1235
Thermal "CI Atom Reactions with Ethylene
(10) (11) (12) (13) (14) (15) (16) (17) (18)
of 31 kcal/mol. J. C. Amphlett and E. Whittle, Trans. Faraday Soc., 63, 2695 (1967). The abstraction of F from CCIF:, by Ci to form CIF is more than 50 kcal/mol endothermic. M. Hudlicky, "Chemistry of Organic Fluorine Compounds", MacMillan, New York, N.Y., 1962, p 297 f f . D. J. Stevens and L. D. Spicer, J . Chem. Phys., 64, 4798 (1976). D. J. Stevens and L. D. Spicer, preprint. T. Smail, R. S.Iyer, and F. S.Rowland, J. Am. Chem. Soc., 94, 1041 (1972). R. L. Williams and F. S. Rowland, J. Phys. Chem., 76, 3509 (1972). J. P. Frank and F. S.Rowland, J . Phys. Chem., 78, 850 (1974). F. S.Rowland, F. Rust, and J. P. Frank, ACS Symp. Ser., in press. Extensive details are given in the Ph.D. Thesis of F. S. C. Lee, University of California, Irvine, Calif., 1975. J. K. Lee, E. K. C. Lee, B. Musgrave, Y.-N. Tang, J. W. Root, and F. S. Rowland, Anal. Chem., 34, 741 (1962).
(19) M. J. Welch, R. Withnell, and A. P. Wolf, Chem. Instrum., 2, 177 (1969). (20) R. S. Iyer, Ph.D. Thesis, University of California, Irvine, Calif., 1973. (21) C. M. Wai, Ph.D. Thesis, University of California, Irvine, Calif., 1967. (22) C. M. Lederer, J. M. Hollander, and I. Perlman, "Table of Isotopes", 6th ed., Wiiey, New York, N.Y., 1967. (23) T. B. Ryves and D. R. Perkins, J . Nucl. Energy, 24, 419 (1970). (24) Y.-N. Tang, W. S.Smith, J. L. Williams, K. Lowery, and F. S.Rowland, J. Phys. Chem., 75, 440 (1971). (25) E. K. C. Lse, Y.-N. Tang, and F. S. Rowland, J . Phys. Chem., 68, 318 (1964). (26) C. M. Wai and F. S. Rowland, J . Phys. Chem., 74, 434 (1970). (27) F. S. C. Lee and F. S. Rowland, J. Phys. Chem., 81, 1235 (1977). (28) F. S. C. Lee and F. S.Rowland, J . Phys. Chem., 81, 684 (1977). (29) F. S. C. Lee and F. S. Rowland, J . Phys. Chem., 81, 1222 (1977). (30) M. Kikuchi, F. S.C. Lee, and F. S. Rowland, unpublishedexperiments.
Thermal Chlorine-38 Atom Reactions with Ethylene F. S. C. Lee and F. S. Rowland" Department of Chemistry, University oi California, Irvine, California 927 17 (Received January 19, 1977) Publication costs assisted by the U.S. Energy Research and Development Administration
The reactions of thermal chlorine atoms with ethylene have been studied using radioactive 38Clatoms from the neutron reaction 37Cl(n,y)38C1 on CC1F3. The 38Clatoms are moderated to thermal energies by multiple collisions with CC1F3(mole fraction >0.9), and then allowed to react with C2H4plus HI or H2S. The only observed product is C2Hj3'C1, representing addition to ethylene to form C2H?'Cl*. The decomposition rate of CzHt8C1* by loss of 38Clwas measured by varying the total pressure in the system. The rates of decomposition and collisional stabilization are equal at a CC1F3 pressure of 800 f 120 Torr. The rate constants for reactions 1, 10, and 15 are in the ratios (hlo/hl) = 0.60 f 0.05 and ( h I 5 / h l = ) 0.45 f 0.05 at 293 K: 38Cl+ CzH4 CzH438C1*(1);38Cl + HI ~ 3 8 c 1 +I (io); 38c1+ H ~ S ~ 3 8 c 1 +SH (15).
