschuikowRoux, J. Phys. Chem., 83,2572 (1979). T. Yano and E . TschuikowRoux, J . Chem. Phys., 72, 3401 (1980). T. Yano, K.-H. Jung, and E. TschuikowRoux, J. Phys. chem., 84, 2146 (1980). V. N. Kondrai ev, “Gas-Phase Reaction Rate Constants”, Akad. Nauk, Moscow, USSR, 1970. P. Cadman, A. W. Klrk, and A. F. Trotman-Dickenson, J . Chem. Soc., Faraday Trans. 7, 72, 1027 (1976). D. Salomon and A. A. Scala, J. Chem. Phys., 82, 1469 (1975). R. Gorden, Jr., R. Doepker, and P. Ausioos, J . ch8m. Phys., 44, 3733 (1966). (a) R. F. Hampon, Jr., J. R. McNesby, H. Akimoto, and I. Tanaka, J. ch8m. Phjys., 40, 1099 (1964); (b) R. F. Hampson, Jr., and J. R. McNesby, IbM., 43, 3952 (1965). J. R. McNesby and H. Okabe, Adv. Photochem. 3, 157 (1964). H. Okabe and D. A. Becker, J. Chem. Phys., 39, 2549 (1963). V. N. Kondrai‘ ev, see ref 9. J. H. Knox and R. L. Nelson, Trans. Faraday Soc., 55, 937 (1959). C. Cillien, P. Goldfinger, G. Huybrechts, and G. Martens, Trans. Faraday Soc., 83, 1631 (1967). W. E. Faiconor and W. A. Sunder, Int. J. Chem. Klnef., 3 , 523 (1971). D. W. Setser and J. C. Hassler, J . Phys. Chem., 71, 1364 (1967). A. B. Callear alnd R. J. CvetanoviE, J. Chem. Phys., 24, 873 (1956). R. J. CvetanoviE, Prog. React. Khet., 2, 39 (1964). Heats of formation (298 K) in kcai mol-’: AH?(CH2CiCH2CI)= 31.0 AH,0(C2H4)= 12.5; AH,0(C2H3CI)= 8.4, taken from D. R. Stuii, E. F. Westrum, Jr., and G. C. Slnke, “ The Chemical Thermodynamlcs of Organlc Compounds”,Wiiey, New York, 1969. A. W. Kirk and E. TschuikowRoux, J. Chem. Phys., 51,2247 (1969). F. Zabel, Int. J . Chem. Kinet., 9, 651 (1977). H. Hippler andl J. Troe, Int. J. Chem. Klnet., 8, 501 (1976). M. A. A. Clyntt and H. W. Cruse, J . Ch8m. Soc., Faraday Trans. 2 , 68, 1281 (1972). G. Chlltz, P. Goldfinger, G. Huybrechts, G. Martens, and G. Verbeke, Chem. Rev., 83, 355 (1963). J. A. Franklin and 0. H. Huybrechts, Inf. J. Chem. Kinst., 1, 3 (1969). 4(CHCICHCI) :Z 0.02, allowance being made due to dlfficuttiis in aC separation. J. Heicklen, J . Am. ch8m. Soc., 87, 445 (1965). M. G. Katz, G. Baruch, and L. A. Raibenbach, Int. J. ch8m. Kinet., 9, 55 (1976). J. A. Kerr and E. Rataiczak. “Third SuDDlementaw Tables of Bimolecular Gas Reactions”, University of Birmingham, Birmingham, England, 1977.
