Environ. Sci. Technol. 1994,28,2306-2313
Tropospheric Reaction Products and Mechanisms of the Hydrochlorofluorocarbons 141b, 142b, 225ca, and 225cb 3nesto C. Tuazon' and Roger Atkinson
Statewide Air Pollution Research Center, University of California, Riverside, California 92521 Fhe C1 atom-initiated photooxidation of CF3CF2CHC12 HCFC-225ca) produced CF3CF2C(O)Cl in 100% yield, shile CFzClCFzCHFCl (HCFC-225cb)yielded CF2ClCF2?O)F (99%) and C(O)FCl(l%). The two-carbon aldeiydes CFClzCHO and CFzClCHO were positively observed is photooxidation products of CFC12CH3 (HCFC-14lb) ind CF2ClCH3 (HCFC-l42b), respectively. Their calcuated 100% formation yields show conclusively that under ower tropospheric conditions the alkoxy radicals CFC12:HzO and CFzClCHzO do not undergo C-C bond scission, jut rather react with 02 to form the aldehydes and HOz. Phe contributions of two competing reaction pathways or the acyl radicals formed from the OH radical-initiated eactions of halogenated aldehydes, CX&O CX3 + CO a) and CX&O 02 CX&(O)OO (b),were determined is 79 f 7 % (a) and 21 f 5 % (b) for the acyl radical CFC12:O and 39 f 3% (a) and 61 f 5% (b) for CFzClCO at 298 < and 740 Torr of air. These results are intermediate and monsistent with those reported in the literature for the torresponding reactions of CCl3CO and CF&O radicals.
+
-
-
the key intermediate species being the haloalkoxy radical CX&YZO. The haloalkoxy radicals can react in the troposphere via three pathways: CX3CYZ0
-
CX,
CX3CYZ0 + 0,
CX,CYZO
-
+ C(0)YZ
-
(C-C bond scissicn) (1) CX,C(O)Y
(0,reaction; Z = H) (2)
CX,C(O)Y + c1 (C1 atom elimination; Z = C1) (3)
When the first-generation product is an aldehyde (CX3CHO) formed from an HCFC or an HFC containing a CH3 group via reaction 2, then a further series of reactions occur, initiated by photolysis and reaction with the OH radical. The OH radical reaction leads to the formation of the acyl radical OH + CX3CH0
ntroduction As a result of the imminent phase-cut of the chloroluorocarbons (CFCs) and the halons, which are believed esponsible for stratospheric ozone depletion (1-41, a lumber of replacement compounds have been identified vhich react in the troposphere and hence do not lead to ransport of chlorine to the stratosphere to any significant xtent (4-6). These replacement compounds include the iydrochlorofluorocarbons (HCFCs) and the hydrofluoocarbons (HFCs) ( 6 , 7 ) ,and their tropospheric reactions vith the hydroxyl (OH) radical result in atmospheric ifetimes of typically 99.99% purity, were donated by Solvay S. A. CFzClCHO was prepared by adding 5 mL of concentrated H2S04 to 10 g of a commercial sample of chlorodifluoroacetaldehyde hydrate (75 % ,Strem Chemicals, Inc.), collecting -5 mL of the most volatile fraction of the mixture under vacuum a t room temperature, and purifying this fraction by trap-to-trap distillation a t 0, -78, and -195 "C trap temperatures. The fraction collected a t liquid N2 temperature (-195 "C) did not show a detectable change in its infrared spectrum upon further fractionation. The frequency maximum of its C=O stretch absorption band in the vapor phase was recorded a t 1778.6 cm-1. Its 1H NMR spectrum showed a triplet a t 6 = 9.26 ppm with JFH = 3.26 cps (av), in agreement with those reported by Yamada et al. (20). CFClzCHO was prepared by LiAlH4 reduction of CFC12C(O)OCH3 (1:4 mole ratio) by a method similar to that described for the preparation of perfluoroaldehydes by Pierce and Kane (21). Analysis of the redistilled sample by infrared spectroscopy detected the presence of 2.7 %
+
CF3CF2CHCi2(HCFC-225col 12.3
CF,CF2 C(0IC I 9.3 C
'roo
400
iioo
1300
I/X
1500
1900
i$oo
zioo
(cm-')
Figure 1. Infrared spectra from CF3CF2CHCI2-Cl2-alr photolysis (run EC-1516). (A) Initial CF3CF2CHC12.(6)Mixture with 75% CF3CF2CHCl2 reacted. (C) From panel B after subtraction of absorptlons by the remaining reactant. Numbers given are concentrations in units of 1013 molecule ~ m - ~ .
