Chlorine-Atom Sensitized Oxidation of CH2C12 and CH3CI
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Chlorine-Atom Sensitized Oxidation of Dichloromethane and Eugenio Sanhuezalb and Julian Heicklen* Department of Chemistry and Center for Air Environment Studies, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received July 1, 1974) Publication costs assisted by the Center for Air Environment Studies
Mixtures of Clz, 0 2 , and either CH2C12 or CH3Cl were irradiated at 3655 8, and 32". The Clz photodissociates and CHC12 or CHzCl radicals are produced uia hydrogen abstraction from the corresponding chlorinated methane. In the CH2C12 system, there is a long chain process, the initial products are HC1, CHClO, CC120, and possibly CO. @{CHClO]= 49 and @,ICC120]= 4.1 independent of CHzC12 or 0 2 pressure or I,. A complete mechanism is proposed. In the CH3Cl system, the initial products are exclusively HC1 and CHClO, the quantum yield of the latter being 2.0 independent of reaction conditions. Thus in this system there is no chain at all, and the mechanism is similar to that for CH3 oxidation, except that no alcohol is produced. In both systems the CHClO produced is removed by chlorine atom attack C1+ CHClO HCl ClCO C1+ CO (10) with h = 7 X lo8 M - l sec-l.
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Introduction As far as we know the oxidation of CHClz and CH2Cl radicals has not been studied, previously. Therefore in our continuing program on halocarbon oxidation, we have studied these oxidations and report the results here. The radicals were prepared from the reaction of C1 atoms with CH2C12 and CH3C1, respectively. Experimental Section Much of the experimental procedure has been described p r e v i o u ~ l y The . ~ ~ ~experiments were performed in a conventional Hg-free high-vacuum glass line equipped with Teflon stopcocks and Viton 0 rings. The light source was a Hanovia medium-pressure mercury lamp. The radiation passed through a Corning 7-54 filter and Pyrex glass to isolate the 3655-8, line before entering the stem of the T-shaped reaction cell. The reaction cell was situated in the sample beam of a Beckman IR-10 infrared spectrometer for continual analysis. Absolute light intensities were obtained from the chlorine-atom sensitized oxidation of CzC14 a t high 0 2 pressures, where the oxidation quantum yield was taken to be 3OOe4 The CHzCl:! was Eastman Kodak Spectro ACS grade, and the fraction volatile at -80" but condensable at -130" was used. The CH3Cl was from the Matheson Co., and the fraction volatile at -80" but condensable a t -196" was used. The chlorocarbon reactants and products were monitored by in situ infrared spectroscopy by monitoring the following bands with the listed peak extinction coefficients (to base 10). Molecule
Band, cm - 1
CHZC12 CHlCl CH3Cl CHClO CC1,O
1320 3020 1380 1783 950
Peak extinction coeff, Torr -1 cm -1 3.05 X 1 . 0 3 x 10-3 4.4
x
10-4
1.5 X 1 . 8 5 x lo-'
Unfortunately with CH3C1, both bands were overlapped by product bands, so that analysis was difficult and not
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very accurate. For short conversions the stronger band a t 3020 cm-l was used, with corrections made for the overlapping HC1 band at 3020 cm-l, using the HCI band at 2810 cm-l to estimate the correction. For longer exposures, the weaker band of CH&l at 1380 cm-l was used. CHClO is thermally unstable, so that a direct measure of its extinction coefficient is not possible. The value above was obtained from the initial rates in the rapid chlorineatom sensitized photooxidation of CHzCl2. Under these conditions thermal decay of CHClO is unimportant, and calibrations were made from mass balance considerations (i.e., CH2C12 consumed equals CHClO CClzO CO produced). The extinction coefficient obtained in this manner is reasonable, being only slightly larger than that of the carbonyl stretch in CC120. CO and COP were analyzed by gas chromatography after irradiation was terminated. For a few runs mass spectral analysis was also made after the termination of irradiation.
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Results
CH 2C12. When mixtures of Cl2, 0 2 , and CHZCl:! are irradiated at 3655 A, the initial major products are CHClO and HCl, with smaller amounts of CC120 and CO being produced. As the photolysis times increase the CHClO yields decrease, ultimately to zero, and the CO yields increase. Thus CO is a secondary product resulting from the decomposition of CHClO. Table I lists the initial quantum yields for CHzC12 consumption and CHClO and CClzO production. The yields show some scatter but essentially are independent of the CH2C12 pressure, the 0 2 pressure, or the absorbed intensity, I,, which were varied by factors of 4.3, 14, and 5.3, respectively. The average quantum yields are -@(CH2C12J= 56 f 4, @(CHClOj= 49 f 3, and @(CC120]= 4.1 & 0.4. Three runs were taken to completion so that all the CHaCl2 and CHClO were consumed. Then if the sole carbon-containing products are CO and CC120, their sum should equal the initial CHzClz pressure. This relationship is well obeyed for the second run in Table I, which was done at a relatively low 0 2 pressure. For two other runs at very high 0 2 pressures, there is a mass deficiency in the products. Presumably under these conditions some COz is The Journal of Physical Chemistry, Vol. 79, No. 1, 1975
a
Eugenio Sanhueza and Julian Heicklen
TABLE I: Chlorine-Atom Sensitized Oxidation of CH2C12at 32 i 2"
Symbol for Figures 1-3
[CHzC12], Torr
[02l, Torr
[Clz], Torr
I*,a mTorr/ see
4.43 7.05 8.46 8.66 8.73 10.7 16.5 18.9
79.5 19.4 5.65 23.0 70.7 53.7 59.6 68.3
9.41 1.80 5.33 5.37 5.29 9.45 2.04 9.37
1.41 0.27 0.80 0.805 0.80 1.42 0.31 1.41
e c m @
0 A
A
-+{ CH2C12)
cP{ CWClO)
50 57 60 53 52 62 54 61 AV = 56 i.4
43 50 52 54 47 50 47 50 AV = 49 i: 3
Footnote
+( CC120) 3.3 4.1 4.2 3.9 3.3 4.5 4.9 4.5 AV = 4 . 1 f. 0 . 4
--
d e c
f
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a Assuming quantum yield of CnCla oxidation to be 300 at high 0 2 pressure. b From initial rates. c a{CO] < 4 after photolysis time of 20 sec. (A[COl f A[CC120])/ - A[CHtC12] = 0.65 a t the end of the run when [CH2C12] 0 and [CHClO] 0. e (A[CO1 A[CC1201)/ - A[CH,CL] = 1.0 a t the end of the run when [CH2C12] 0 and [CHClO] 0. Photolysis time = 500 sec. (A[COl A [ C C ~ ~ O ] ) ~ A [ C H Z=C0.85 I ~ I at the end of the run when [CH2C12] 0 and [CHClO] 0. Photolysis time = 220 sec.
