Gutman et ai.
1-buten-1-yl and 1-buten-4-yl radicals were taken from 1butene,31 in which the torsional mode about the central C-C bond was lowered to 102 cm-l as pointed out by Pearson and Rabinovitch,15 and the following three frequencies were removed: CH stretch, 3011 cm-l, and two HCH bends, 1443 and 913 cm-l, for 1-buten-1-yl; 2972,1045, and 1470 cm-l for 1-buten-4-ylradicals. For the calculation of Iz,, the association complex was specified as
D. C. Tardy, lni. J. Chem. Kinei., 6, 291 (1974). L. Endrenyi and D. J. LeRoy, J. Phys. Chem., 70, 4081 (1966). K. W. Watkins, J. Am. Chem. Sac., 93, 6355 (1971). K. W. Watkins and L. A. Ostereko, J. Phys. Chem., 73, 2080 (1969). K. W. Watkins, J. Phys. Chem., 77, 2938 (1973). (IO) K. W. Watkins and D. K. Olsen, J. Phys. Chem., 76, 1089 (1972). (11) K. W. Watkins and L. A. O'Deen, J. Phys. Chem., 75, 2665 (1971). (12) W. P. L. Carter and D. C. Tardy, J. Phys. Chem.. 78, 1245 (1974). (13) W. P. L. Carter and D. C. Tardy, J. Phys. Chem., 78, 2201 (1974). (14) T. Ibuki, T. Murata, and Y. Takezaki, J. Phys. Chem., 78, 2543 (1974). (15) M. J. Pearson and B. S. Rabinovitch, J. Chem. Phys., 42, 1624 (1965). (16) C. W. Drew and A. S. Gordon, J. Chem. Phys., 31, 1417 (1959). (17) (a) J. 0. Terry and J. H. Futrell, Can. J. Chem., 45, 2327 (1967); (b) J. Grotewold and J. A. Kerr. J. Chem. SOC.,4337 (1963); (c) Y. Ined, J. CHsCH2- -CH=CH Phys. Chem., 74, 2581 (1970); (d) R. A. Holroyd and T. E. Pierce, ibid., 68, 1392 (1964). and the isomerization complexes as (18) J. A. Kerr and A. F. Trotmann-Dickenson. "Progress in Reaction Kinetics", Vol. 1. Pergamon Press, New York, N.Y., 1961. (19) B. S. Rabinovitch and D. W. Setser, "Advances in Photochemistry", C% ,, Vol. 3, Wiley, New York, N.Y., 1964. (20) D. G. L. James and G. E. Troughton, Trans. Faraday SOC., 62, 145 CH, H and ,H, (1966). e , \ (21) N. M. Emanuel' and D. G. Knorre, "Chemical Kinetics," Wiley, New CH?-CHCH=CH, HC=CH York, N.Y., 1960, p 58. (22) M. H. Jones and E. W. R. Steacie, Can. J. Chem., 31, 505 (1953). (23) S. W. Benson, "The Foundation of Chemical Kinetics", Wiley, New and modifications of their frequency assignments were York, N.Y., 1960. made according to those by Rabinovitch and c o ~ o r k e r s . ~ J ~(24) A. F. Trotmann-Dickenson and E. W. R. Steacie, J. Chem. Phys., 19, 169 (1951). The modified frequencies are given in Table VI. These (25) A. F. Trotmann-Dickenson and G. S. Milne, Natl. Stand. Ref. Data Ser., frequencies give log A(sec-l) = 12.11 and log A(sec-l) = Natl. Bur. Stand., No. 9 (1967). 12.68 for 1,4-and 1,2-H atom shifts, respectively. (26) P. v. R. Schleyer, J. E. Williams, and K. R. Blanchard, J. Am. Chem. SOC.,92, 2377 (1970). (27) J. 0 . Hirschfelder. C. F. Curtiss, and R. B. Bird, "Molecular Theory of References and Notes Gasses and Liquids", Wiley, New York, N.Y., 1964. (1) E. A. Hardwidge, C. W. Larson, and B. S.Rabinovitch, J. Am. Chem. (28) C. W. Larson and B. S.Rabinovitch, J. Chem. Phys., 50, 871 (1969). SOC., 92, 3278 (1970). (29) D. M. Golden and S.W. Benson, Chem. Rev., 69, 125 (1969). (2) K. W. Watkins and D. R. Lawson. J. Phys. Chem., 75, 1632 (1971). (30) J. A. G. Dominguez and A. F. Trotmann-Dickenson. J. Chem. Sac.. 940 (3) C. W. Larson, P. T. Chua, and E. S. Rabinovitch, J. Phys. Chem., 76, (1962). 2507 (1972). (31) L. M. Sverdlov. M. G. Borisov, and N. V. Tarasova, Opt. Specfrosk., 5, (4) K. W. Watkins, Can. J. Chem., 50, 3738 (1972). 354 (1958). (5) (6) (7) (8) (9)
Direct Identification of Reactive Routes and Measurement of Rate Constants in the Reactions of Oxygen Atoms with the Fluoroethylenes James R. Gilbert, Irene R. Slagle, Ronald E. Graham, and David Gutman" Department of Chemistry, Illinois Institute of Technology,Chicago, Illinois 606 16 (ReceivedJune 13, 1975) Publication costs assisted by the Petroleum Research Fund