CFC12C(O)OCH3as impurity. Further evaluations of the purities of the CFClzCHO and CFzClCHO preparations are presented in the Results and Discussion. The C=O stretch frequency of gaseous CFClzCHO was recorded at 1782.7 cm-l, and its lH NMR spectrum showed a doublet a t 6 = 9.12 ppm with JFH= 5.0 cps, in agreement with the data of Yamada et al. (22). The ester CFC12C(O)OCH3 was obtained by heating a mixture of CC13C(O)OCH3 (Aldrich, 99%) and SbF3 (3:l mole ratio; with a catalytic amount of Br2 added) and continuously distilling CFC12C(O)OCH3 as it was formed. Results and Discussion
Products of C1 Atom-Initiated Photooxidation of CF3CFzCHClz and CFzClCFzCHFCl. CF3CF2CHC12 (HCFC-225ca). An irradiation experiment (run EC-1516) was carried out with a mixture of 1.23 X 1014 molecule ~ ~ m of-CF3CF2CHC12 ~ and 1.05 X 1015molecule ~ m of-Cl2 in air. Only one product was observed, based on a detailed comparison of the spectra at various stages of the irradiation. The C=O stretch absorption frequency observed a t 1815 cm-l in the product spectrum (Figure 1) is consistent with those of acyl chlorides (23) and thus with the expected product CF3CF2C(O)Cl. The spectrum of this product (Figure 1C) is in good agreement with that obtained by Sat0 and Nakamura (24) from a similar C1initiated photooxidation of HCFC-225ca. The plot of the integrated band intensity of the C=O stretch absorption against the amount of CF3CFzCHClz consumed was linear (correlation coefficient = 1.000). On the basis of a unit yield, the integrated absorption coefficient (base 10) for the C=O stretch absorption band of CF&F&(O)Cl in the range 1750-1880 cm-l (baseline corrected) was calculated from the slope of this plot to be (1.25 f 0.01) x 1O-l' cm molecule-l. The product CF3CF2C(O)Cl is expected (6) from the following sequence of reactions:
+
+
CF3CF2CHC12 C1- CF3CF2CC12 HC1
(9)
Environ. Sci. Technol., Vol. 28, No. 13, 1994 2307
il
1 ii
0.95
Ln
CF2CICFzCHFCI (HCFC-225cb)
i n
01
i2.3
f
0
B
Irradiation Time
&&kL
m ,
,
,
,
,
4.5min
0
, _, ,
W U
z
E?
D
m m
a ~ O O
Boo
1 io0
1500
1500
i-foo
1400
eioo
Flgure 2. Infrared spectra from CF2CICF2CHFCI-Cl2-air photolysis (run EC-1517). (A) Initial CF2CICF2CHFCI.(B) Mixture with 76% CFpCICF2CHFCIreacted. (C) From panel B after subtraction of absorptions by the remaining reactant. Asterisk denotes a superimposed minor absorption by C(0)FCI (see text). Numbers given are concentrations in units of 10'3 molecule cm4.
+ 0,
-
2CF3CFzCC1,00 CF3CF,CC1,0
-
! .Ornin
2CF3CFzCC1,0 + 0,
CF3CF,C(0)Cl + C1
-
0.09 0 0
0.03
t ; 0.5 rnin
CFCIzCHO
C(0IFCI
I
0
'1 00
1750
1800
I/x
CF3CFZCC1,00
(10) (11)
(12)
~
CF,ClCF,C(O)F
+ C1
(13)
The small amount of C(O)FC1 observed indicates that decomposition via C-C bond cleavage occurs to a minor 2308
0.10
0
M
The haloalkoxyradical CF3CF2CC120could also possibly decompose via C-C bond scission to produce the CF3CF2 radical and C(0)ClZ. The strong infrared band of C(0)Clz at 850 cm-1 would have been evident in the product spectrum shown in Figure 1if it was formed in significant amounts. The upper limit yield of phosgene from HCFC225ca in the above experiment is estimated to be 50.5 % CF2ClCF2CHFCl (HCFC-225cb). A mixture of 1.23 X 1014molecule ~ m of- CF2ClCF2CHFCl ~ with 1.26 X 1015 molecule ~ m of- Cl2 ~ in air was photolyzed (run EC-1517). The infrared spectra (Figure 2) showed one major product, with the position of its C=O stretch absorption at 1886 cm-1 being consistent with those of acyl fluorides (23)and with the expected product CF2ClCFzC(O)F. The spectrum of this product (Figure 2C) is in good agreement with that obtained by Sat0 and Nakamura (24) from a similar experiment with HCFC-225cb. C(0)FCl was also identified as a minor product by its absorption at 1094 cm-l, and its yield was determined to be 1.0 f 0.1 5%. A linear plot (correlation coefficient = 1.000) was obtained for the integrated band area of the 1886 cm-I C=O stretch of CF&lCF&(O)F versus the amount of CFzClCFzCHFCl reacted less the amount of C(0)FCl formed. The integrated absorption coefficient in the range of 1786-1986 cm-1 (baseline corrected) was determined from the slope of this plot as (1.33 f 0.01) x 1O-l' cm molecule-l. The haloalkoxy radical CFzClCFzCFClO is expected to be formed from CFzClCFzCHFCl via steps analogous to reactions 9-1 1and to undergo C1 atom elimination to yield the major product CF&lCF&(O)F: CF,ClCF,CFClO
I
(ern-')
I/X
CF,CFzCC1,
-
Environ. Sci. Technol., Voi. 28, No. 13, 1994
1850
1900
I
1950
k"")
Figure 3. Product spectra from the CI atom-initiated photooxidation of CFC12CH3(run EC-1543). Numbers given are concentrations in units ~ . ordinates of the upper plots are offset for of l o i 3molecule ~ m - The clarity.