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-
+
N
-
A
0 IO 20 30 40 50 60 70 80 90 IO0 I10 I20 130 I40 150 160 170 180 190 200210 220230240 PF0TC)LYSIS T I M E , SEC
the loss of CH2C12divided by I, vs. the photolysis time in the chlorine-atom sensitized oxidation of CH2CIPat 32'. The symbol key is listed in Table I.
Figure 1. Plot of
also produced, which could account for the deficiency. We did not analyze for CO2, since on our chromatographic column, CClzO interfered with the analysis. More complete time histories of the data in Table I are displayed in Figures 1-3. The loss of CH2C12 is shown in Figure 1. When normalized for I,, all the data can be fitted by the same straight line. The best fit of the data shows a slight induction period of about 4 sec, but this is probably an experimental artifact. The slope of the line, which is now heavily weighted by the longer conversion experiments, gives -@{CH&) = 62, slightly higher than the value of 56 obtained from Table I. Figure 2 displays the CC120 data. All the data can be fitted by a straight line passing through the origin. The slope of the line gives @{CC120)= 4.4, again slightly higher than the value of 4.1 obtained from Table I. Figure 3 shows three typical time histories for CHC10. This molecule is an initial product, but the CHClO pressure passes through a maximum, and then falls to zero as the irradiation proceeds. The peak CHClO pressures, [CHClO],,,, and the corresponding CHzClz pressures, [CH2C12Imax,at the [CHClO] maximum are The Journal of Physical Chemistry, Vol. 79. No. 1 , 1975
[CHClO Lax, Torr 1.25 3.15 5.45
[CH,CL],,,, Torr
[CHzCLIn.ax/ [CHClO],,,
2 .o 5.20 9.45
1.6 1.65 1.74
CH3C1. When mixtures of CH3C1, 0 2 , and Clp are irradiated at 3655 A,the main products are CHClO and HCl. CO is also produced with a quantum yield 1 0 . 2 for short conversions, and, as with CH2C12, it is a secondary product resulting from the decay of CHC10. The COS produced was negligible, its quantum yield being about 0.01. Mass spectral analysis also showed the presence of HzO as a product as well as mass spectral cracking peak at m/e 79-81 (ClCO2') and 80-82 (ClCH02+). These high mass peaks suggest CClH202H and/or (CClH20)Z as products. Absent were peaks at mfe 31. 65, and 66, indicating that CClH2OH was not a product. Formic acid, HCOOH, was also produced as a secondary product, and can be attributed to CHClO decay in the presence of H20.j There was no evidence for either CC120 or CH20 production.
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Chlorine-Atom Sensitized Oxidation of CH2CI2 and CH3CI
TABLE 11: Chlorine-Atom Sensitized Oxidation of CH,,CI at 32 i 2"
[CHaCl], TOIT
[02], Torr
[ C ~ PTorr ],
358 64.3 20.4 37.2 65.3 62.2 59.7 43.8 104
9.41 9.76 9.49 5.68 1.88 9.49 19.6 9.45 30.2
5.72 6.90 7.13 13.1 15.8 16.3 20.7 33.8 60.0
mTorr/sec 1.41 1.46
- @(CHsClj
-2.2 -2.0 -1.8
i ' . ~ 0.85 0.28 1.42 2.94 1.42 4.5
-1.7 -2
.o
+{CHCl0)b 2 .o 2.05 2 .o 2.05 2.1 2 .o 1.9 2 .O 1.81
Footnotes C
d C
f
e
d,g d
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Assuming quantum yield of C&ld oxidation t o be 300 at high 02 pressure. From initial rates. c - @{CH,Cl)estimated a t high conversion. - @{CH,Cl}estimated from initial rates @( CO} < 0.2 after photolysis time of 200 sec. ' @ { CO?} after photolysis time of 1252 sec. @( HCOOH] 0.8 after photolysis time of 1500 sec followed by dark thermal decay of 0.2, @{ COP{ 10-2. CHClO to completion. @ { C O ] a