The room temperature reactions between oxygen atoms and monofluoro-, difluoro (1,l and 1,2)-, trifluoro-, and tetrafluoroethylene have been studied in crossed jets to directly identify their reactive routes. Free radical and stable products were detected using photoionization mass spectrometry and were assigned to WXC=CYZ WXC: CYZO, :O reactive routes. The routes identified were of three types: :O WXC=CYZ WXYC. .CZO (or Z. CO), :O WXC=CYZ WX + C2YZ0, where W, X, Y, and Z are either H or F atoms. Individual 0 fluoroethylene reactions were found to proceed by either one, two, or three of these distinctly different kinds of routes. The results of this study are interpreted using an expanded version of a mechanism proposed by CvetanoviE for 0 olefin reactions. Overall rate constants for several of these reactions were also measured a t 300 K and are reported.
-
+
+ +
-
+
+
-
+
+
+
Introduction The reactions of oxygen atoms with unsaturated organic molecules proceed via the formation of extremely energy rich adducts which are capable of decomposing or undergoing rapid internal rearrangements.' These reactions have The Journal of Physical Chemistry, Vol, 80, No. 1, 1976
long been recognized as important steps in thermal and photochemical combustion mechanisms and are now receiving new attention as potential chemical lasers.2 Although today there is considerable information on the overall rate constants for many of these reactions, knowledge of the products they produce is still quite parse.^,^ We are
Reactions of Oxygen Atoms with the Fluoroethylenes
15
currently investigating the reactions of 0 atoms with alkenes and alkynes both in crossed jets and in a flow system specifically to directly detect and identify the reactive and stable intermediates they produce as well as to determine the importance of the various reactive routes by which these same reactions p r ~ c e e d . ~ We - ~have now completed a study at ambient temperature of the reactions of 0 atoms with each of the fluorinated ethylenes (FE) and have identified their open reactive channels. The results of this study are reported here. The two extreme members of the series of reactions 0 + C2HxF, (x = 0-4, y = 4 - x) proceed by entirely different paths. Products of the 0 C2H4 reaction have been detected and identified in experiments using crossed jets and the reaction was shown to proceed by two routes, a principal one involving internal H-atom migration in the 0.CzH4 adduct followed by decomposition into two free radicals, and a secondary one in which H2 is eliminated from the excited add~ct~,~ CH, + CHO (1) 0 C,H, [C,H,O]* H2 HZC,O (2)
+
+
-
€
+
+
The 0 C2F4 reaction has been extensively studied and there is strong indirect evidence that it proceeds exclusively by direct C-C bond cleavage of the 0-CzF4 adduct to produce difluorocarbenes
0 + CzF4
-
[CzF40]*
+
CFz
+ CF2O
(3)
The present study was undertaken partially because it was anticipated that the knowledge gained of the mechanistic differences among the 0 CzH,F, reactions might indicate some of the factors which govern access to the different kinds of energetically allowed reactive routes in the reactions of 0 atoms with unsaturated organic molecules. We feel some additional details were revealed regarding how these reactions proceed, and we have discussed our results with an expanded version of a mechanism first proposed by Cvetanovi6.l
+
Experimental Section Two kinds of experiments were performed on each 0 fluoroethylene reaction. First each was studied in high-intensity crossed jets to directly detect and identify products of the reaction. Second, the room temperature rate constant was measured using a fast-flow reactor. In the crossed-jet experiments, an uncollimated and undiscriminated room temperature beam containing -5% 0 atoms intersected a similar beam containing the FE. Products scattered in the direction of the 0-atom beam were detected with a photoionization mass spectrometer. Details of the crossed-jet reactor, the detection system, the experimental conditions, and the data reduction procedure followed are all as described p r e v i ~ u s l y . ~Special -~ tests to assure that detected products were not the result of secondary reactions or the result of possible hyperthermal kinetic energy of the reactant molecules were done as in previous studie~.6-~ The FE gases used were obtained from PCR Inc. (Gainesville, Fla.) except 1,l-difluoroethylene and CzF4 which were obtained from Matheson Gas Co. All F E reactants were condensed with liquid Nz,degassed, and fractionally distilled, the middle third being retained for the experiments.