extent:
-
CFzCICFzCFC1O
CFzCICF, 4- C(0)FCl
(14)
The CFzClCFz radical formed in step 14 could then react with 0 2 and undergo a sequence of reactions (see, for example, refs 6 and 14) to yield two molecules of C(0)F2. While C(O)F2 was possibly formed at concentrations comparable to those of C(O)FCl, its detection at these levels from the product spectra was much less certain than that of C(0)FCl due t o the relatively weaker features of the infrared bands of C(O)F2. Aldehyde Yields from C1 Atom-Initiated Photooxidations of CFClzCH3 and CFzClCH3. CFCl2CH3 (HCFC-14lb). The determination of the yield of CFC12CHO from the C1 atom-initiated photooxidation of CFC12CH3 required the use of high concentrations of the parent compound due to the much faster reaction of C1 atoms with the aldehyde (26,26)relative to that with the parent HCFC (27). The use of a highly purified sample of CFC12CH3 (>99.99%, Solvay S. A.) was critical since preliminary experiments using samples of lesser purities (599.5 96) indicated the formation of other C=O containing products which interfered with the measurement of CFClzCHO by infrared spectroscopy. A photolysis experiment with initial concentrations of 4.92 X 10l5 molecule of CFC12CH3 and 1.26 X lo1* molecule ~ m of- Clz ~ in air was carried out in the 5800-L chamber (run EC-1543). Intermittent irradiation was employed to allow for longer signal averaging in recording the infrared spectra during the intervening dark periods. A time series of spectra from this run is presented in Figure 3, which shows the rapid accumulation of C(0)FCland an apparently much slower buildup of CFClzCHO with irradiation time. (CO and C02 were also observed as products.) These products could only be measured in this spectral range due to the presence of saturated absorptions by the parent compound in most of the other spectral
Table 1. Product Yields during C1 Atom-Initiated Photooxidation of CFC12CHa (Run EC-1543)s irradiation time (min)
%
[C(O)FCllob,b
0.5
0.027
1.0
0.10
1.5 2.5
0.20 0.44 0.95 1.41 2.13 2.77 3.48
4.5
6.5 9.5
12.5 15.5
[CFC12CHOl0b,b 0.094 0.14 0.18 0.21 0.22 0.23 0.23 0.22 0.21
Fd
reactionc 0.025 0.049
[CFC1zCHOI& 0.13 0.25 0.42 0.74 1.4 2.0 2.9 3.5 4.1
1.36 1.77
2.32 3.50 6.26 8.61 12.5 16.0 19.6
0.077
0.13 0.24 0.33
0.48 0.61 0.75
*
a Initial concentrations: [CFClzCH& = 4.92 X 1015molecule ~ m and - ~[Cl& = 1.26 X 1014molecule ~ m - ~Concentration . in units of l O l 3 + [CFC12CHO],bB]/[CFClzCH& Calculated multiplication factor to correct the measured CFClzCHO molecule ~ m - ~100([C(O)FC1],,b8 . concentrations for reaction with C1 atoms (see text).
an intrinsic yield of CFCl2CHO from CFC12CH3 that is large relative to that of C(0)FCl. The buildup of the latter results from the rapid conversion of the aldehyde to C(0)FC1 by reaction with C1 atoms: CFC1,CHO
+ C1-
+ -
CFC1,CO
CFC1,CO
CFC1,
+ HC1
+ CO
(20) (21)
M
CFC1,CO
CFCI2CHO
0.45
0 0 0
0,
2CFC12C(0)00 CFCl,CO,
01%
1800
lj50
1650
1900
1950
(crn")
I/X
Flgure 4. Reference spectra of CFCl&HO and C(0)FCi. Numbers ~ . length given are concentrations in units of IOi3 molecule ~ m - Path = 57.67 m; resolution = 0.7 crn-'. The ordinates of the upper plot are offset for clarity.
