Photolysis of 1,P-Dichloroethane
The Journal of Physical Chemistry, Vol. 83, No. 20, 1979 2569
Primary Processes in the 147-nm Photolysis of 1,2-Dichloroethane D. Salornon, A. W. Kirk, and E. Tschuikow-Roux" Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N IN4 (Received February 20, 1979)
1,2-Dichloroethanewas photolyzed at 147 nm in the pressure range 0.6-54 torr. The effects of added NO, HI, and large pressures of CF4were also investigated. The major primary process is the molecular production of ethylene (4CaH, = 0.77). The molecular elimination of HCl also occurs but to a lesser extent. The secondary production of 1,1,2-trichloroethaneand an equal proportion of "scavengable" vinyl chloride has been attributed to the dispropriation of CHzCICHC1.radicals by chlorine atom transfer. The quantum yields for the primary molecular processes are compared with those observed in the vacuum ultraviolet photolysis of other haloethanes.
Introduction The 147-nm photolysis of CH3CH2C1,1CH3CHC12,'and CH3CC1t are characterized by the predominant molecular elimination of HC1. To this extent the ultraviolet photochemical decompositions resemble those of thermal and chemical activation systems involving ground electronic Similarly, the 147-nm photolysis of CH3CFzCl also gives rise to mainly HC1 elimination.' However, the substitution of fluorine atoms p to the chlorine atom gives rise to primary processes which differ from those of the ground state. Thus at 147-nm CHzFCH2Cl not only eliminates HC1 but also a significant fraction decomposes by FC1 eliminationa8 Moreover, HF is the preferred molecular elimination from the ground stateagFurther p fluorine substitution (CF3CH2C1) reduces the overall quantum yield of molecular processes at 147 nm, while at shorter wavelengths (123.6 nm) FC1 elimination is the major primary process.1° Since the substitution of fluorine atoms /3 to the chlorine atom had increased significantly the fraction of molecular halogen elimination, the investigation of the 147-nm photolysis of the chlorine analogue of CH2FCH2C1,namely, CH2C1CHzC1,was a logical extension of our work. Experimental Section The basic apparatus and xenon resonance lamp used in these experiments have been described previously.2 The chromatic purity of the lamp was routinely checked with a 0.3-m McPherson (218-0.3 m) vacuum ultraviolet monochromator. The intensity of the 129.6-nm resonance line never exceeded 1% of that of the 147-nm line. Ethylene was used as an actinometer, the quantum yield for the production of acetylene being taken as unity at 147 nm." Because the window tended to accumulate a polymer buildup, actinometry was performed at the beginning and end of each run. Between runs the LiF lamp window was cleaned with a slurry of A1203in CC1,. Total conversion was always less than 0.1 % . Product analysis was by isothermal (125 "C) gas chromatography (Hewlett-Packard5830A twin F.I.D.) with a 3m Porapak N column with a helium flow rate of 25 cm3/min. Ten runs were analyzed specifically for 1,2,3,4-tetrachlorobutane with a 30-cm Durapak column (Carbowax 400 on Porasil S) but at the expense of the acetylene yield which could not be determined under these conditions. Product identification was by comparison of retention times with those of authentic samples. However, the production of 1,1,2-trichloroethanewas confirmed by mass spectrometry. The dichloroethane was obtained from the Fisher Scientific Co. and purified to better than 99.99% by using 0022-365417912083-2569$0 1.OO/O
a 125-plate temperature-controlled spinning band fractionating column (B/R Glass Inc.). Nitric oxide, C12,HI, and CF4 were all from the Matheson Co. and, apart from low temperature trap-to-trap distillations, these compounds were used without further purification. Precise measurements of low pressures were made by using a fused quartz Bourdon gage (Texas Instruments, Model 145).