-
-
-
Introduction The only exothermic reaction with gaseous ethylene available to thermal chlorine atoms is addition to form a chloroethyl radical as in C1 + C,H,-
CH,CH,Cl*
(1)
The hydrogen abstraction reaction 2 is endothermic by 6 Cl
+ C,H,
+
HC1 + C,H,
(2)
kcal/mol and is an unlikely process a t room temperature.' Almost all C1 atoms a t 300 K must then either react by (1) or find some other molecule with which to react in ethylene-containing systems. The possible subsequent reactions of CzH4C1*radicals include the reverse of the formation reaction, as in (3), and collisional stabilization C,H4C1* C,H,Cl*
C1 + C,H4 + M - C,H,Cl + M
-+
(3)
(4)
of the excited radical, reaction 4. The loss of H from C2H4C1*is highly endothermic and cannot occur with radicals formed by addition of thermal chlorine atoms to ethylene.' Earlier studies of the atomic chlorine-ethylene reaction were concerned with the relative reactivity of ethylene and the various chloroethylenes, and showed that the rates of reaction with all except C2C14were comparably rapid, and had low activation energies ( 1 2 kcal/rn01).~-~ These studies thereby confirmed that reaction 1 is quite rapid, and that an appreciable fraction of the radicals from (1) did not undergo the reverse reaction 3. Wijnen has studied the reactions of stabilized C2H4C1 using the photolysis of phosgene, C0Cl2, as the source of
atomic ~ h l o r i n e .The ~ chief C2H4C1product observed was 1,4-dichlorobutane from the combination of two such radicals, as in (5). Wijnen et al. also used ultraviolet (5)
SC,H,Cl -.+ CH,ClCH,CH,CH,Cl C1 transfer C,H,Cl t C,H4C1- C,H, t C,H,Cl,
(6)
H transfer C,H,Cl + C,H,Cl-
(7 1
C,H,Cl
+ C,H,Cl
photolysis of C C 4 as a C1-atom source for reaction with ethylene.6 In these experiments, disproportionation products from C1 and H transfer between C2H4C1radicals were observed, as in (6) and (7). The yields of the observed two-carbon products (C2H3C1,C2H5C1,CzH4C12) indicated that the disproportionation processes 6 and 7 were 50.1 as probable as combination 5 . The ultraviolet photolysis of CC14has recently been shown to lead in some cases to CClz + Clz rather than CCl3 + Cl,7 but the implication of this observation for the interpretation of C1 + ethylene reactions has not yet been examined. The loss of C1 from excited C2H4C1*was not explicitly considered in Wijnen's work. A further study of the C1+ C2H4system has been made by Heicklen, again using the photolysis of COClz as the source of atomic chlorine.8 These data showed a much higher disproportionation/combination ratio (0.45) and indicated that the H-atom transfer of (7) was four times as probable as the C1-atom transfer of (6). Heicklen also suggested the reaction of two C4H8C1radicals with one another, as in PC,H,Cl
-+
C,H,Cl
+ C,H,Cl+
C,H,
(8)
The Journal of Physical Chemistry, Vol. 81, No. 13, 1977
F. S. C. Lee and F. S. Rowland
1236
We have utilized thermal 38Clatoms formed by thermal neutron irradiation of CC1F3' for study of the atomic chlorine-ethylene system. The initially energetic 38Cl atoms are thermalized by multiple collisions with CC1F3 (mole fraction >0.9) prior to reaction with ethylene. In the presence of hydrogen-donor radical scavengers (e.g., HI or H,S), the stabilized chloroethyl radicals from the 38Clanalogue of (4)are converted to CzH2'C1 and assayed in this form, as in CH,CH,Wl
+ HI
-
CH,CH,38C1 + I
COUNTS MINUTE 8,0004 1 Ar (x
7,000
-
6,000-
'/41 PRESSURES ( T O R R ) CCIF, C2H4
: 4000 :
133
(9)
However, the inclusion of HI (or H2S) furnishes a competitor molecule for ethylene in reaction with the initial 38Clatoms, and increasing concentrations of HI (or H2S) can be expected through reaction 10 to reduce the yield ~ c + iHI .+ ~ 3 8 c 1 +I
(10)
of C2H43'C1*from (l),and thereby of CzH:C ' 1 from (9). Some earlier experiments with radioactive C1 atoms and ethylene were performed by Spicer and Wolfgang who used a different nuclear reaction 40Ar(y,p)39C1,and chose a mixture of CzH4 and I2 as a scavenger combination in argon-alkane mixtures." They found more than 90% of the 39Clactivity as CzH2'C1I in a typical sample containing 600 Torr of Ar, 150 Torr of CH4, 20-40 Torr of CzH4,and traces of Is. Obviously almost all of the 39Clatoms in this system followed the reaction route of (l),(4),and then reaction of C2Ht9C1with I2to form C2Ht9C11,and strongly suggests that the bulk of these reactions were those of thermalized 39Cl atoms. The relative nonreactivity of methane toward thermal 39Cl is consistent with the measured rate constants for that abstraction reaction.">l2 Very recently Stevens and Spicer have applied 38Clfrom neutron irradiation of CC12F2to the study of 38Clreactions with H213and C2H4.14