regions. Reference spectra of C(0)FCl and CFClzCHO are shown in Figure 4. The CFCl2CHO calibration was based on a prepared sample (see Experimental Section; also cf. below), while the C(0)FCl calibration was based on a 100% yield from the C1atom-initiated photooxidation of CHFCl2 (14). The observed yields of CFCl2CHO and C(0)FCl are reported in Table 1. The formation of the alkoxy radical from CFC12CH3 is expected from the reactions CFCl,CH,
+ C1-
CFCl,CH,
+ 0,
2CFC1,CH200
CFCl,CH,
-
+ HC1
(15)
M
-
CFC1,CH200
2CFCl,CH,O
+ 0,
(16) (17)
The alkoxy radical can then decompose by C-C bond scission (reaction 18) or react with 02 (reaction 19): CFCl,CH,O CFCl,CH,O
-
+ 0,
+ HCHO CFC1,CHO + HO,
CFC1,
-
(18) (19)
The relative contributions of these two reactions were not determined in previous studies (8,14) since both pathways ultimately yield C(O)FCl, which was the only product that could be reliably measured. As shown in detail later, the product data depicted in Figure 3 actually correspond to
CFCl,C(0)00
(22)
2CFC1,C02 + 0,
(23)
CFC1,
+ CO,
The CFCl2 radical then reacts with 0 2 leading, via a sequence of reactions, to the formation of C(0)FCl in unit yield (14). The amounts of aldehyde formed (Table 1) can be corrected for reaction with C1 atoms following the method described by Atkinson et al. (28). The required inputs are the ratio of the rate constants for reaction of the C1 atom with CFClzCHO and CFCl2CH3 and the percentage of CFC12CH3 reacted. In the present calculation, the total amount of CFClzCH3 reacted was assumed to be the sum of the observed C(0)FCl and CFClzCHO concentrations which, for the experimental conditions employed (run EC1543),did not exceed 0.8% of the initial reactant throughout the total of 15.5-min irradiation. The published rate constants are k(Cl+ CFCLCHO) = (5.7 f 1.2) x lo-', cm3 molecule-'s-l (25) and k(C1+ CFC12CHs) = (2.2 f 0.4) x lO-I5 cm3 molecule-l s-1 [average of literature values from Tuazon et al. (27) and Wallington and Hurley (29)l. Hence, the rate constant ratio is k(C1+ CFCl&HO)/k(Cl + CFC12CH3) = 2600 f 700. The calculated multiplication factors (F)to take into account secondary reactions of CFC12CHO are given in Table 1 and were applied to the observed CFCl2CHO concentrations to obtain the corresponding concentrations corrected for reaction with C1 atoms. The plot of [CFC12CHOI,,,, vs ([C(O)FCll,b, + [CFC12CHO0b,J for the rate constant ratio 2600 is presented in Figure 5, with the slope of the straight-line plot indicating a CFClzCHO yield of 116%. Also included in Figure 5 is a similar plot for an assumed rate constant ratio of 2200, which is well within the uncertainty of the experimental value and which leads to a 97% yield of CFClzCHO from CFC12CH3. Thus, within the experimental errors and/or uncertainties in the published rate constants, the data presented in Table 1 Envlron. Scl. Technol., Vol. 28, No. 13, 1994 2309
I
0
;lJ700
0.0
le00
1850
1900
195oooo
I / X (cm-') Flgure 7. Reference spectra CF2CICH0, CF,CIC(O)OH, and C(0)F2. Numbers given are concentrations in units of I O l 3 molecule cm4. Pathlength = 57.67 m; resolution = 0.7 cm-l. The ordinates of the upper plots are offset for clarity.
-
lrrodiotton
Time
7 rnin
3 min
2 rnin u
while that of C(O)F2 was based on a 100% yield from the C1 atom-initiated photooxidation of CHFzCl (13, 1 4 ) . Analogous to the case of CFClzCH3, it is evident from the progression in the relative yields of CF2ClCHO and C(0)F2 in Figure 6 that CFzClCHO is the primary product, and C(O)F2 could be expected as a secondary product which is efficiently formed from CFzClCHO. C(O)F2 (apart from CO and C02) was the only product directly observed in the previous studies (10, 14). The reactions leading to the formation of the alkoxy radical CFzClCHzO from CFzClCH3 are analogous to reactions 15-17 for CFClzCH3. The larger yield of CF2ClCHO than C(O)F2 during the very early stage of the irradiation indicates that the dominant reaction channel for the alkoxy radical is reaction with 02 CF2C1CH20+ 0,
I min
'
0 0
CF2C I C HO
I
if50
-
CF2C1CH0 + HO,
and that decomposition via C-C important:
C(0)FZ
bond breakage is not
i , CF2C1CH20- CF2C1+ HCHO
0
,1700
1750
18CO
1850
1000
1350
2600
(ern-') Flgure 6. Product spectra from the CI atom-initiated photooxidation of CF2ClCH3(run EC-1541). Numbers given are concentrations in units of I O ' ? molecule ~ m - The ~ . ordinates of the upper plots are offset for clarity. i/X