Results The two primary products are C2H4and C2H3C1. Although a significant amount of CH2ClCHC12is observed at low pressures, it is obviously a secondary product of free radical origin since it is not observed in the presence of nitric oxide. The quantum yield of C2H4 decreases steadily with increasing total pressure (Table I, runs 1-21) as does the overall yield of primary products. Additions of radical scavengers such as NO and HI produce no further decreases in the yield of CzH4 at any pressure. In fact, in some instances, the yield of C2H4 appears to be a little larger. The yield of C2H3Cldecreases quite rapidly with increasing total pressure at the lower pressures (runs 1-11), but this decrease becomes more gradual with the addition of inert gas (runs 12-21). The addition of NO (or HI) decreases the low pressure yield such that the decline in the yield of the residual C2H3C1with increasing pressure is then gradual over the whole pressure range. It is thus apparent that there are two sources of C2H3C1,one of which is of free radical origin, the yield of which decreases rapidly with increasing pressure, while the second is a primary source and is less affected by pressure. The quantum yields of C2H2are not large and are also not very reproducible. However, since the yields do appear to decrease somewhat as the pressure is increased and the addition of NO has little or no effect, one may conclude that the C2H2is produced in a molecular process directly related to the production of CzH4 and/or C2H3C1. The fairly rapid decline in the yield of CHzCICHClzwith increasing pressure (in the absence of NO) closely parallels the decreases in the yield of radical produced C2H3C1with increasing pressure. Thus not only must one account for the formation of a product molecule containing three chlorine atoms, but also the explanation must provide for the additional and probably simultaneous formation of C2H3C1. The remaining trace products are 1,2,3,4-tetrachlorobutane, ethyl chloride, and trans-1,2-dichloroethylene. Only three of the ten runs previously mentioned produced measurable amounts of tetrachlorobutane (runs 6,10, and 17). Nevertheless, we are confident it is a mechanistically 0 1979 American Chemical Society
2570
D. Salomon, A. W. Kirk, and E. Tschuikow-Roux
The Journal of Physical Chemisfty, Vol. 83, No. 20, 1979
TABLE I: Photolysis of CH,ClCH,Cl at 147 nm quantum yield
@'
1,2,3,4-
run no.
Pc,H,c~,,torr
1
a
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
0.58 1.09 1.22 1.36 2.28 5.28 8.05 8.59 9.01 14.9 53.6 4.67 4.33 4.47 4.15 4.38 4.01 4.67 4.85 4.45 5.59 0.98 1.21 5.10 5.16 7.58 22.4 5.29
29
5.05
30
5.14
31
5.33
32
5.00
33
5.37
34 35 36 37 38 39 40 41
0.57 0.98 2.42 5.07 10.5 25.1 59.6 4.78
42
5.25
Padd, torr
CF, CF, CF, CF, CF, CF, CF, CF, CF, CF,
50.8 91.9 97.0 214 258 272 305 492 590 766 NO 0.13 NO 0.25 NO 0.47 NO 0.53 NO 1.16 NO 1.74 CF, 16.9 NO 1.08 CF, 289 NO 0.76 CF, 5 2 1 NO 0.90 CF, 595 NO 1.03 CF, 732 NO 0.83 CF, 763 NO 1.07 HI 0.08 HI 0.07 HI 0.29 HI 0.99 HI 1.15 HI 3.60 HI 6.25 CF, 30.1 HI 1.03 CF, 217 HI 1.15
C,H,Cl
C,H,
CH,ClCHCl,
0.70 0.72 0.74 0.73 0.74 0.70 0.70 0.70 0.70 0.70 0.71 0.67 0.65 0.65 0.60 0.60 0.60 0.56 0.52 0.47 0.49 0.74 0.75 0.75 0.76 0.73 0.70 0.65
0.35 0.32 0.28 0.24 0.24 0.30 0.24 0.25 0.25 0.24 0.19 0.20 0.15 0.15 0.14 0.13 0.14 0.14 0.13 0.12 0.13 0.17 0.17 0.17 0.16 0.15 0.16 0.14
0.069 0.069 0.061 0.064 0.061
0.20 0.18 0.18 0.17 0.15 0.17 0.15 0.14 0.14 0.12 0.08 0.06 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.60
'ZH4
0.061 0.052 0.048 0.048 0.041 0.039 0.031 0.036 0.040 0.067 0.031 0.039
C,H,Cl,
-,0.02 a
a 0.03
a a 0.03,
a a
0.01 0.046
0.00
0.14
0.028
0.00
0.59
0.14
0.040
0.00
0.57
0.14
0.032
0.00
0.55
0.13
0.045
0.54
0.13
0.037
0.76 0.75 0.78 0.80 0.77 0.75 0.76 0.74
0.18 0.18 0.17 0.16 0.16 0.18 0.18 0.15
0.047 0.056 0.051 0.047 0.059 0.056 0.056
0.62
0.14
0.049
a
Sought but not detected.
TABLE 11: Primary Processes in the 147- and 123.6-nm Photolyses of Haloethanes molecule
,
%
@"C1
CH CH, C1 CH,CHCl, CH,CC1, CH,ClCH, C1 CH, FCH, C1 CF, CH, C1 CF,ClCH,
0.6 0.6 0.6 0.2 0.4 0.2 0.8
CH, FCH,Cl CF,CH, C1 CF,ClCH,
0.5 0.3 0.8
0.2
@'HZ
@'bondfiss
comment
0.3
pressure dependent
1 4:7 nm 0.1 small
0.3
small small
1 2
3
this work
-0.8 0.3 0.1 (FCl)
ref
small
0.2
-0.3 small
small (FCl)
some HF elimn some HF elimn no H F elimn
8 10 7
some HF elimn some H F elimn some HF elimn
17
123.6 nm 0.3 0.4 (FC1)
small
small (FCl)
significant product. Small yields of trans-C2H2C12. (4 N 0.01) were observed at low pressures (