Experimental Section The procedures for forming 38Cl (tlI2 = 37.3 min) by thermal neutron irradiation of 37Clcontained in CC1F3or CC12F2have been described el~ewhere.~ The difficulties encountered with radiation damage problems in such irradiations are discussed there in detail. Essentially such irradiations are most susceptible to radiation damage alteration of the observed product spectrum at high reactor power (i.e., high neutron flux), long reactor exposure times, and low absolute concentrations of reactive species, e.g., HI. Some of the low concentration HI experiments described here suffered from radiation removal of HI, and are disregarded in the quantitative evaluations. When CC1F3, C2H4, and HI or H2S are irradiated, the chief 38Cl-labeledproduct is C2H;'C1, with lesser amounts of CH?'Cl and CH2=CH3'C1 also being formed. However, when the mole fraction of CClF3 exceeds 0.90, the yields of CH338C1and CH2=CH3'C1 are ne ligibly small, indicating that they require nonthermal "Cl atoms for their formation. The reactions of energetic %l atoms with ethylene have also been studied through the use of CC1F3-C2H4mixtures with the mole fraction of CClF3 varying from 0.1-0.8.'' The analysis of the radioactive products in CC1F3ethylene mixtures in highly moderated systems (mole fraction of CClF, >0.9) is carried out by radiogas chromatography using two chromatographic columns in series, 50-ft dimethylsulfolane (DMS) and 24-ft propylene carbonate on alumina (PCA). The PCA column was necessary for the separation of 41Ar and C3'C1F3, with the former serving as an internal neutron flux monitor for absolute calculation of 38Clyields through the 40Ar(n,y)41Arnuclear reaction. The Journal of Physical Chemistry, Vol. 81, No. 13, 1977
RETENTION T I M E ( M l N 1
Flgure 1. Radiogas chromatogram of %I products from ''CI-atom reactions in CCIF31C2H41HI/Armixture. Analysis with dimethylsulfolane and propylene carbonate-on-alumina columns. The PCA column is removed from the flow stream during the analysis, and the peak for C3'CIF3 never emerges.
After injection of the irradiated sample, both 41Arand C3'C1F3 passed rapidly through the DMS column and into the PCA column. After the entire system had been tested with a few samples, the C3'C1F3 yields were no longer measured, and the PCA column was removed from the flow stream as soon as 41Ar had emerged from the PCA column and had been assayed. The other radioactive molecules then passed directly from the DMS column into the gas proportional counter for radioassay while the C3'C1F3 peak was usually left in the PCA column and not analyzed. A series of separate experiments had demonstrated that the yield of C3'C1F3 was consistent, varying from about 0.5% at 500 Torr to 1.0% at 4000 Torr. Only about 7 % of the 38Clradioactivity present as CzH538C1at injection is still present during assay 140 min later. A radiogas chromatogram of the analysis of a typical CC1F3-CzH4sample is shown in Figure 1. The small yield of CF31281is formed from "'1 atoms created by the lZ7I(n,y)'*'I nuclear reaction on the HI scavenger. All of these experiments were carried out at room temperature, about 293 K in the reactor irradiation facility.
Results and Discussion Approximately 97% of the total 38Cl radioactivity formed by neutron irradiation of CC1F3 is available as thermal 38Cl.9 In the present experiments the only measured product from these thermal atoms is CzH2'Cl. About 3% of the 38Clradioactivity is observed as reaction products from hot 38Clreactions with CC1F3, while traces are formed from hot 38Cl reactions with CzH4. The remainder of the 38Cl is presumed present as H3%l from reaction 10, but was not directly assayed in these experiments. The yields of these various 38C1-labeledorganic molecules are summarized in Table I for a series of experiments at a total pressure of about 4100 Torr. Similar experiments at 1000 and 640 Torr are given in Table 11. The observed yields of C2H:C '1 are consistent with a mechanism involvin competition between reactions 1 and 10 for thermalized FC1 atoms, and between decomposition (3) and stabilization (4) for the excited CzH:C ' 1* radicals '1 radicals are all formed in (1). The stabilized CzH:C assumed to be converted to C2H2'C1 by reaction 9. If reaction 1 occurs only for thermal 38Cl atoms, then the C2H:C ' 1* radicals are essentially monoenergetic and can be effectively described by an average decomposition rate, k3. The stabilization rate is then given by a rate constant proportional to pressure, k4M, and the rates of formation
Thermal 38CIAtom Reactions with Ethylene
1237
TABLE I: "Cl Radioactive Product Yields with Ethylene and HI as Competitors in Highly Moderated CClF, Systems, Total Pressure 4100 T o r P CClF,
"2
H4
HI
Ar [HII/IC,H,I Total pressure
Sample Composition Pressure. Torr 4000 3960 4000 26.3 9.9 80 60.0 20.4 80 14.3 15.6 14.9 2.1 1.0 2.3 4100 4000 4180
3970 40.0 120 15.1 3.0 4140
4000 28.1 160 14.8 5.7 4200
4000 133 27.6 13.7 0.21 4170
4000 133 26.4 10.8 0.20 4170
b 0.62 ?: 0.01 0.68 f 0.01 0.15 i 0.02