+
+
The overall rate constants of the 0 FE reactions were measured at 300 f 2 K using a fast-flow reactor described b e f ~ r e Oxygen .~ atoms were generated by flowing He 0 2 gas mixtures through a microwave discharge. Rate constants were obtained by monitoring the first-order decay of F E along the reactor in the presence of a large 0-atom excess. In the experiments involving fluoroethylenes which were found to produce carbenes (CHF or CF2) in their reactions with 0 atoms (1,2-CzHzFz, C2HF3, CzF4) extremely high [O]/[FE]o (>go) had to be used to obtain “rate constants” which were independent of reactant concentrations. A similar observation was reported by Huie, Herron, and Davis in their flow-reactor study of these same reactions.1° Our sensitivity for detecting C2F4 by photoionization was lower than for the other fluoroethylenes, so we were unable to accurately monitor ClF4 under the required conditions and to obtain accurate rate constants for the 0 CzF4 reaction. A possible explanation of this interference is that carbenes add so readily to the FE reactants that the subsequent reaction of these products cannot be adequately suppressed until extremely low F E concentrations are used. By contrast, carbenes were not detected as products 1,1-C2HZF2 reaction and measurements of its of the 0 rate constant required only the normal [O]/[FE]o ratios which are needed to assure negligible depletion of 0 atoms during reaction (-20). The results of the crossed-jet experiments are given in Table I, and the measured rate constants are listed in Table 11.
+
+
+
Discussion Overall R a t e Constants. The overall rate constants for the 0 F E reactions measured in this study at room temperature are in excellent agreement with those measured by Huie, Herron, and Davis in a similar flow reactor study.1° Jones and Moss have determined rate constant ratios for all the 0 FE reactions relative to the 0 2-(trifluoromethy1)-propene reaction.12 Using their data we have calculated quotients of the 0 FE rate constants divided by that for 0 C2H3F. These ratios are in good agreement with similar quotients using the results of our study. A comparison of these ratios as well as of the overall rate constants is given in Table 111. Reactive Routes. Every product detected and identified in the study of each 0 fluoroethylene reaction has been assigned to a reactive route. These routes are listed in Table I. Those products which were not detected in each of the routes listed generally could not be detected either due to their having ionization potentials which were too high to be detected with our photoionization mass spectrometer (H, F, CO, HF, H2, F2, CHFO, and CF20), or due to their having mass numbers which are a t or adjacent to that of the reactant (e.g., CFO in the 0 CzH3F reaction), or due to their having the same mass number as that of an impurity in one of the reactant beams (e.g., CHO in most of the reactions). There have been no earlier studies of these reactions under conditions where subsequent reactions of initial products were suppressed and hence comparison of our results can only be done with the indirect evidence for reactive routes reported by others. What secondary evidence exists generally supports the results of this study. Heicklen and coworkers have extensively studied the 0 + C2F4 reaction through measurements of the ultimate stable products produced (CFzO and c - C ~ F ~ ) Their .’ results suggested to
+
+
+
+
+
+
+
The Journal of Physical Chemistry, Vol. 80, No. 1, 1976
16
Gutman et al.