indicate that the alkoxy radical CFC12CH20 does not undergo C-C bond scission and that a t room temperature in air it reacts solely with 0 2 to form CFC12CHO. CF2C1CH3 (HCFC-142b). The sample of CF2ClCH3 used in these experiments was donated by Solvay S. A. and had a stated purity of >99.99%. Even higher concentrations of CF2ClCH3 than those used for CFC12CH3 were needed to detect and measure the corresponding product aldehyde. Thus, the spectral data presented in Figure 6 were obtained from the irradiation of an air mixture of 2.56 x 10l6molecule ~ mof-CFzClCHs ~ and 1.26 X 1014molecule ~ m of- Clz ~ (run EC-1540). As depicted in Figure 6, three products were identified and measured: CF2ClCH0, C(O)F2, and CFzClC(0)OH. Reference spectra of these three products are presented in Figure 7. The CFzClCHO calibration was based on a prepared sample (see Experimental Section; also cf. below), 2310
Environ. Scl. Technol., Vol. 28, No. 13, 1994
(25)
(26)
The results for the C1 atom-initiated photooxidation of CF2ClCH3 are summarized in Table 2. The factors, F , which correct the yields of CFzClCHO for reaction with C1 atoms, were derived using the published rate constants k(C1 CF2ClCHO) = (4.5 f 0.3) x 10-l2 cm3 molecule-' s-1 (25) and k(C1 + CF2ClCH3) = (3.8 f 0.6) X cm3 molecule-l s-l [average of literature values from Tuazon et al. (27) and Wallington and Hurley (2911, with a rate constant ratio k(C1+ CF2ClCHO)/k(Cl+ CF2ClCH3) of 11 800 f 2000. A very small fraction of the starting compound was consumed (-0.1 % with 11-min total irradiation time) such that the amount of CFzClCH3 reacted was calculated by the sum of the observed C(O)F2, CF,ClC(O)OH, and CF2ClCHO concentrations. The plot Of [CF2ClCHOIcorr VS ([C(O)F2Iobs + [CF2ClC(O)OHIobs [CF2C1CHO],b,) for the rate constant ratio 11800 is presented in Figure 8, with the straight-line plot indicating a CF2ClCHO yield of 167%. A very good fit to the experimental data and a prediction of 100% yield of CF2ClCHO from CF2ClCHs were obtained with an assumed rate constant ratio of 7400 (Figure 8),the latter ratio being approximately 40 % lower than the value of 11800 derived
+
+
Table 2. Product Yields during C1 Atom-Initiated Photooxidation of CF2ClCHs (Run E C - K I ~ ~ ) ~
irradiation time (min)
%
[C(O)Fzlobsb
0.5 1 1.5 2 3 4 5
[CFZC~C(O)OH]O~,~ [CF2ClCHO],b,b
0.013 0.080 0.17 0.27 0.49 0.72 0.95 1.35 1.85 2.28
7 9 11
0.11 0.18 0.25 0.28 0.33 0.33 0.33 0.33 0.35 0.36
0.020 0.027 0.049 0.069 0.091 0.13 0.18 0.21
reactionC
Fd
0.005 0.010 0.017 0.023 0.034 0.044 0.054 0.071 0.093 0.11
1.32 1.70 2.32 2.91 4.09 5.22 6.39 8.39 11.0 13.0
[CFZC~CHOICO~' 0.15 0.31 0.58 0.81 1.4 1.7 2.1 2.8 3.9 4.7
Initial concentrations: [CFZCICH~IO = 2.56 X 1OI6 molecule ~ m and - ~[Cl~lo= 1.26 X 1014molecule cm3. Concentration in units of 1013 + [CFzC1C(O)OHl0b,+ [ C F Z ~ I ~ H O I ~ ~ , ) / [ C F ~ CCalculatedmultiplication ICH~~~. factor tocorrect CFzClCHO molecule ~ m - ~100([C(0)Fzlob, . concentrations for reaction with C1 atoms (see text).
-
slope = 1.67 k 0.04 corr. coeff. = 0.999
3.2
I
/
2.4
1.6
0.8
0.0 0.0
0.8
2.4
1.6
-A[CF,ClCH,],
lOI3
3.2
4.0
molecule cm
4.8
-3
Flgure 8. Yields of CF2CICH0from CF2CICHscorrected for its reaction with CI: (A)with the rate constant ratio of 11 800 derivedfrom published data; ( 0 )with an assumed rate constant ratio of 7400 (see text).
from the published rate constants. Such a change in the rate constant ratio is not unlikely since, for example, the two recent measurements of the rate constant for the reaction of the C1 atom with CF3CHO differ by 50 % (25, 26). However, the overestimate in the corrected CFzClCHO concentrations may not necessarily be entirely due to the uncertainties in the published rate constants and the experimental results. The ensuing reaction of the CF2ClCHO with C1 atoms can, in fact, lead to some regeneration of the aldehyde: CF2C1CH0 + C1- CF2C1C0 + HC1
(27)
-
(28)
CF2C1C0 + 0,
M
CF2C1C(0)00
-
CF2C1C(0)00+ CF2C1CH200 CF2C1C(0)OH+ CF2C1CH0 + 0, (29) The significant amounts of the CF&lC(O)OH formed (Table 2) suggest that the formation of the aldehyde via the above reaction of the peroxy species may be significant, and the contribution of this reaction would lead to an overestimate of the correction for loss of CF2ClCH0 by reaction with the C1 atom.
Our present product data for the C1 atom-initiated reactions of HCFC-141b and HCFC-l42b, leading within the experimental uncertainties to unit yields of the corresponding aldehydes CFClzCHO and CF2ClCH0, are consistent with the time-resolved measurements of Zellner et aZ. (30). Zellner et aZ. (30) used laser absorption and laser-induced fluorescencemethods to monitor OH radicals and NO2 after the laser photolysis formation of the haloalkyl radical in the presence of 0 2 and NO. For both HCFC-141b and HCFC-l42b, Zellner et al. (30)concluded that reaction of the alkoxy radical CF,C13-xCH20 with 0 2 would dominate under atmospheric conditions and that the C-C bond scission pathway will account for 5 f 5 % of the overall alkoxy radical reaction for HCFC-141b and 8 f 8% for HCFC-142b in the lower troposphere. Additionally, Hayman (31) has reported the formation of CF2ClC(O)OH from the C1 atom-initiated reaction of HCFC-l42b, in agreement with our present observation (Figure 6), indicating that the intermediate alkoxy radical cannot solely decompose. C1 Atom-Initiated Photooxidation of CFC1,CHO and CF2ClCHO. Further details of the atmospheric transformations of HCFC-141b and HCFC-142b were studied by examining the reactions of their respective aldehyde products CFCl2CHO and CF2ClCHO. Photooxidation experiments were conducted in the 5800-L chamber at 298 K in 1 atm of synthetic air containing (1.1-1.2) x 1014molecule ~ m of- the ~ aldehyde and (6.37.3) x 1013molecule ~ m of- Cl2, ~ with the irradiation being carried out intermittently. In addition to the products C(0)FCl (from CFC12CHO) and C(O)F2(from CF2ClCHO), particular attention was given to the accurate measurements of CO and C02 formed during these reactions. The spectrometer's 220-L enclosure, and hence the optical path outside the reaction chamber, was purged of atmospheric CO and C02 with a constant flow (20 L min-l) of N2 gas (head gas from a liquid N2 tank) for 1 3 h prior to the irradiations, and the equilibrated N2 flow was maintained throughout the experiments. Calibration curves for CO and C02 in the concentration - ~ constructed by range (1.2-12) X l O I 3 molecule ~ m were injection of known amounts of CO and C02 into the 5800-L chamber and recording their infrared spectra at the 57.67-m path length employed in the irradiation experiments. At the spectral resolution employed (0.7 cm-1, fwhm), the plots of the intensities of the well-resolved absorption bands of CO a t 2143 cm-l and C02 at 2349 cm-1 against concentrations displayed some (known and expected) nonlinearity. The absorption band of CO2 a t 667 Environ. Sci. Technol., Vol. 28, No. 13, 1994 2311
Table 3. Reactant and Product Concentrationse during C1 Atom-Initiated Photooxidation of CFClzCHO (Run EC-1547)
Table 4. Reactant and Product Concentrationse during C1 Atom-Initiated Photooxidation of CFzClCHO (Run EC-1546)
irradiation time (min) [Clz]~ [CFClpCHOlb [C(O)FCll
irradiation time (min)
7.3
0
0.5 1.0 2.0
[CO]
[COzl
3.89 7.11 8.31
1.03 2.09 2.66
11.1
6.13 2.12
0.22
4.55 8.34 9.96
In units of 10'3 molecule ~ m - b~Based . on the partial pressure measurement of the initial sample injected (see text).
cm-l, which gave a more linear calibration plot, was also employed in the measurements. The yields of CO and C02 from the organic impurities in the diluent air were estimated by irradiation of an air mixture of 7.3 X 1013molecule ~ m of- Clz. ~ The concentrations of COz formed were 4.2 X 10l1 molecule ~ m and - ~ 1.2 X 10l2molecule cm-3 after total irradiation times of 1 and 2 min, respectively. Corresponding CO levels were not detectable ( I 1 X 10l2 molecule cm-3) within this irradiation period. These levels of CO and COz arising from the organic impurities of the diluent air were negligible compared to those produced by the C1 atominitiated reactions of the aldehydes in the experiments described below with comparable periods of irradiation. CFCl2CHO. The results of the photolysis experiment with the CFCl2CHO-Cl2-air mixture (run EC-1547) are summarized in Table 3. As noted in the Experimental Section, the exact purity of the prepared sample could not be determined by infrared spectroscopy except for the presence of -3 % CFClzC(O)OCH3. The concentration of the latter compound did not change significantly during the irradiation. The amounts of C(0)FCl formed were equal to 92% of the amounts of CFClzCHO sample reacted, and thus for an expected 1:l yield of C(0)FCl from the aldehyde, the above value is the best estimate of the purity of the CFClzCHO preparation. As noted above, CO formation from CFClzCHO follows from reactions 20 and 21, with further reaction of CFClz yielding C(0)FCl. C02 would be produced if CFClzCO formed in reaction 20 also reacts with 0 2 as shown in reactions 22-24. In general agreement with the occurrence of reactions 20-24, the sums of CO and COZ formed approximated the amounts of C(O)FC1 formed, being 10% higher than the latter (Table 3). CO could possibly react with C1 atoms, but under the present experimental conditions it can be shown from the rate constants for the C1 + CO M reaction (19) and the C1 CFClzCHO reaction (25) that the correction is negligible. Hence, it is estimated from the data of Table 3 that 79 f 7 % of the CFClzCO radicals reacted via decomposition to yield CO and the CFC12 radical, while 21 f 5% reacted with 0 2 resulting in the formation of COz and the CFC12 radical. The errors given for the yields include the two least-squares standard deviations and the errors due to the estimated f 5 % uncertainty in the calibration for each of CQ, C02, and C(0)FCl. CF&lCHO. The results of the CF2C1CHO-C12-air photolysis (run EC-1546) are summarized in Table 4. The amounts of C(0)Fz formed corresponded to 98% of the CFzClCHO sample reacted, and on the basis of the expected 1:1 yield, this value is a good estimate of the purity of the CFzCICHO sample prepared. The higher purity of this sample, compared to the CFClzCHO sample above, resulted in much better agreement (98-101 7a )
-
+
2312
Envlron. Sci. Technol., Vol.