TABLE I: List of Products Detected and Assigned Reactive Routes for 0 + Fluoroethylene Reactions Products detecteda
Reaction 0 + HFCCH,
CH, b CH,F. CHO
Assigned reactive routes
C,HFOb
CH, + CFO ( F + CO) CH,F + CHO C,H,O + H F C,HFO + H,
0 + F,CCH,
CHF, C,HFO
CHF, + CHO ( H + CO) C,HFO + HF
0 + HFCCHFc
CH,F C,HFO CHF
CH,F + CFO ( F + CO) C,HFO + HF CHF + CHFO (CO + HF)
CF,d CHF
CF, + CHFO (CO + HF) CHF + CF,O (CO + F,)
C,H20
(cis and trans) 0 + HFCCF,
TABLE 111: Comparison of Overall Rate Constants for 0 + Fluoroethylene Reactions
0 + F,CCF,
CF, CF, + CF,O Products sought with photoionization energies of 11.6 eV (Ar lamp), 10.2 eV (H lamp), and 9.5 eV (0 lamp). b These products probably of minor importance. Ion signal -1%that detected for all products combined. C Ion signals detected from reactions of cis and trans isomers were not significantly different. Differences in relative ion signals among products were measurable to i30% with 95% confidence. d The detection of CF, may possibly indicate the presence of the reactive route 0 + C,HF, + C,F,O (CF, + CO) + HF. The molecule C,F,O is believed to be unstable and decomposes into CF, + CO.”
This study 1013k,
Huie, Herron, and Davisa
Jones
1013k,
Mossb
and ...~.
Fluoro-
cm3 p-l k/ cm3 p-’ k/ k/ sec-’ ~ C , H , F sec-’ ~ C , H , F ~ C , H , F H,CCHF 4.1C 1.00 4.36 1.00 1.00 HFCCHF (cis) 3.7 0.90 0.7 6 (mixture)d 4.48 1.03 (trans) 5.8 1.41 1.38 H,CCF, 3.1 0.76 3.63 0.83 0.52 8.3 2.02 1.40 HFCCF, a Reference 10, T = 307 K. b From data in ref 12, T = 298 K. C k for 0 + C,H,F reaction from ref 7. d An equilibrium mixture of cis and trans isomers would contain about 82% cis isomer (calculated using equilibrium constant in ref 13). ethylene
a
TABLE 11: Results of Experiments t o Measure the Overall Rate Constant for 0 + Fluoroethylene ReactionsaJb Flow
Pressure,c Torr
velocity, m/sec
1.45 1.46 1.46
11.8
10-’4[0],
p cm-,
lo-”[ Fluoroethylene],, p cm-,
1013k,d
cm3 p-’ sec-’
0 + trans-1,2-C,H,F2
11.9 11.9
3.25 2.96 9.55 9.31 9.03 7.17 Av h for 0 + trans-1,2-C,H,F,
6.1 5.6 5.7 5.8
0 + c~s-~,~-C,H,F,
1.45 1.46 1.46
11.8
1.49 1.49 1.46 1.46 1.46 1.46 1.46 1.46 1.46
12.0 12.0 12.6 12.6 12.6 12.6 12.6 12.7 12.7
11.9 11.9
3.25 2.94 9.55 9.44 9.03 7.10 Av k for 0 + cis-1,2-C,H,F,
3.8 3.6 3.8 3.7
0 + l,l-C,H,F,
3.36 3.36 10.8 10.8 10.8 10.8
16.2 6.57 10.7 15.8 7.5 53.
3.3 3.2 2.6 2.8 2.5 2.8 3.6 4.0 3.0 3.1
4.1 11.2 4.1 5.32 4.6 5.45 Av k for 0 + l,l-C,H,F, 0 + C,HF, 1.46 12.8 7.84 6.52 9.1 1.45 12.6 3.51 2.67 7.8 1.45 12.6 3.58 4.03 8.2 1.46 12.8 3.59 3.42 8.2 Av k for 0 + C,HF, 8.3 a Rate constants for 0 + C,H,F reaction reported in ref 7. Average k for 0 + C,H,F at 302 i 2 K is 4.1 X lo-’: cm3 p-’ sec-’. b T i n all experiments 300 t 2 K. C Average pressure along flow reactor. Total pressure drop along tube less than 5%. d Average rate constants are estimated accurate to t 20%. The Journal of Physical Chemistry, Vol. 80, No. 1, 1976
the authors that the only route of this reaction is the one directly observed in this study, reaction 3. Mitchell and Sim o n as ~ well ~ ~ as Tyermanlj have seen CF2 in absorption in the flash initiated 0 C2F4 reaction and also concluded it was produced by reaction 3. Mitchell and Simons have also studied the 0 1,l-difluoroethylene reaction in a similar flash-initiated experiment, and they also observed CF2 production from this reaction.14 They conclude that the route
+
+
0 + HzCCFz
-+
CH20
+ CF2
(4)
is an important one. As we did not detect CF2 in our study of this reaction, we conclude that this route, if present, must be a minor one (