0
12.3 8.98 4.08 1.06
6.3
0.5 1.0 1.5
3.25 8.07 11.0
[CO]
[COzl
1.30
1.97
3.10 4.18
4.85
6.67
a In units of l O I 3 molecule ~ m - ~Based . on the partial pressure measurement of the initial sample injected (see text).
between the sums of the CO and C02 formed and the amounts of C(O)F2 produced. This is expected from the reaction sequence totally analagous to 20-24, with further reactions of the CFzCl radical leading to the formation of C(O)F2 (13, 14). The results presented in Table 4 indicate that, a t 298 K and 740 Torr total pressure of air, 39 f 3 % of the CF2ClCO radicals undergo decomposition as measured from the CO formation and 61 f 5 % react with 0 2 as estimated from the CO2 formation. The stated errors in the yields are the total of the two least-squares standard deviations and the errors arising from the estimated f 5 % uncertainty in the calibration for each of CO, CQ2, and C(O)F2. Our present data concerning the decomposition versus reaction with 0 2 of the CFClzCO and CFzClCO radicals are consistent with the data reported by Barnes et al. ( 17) for the corresponding reactions of the CC13CO and CF3CO radicals. Barnes et al. (17)determined that under all tropospheric conditions the CC13CO radical decomposes to CO and the CC13radical, while the CF3CO radical reacts with 02 to form the trifluoroacetyl peroxy radical CF&(O)OO. Our data for the CFClzCO and CFZClCO radicals then form a trend toward decomposition being more important the more C1 atoms are present in the CX3CO radical. Under lower tropospheric conditions, the CFClzCO radical formed from HCFC-141b will predominantly decompose, while the CF2ClCO radical formed from HCFC-142b will undergo both decomposition and addition of Oz. These reactions of the intermediate acyl radicals are important since the further reactions of the acyl peroxy radicals can lead to the CZacids CFC12C(O)QH and CF2ClC(0)OH via reaction with the HO2 radical: CFCI,C(O)OH CFCI&(O)OO
+
+ 0,
(30a)
HOz
CFCI,C(O)OOH
+
28, No. 13, 1994
[Clz]~ [CF~CICHOlb [C(O)Fzl
+ 0,
(30b)
Conclusions The product data presented here for the C1 atominitiated reactions of HCFC-141b and HCFC-142b provide the needed information for the elucidation of the tropospheric chemistry of these two HCFCs. Our data show that the applicable series of reactions for these two HCFCs are as illustrated for HCFC-141b in Scheme 1. In Scheme 1, it is indicated that the peroxy radical RCHz00 and peroxyacyl radical RC(0)OOwill predominantly react with NO under tropospheric conditions instead of undergoing the self-reactions which prevailed in the laboratory systems employed in this study. It should be noted that data concerning the photodissociation quantum yields of the aldehydes are also required before a reasonably complete understanding of the detailed atmospheric chemistry of HCFC-141b and HCFC-142b is reached. Additionally,
Scheme I CFCI2CH2 + H g 0
OH + CFCL2CH3
(7) (8)
HO CFC12CH200H \
CFC12CHp00
(9)
OH
(10)
CFCl2CH20
(11)
\ lo2
(12)
CFCI2CHO
(13) (14) (15) (16) NO
CFC12C(0)OON02
?=$
J
NO
CFC12C(0)00
-j-CFC12
0 ----z C(0)FCI
(17)
ing Project Report No. 20, Vol. 11, Appendix: AFEAS Report; World Meteorological Organization: Geneva, Switzerland, 1990. Hoffman, J. S. Ambio 1990, 19, 329. Edney, E. 0.;Gay, B. W., Jr.; Driscol1,D. J. J.Atmos. Chem. 1991, 12, 105. Wallington, T. J.; Hurley, M. D.; Ball, J. C.; Kaiser, E. W. Environ. Sci. Technol. 1992, 26, 1318. Edney, E. 0.;Driscoll, D. J. Int. J.Chem. Kinet. 1992,24, 1067. Sehested, J.; Wallington, T. J. Environ. Sci. Technol. 1993, 27, 146. Edney, E. 0.;Driscoll, D. J. Water, Air, Soil Pollut. 1993, 66, 97. Tuazon, E. C.; Atkinson, R. J.Atmos. Chem. 1993,16,301. Tuazon,E. C.; Atkinson, R. J.Atmos. Chem. 1993,17,179. Franklin, J. Chemosphere 1993, 27, 1565. Ball, J. C.; Wallington, T. J. J.Air Waste Manage. Assoc. 1993, 43, 1260. Barnes, I.; Becker, K. H.; Kirchner, F.; Zabel, F.; Richer, H.; Sodeau, J. Kinetics and Mechanisms for the Reactions of
Halogenated Organic Compounds in the Troposphere; CFCl2C(O)OOH
and/or
CFClgC(0)OH
we have shown that the major, if not only, reaction products of the 3-carbon HCFCs HCFC-225ca and HCFC-225cb are the expected CBcarbonyl compounds. Acknowledgments
The authors gratefully thank the SPA-AFEAS for financial support through Contract CTR91-28/P91-082 (Dr. Igor Sobolev, Project Monitor) and gratefully acknowledge the cooperation of the following individuals and their respective organizations in providing high-purity samples for this study: Dr. Seiji Shin-ya (Asahi Glass Co., Ltd), Dr. Hillel Magid (Allied-Signal, Inc.), and Dr. James Franklin (Solvay S. A.). Thanks are due Dr. Janet Arey for the NMR characterization of the haloaldehyde samples. Although the work described here has been funded by the Alternative Fluorocarbons Environmental Acceptability Study (AFEAS), it does not necessarily reflect the views of the companies or representatives of the companies participating in AFEAS, and no official endorsement should be inferred. Literature Cited (1) Rowland, F. S. Ambio 1990, 19, 281. (2) Rowland, F. S. Environ. Sci. Technol. 1991, 25, 622. (3) Anderson, J. G.; Toohey, D. W.;Brune, W. H. Science 1991, 251, 39. (4) Scientific Assessment of Ozone Depletion: 1991; World Meteorological Organization Global Research and Monitoring Project Report No. 25; World Meteorological Organization: Geneva, Switzerland, 1992. (5) ScientificAssessment of Stratospheric Ozone: 1989;World Meteorological Organization Global Research and Monitoring Project Report No. 20, Vol. I; World Meteorological Organization: Geneva, Switzerland, 1990. (6) ScientificAssessment of Stratospheric Ozone: 1989;World Meteorological Organization Global Research and Monitor-
STEP-HALOCSIDE/AFEAS Workshop, Dublin, Mar 2325, 1993; pp 52-58. (18) Atkinson,R.;Baulch,D.L.;Cox,R.A.;Hampson,R.F., Jr.; Kerr, J. A; Troe, J. J.Phys. Chem. Ref.Data 1992,21,1125. (19) DeMore, W. B.; Sander, S. P.; Howard, C. J.; Ravishankara, A. R.; Golden, D. M.; Kolb, C. E.; Hampson, R. F.; Kurylo, M. J.; Molina, M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling; Evaluation No. 10; J P L Publication 92-20; J e t Propulsion Laboratory: Pasadena, CA, 1992. (20) Yamada, B.; Campbell, R. W.; Vogl, 0. J.Polym. Sci. 1977, 15, 1123. (21) Pierce, 0. R.; Kane, T. G. J.Am. Chem. SOC.1954, 76,300. (22) Yamada, B.; Campbell, R. W.; Vogl, 0. Polym. J. 1977, 9, 22. (23) Pouchert, C. J. The Aldrich Library of Infrared Spectra, 1st ed.; Aldrich Chemical Co.: Milwaukee, WI, 1989 Vol. 3. (24) Sato, H.; Nakamura, T. Nippon Kagaku Kaishi 1991,548. (25) Scollard, D. J.; Treacy, J. J.; Sidebottom, H. W.; BalestraGarcia, C.; Laverdet, G. LeBras, G.; MacLeod, H.; TBton, S. J.Phys. Chem. 1993,97,4683. (26) Wallington, T. J.; Hurley, M. D. Int. J.Chem. Kinet. 1993, 25, 819. (27) Tuazon, E. C.; Atkinson, R.; Corchnoy, S. B. Int. J.Chem. Kinet. 1992,24, 639. (28) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J. Phys. Chem. 1982, 86, 4563. (29) Wallington, T.J.; Hurley, M. D. Chem. Phys. Lett. 1992, 189, 437. (30) Zellner, R.; Hoffmann, A,; Mors, V.; Malms, W. Kinetics and Mechanismsfor the Reactions of Halogenated Organic Compounds in the Troposphere; STEP-HALOCSIDE/ AFEAS Workshop, Dublin, Mar 23-25, 1993; pp 80-87. (31) Hayman, G. D. Kineticsand Mechanismsfor the Reactions of Halogenated Organic Compounds in the Troposphere; STEP-HALOCSIDE/AFEAS Workshop, Dublin, Mar 2325, 1993; p p 65-74.
Received for review February 22, 1994. Revised manuscript received August 22, 1994. Accepted August 23, 1994.' e Abstract published in Advance ACS Abstracts,October 1,1994.
Envlron. Scl. Technol., Vol. 28, No. 13, 1994
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