J. Phys. Chem. 1981, 85, 2488-2492
2488
Department of Energy, Office of Basic Energy Sciences, Division of Materials Science, Contract DE-ACO27831304993, and to the Northwestern University Materials Research Center, supported by the National Science
Foundation-Materials Research Laboratories, Contracts DMR76-80847 and DMR79-23573, in whose Central Facilities the Mossbauer, X-ray, and transmission microscopy studies were performed.
Thermal F/X Atomic Substitution Reactions with Methyl Halides (X = CI, Br, I) R. Subramonla Iyer and F. S. Rowland” Department Of Chemistry, Universify of Caiifornla, Ifvine, Cailfornk, 92717 (Received: April 11, 1980;In Flnal Form: May 11, I98 I )
The thermal substitution of F for X in CH3X (X = C1, Br, I) at 283 K has been observed by using radioactive 18Fatoms moderated to thermal energies by multiple collisions in gaseous SF6at 3000 torr. The relative rate constants for these reactions have been measured in competition with addition to C2H2and/or abstraction from HI, and have been converted to absolute rate constants by assuming a rate constant of X lO-’O cm3 molecule-l s-’ for F + C2H2.The rate constants for the thermal substitution of F/X in CH3X increase as the exothermicity of the replacement reaction increases, and have the following rate constants at 283 K, in units F + CH3Br CH3F + Br, kS = (1.7 f 0.3) of cm3molecule-’ s-l: F + CH31 CH3F + I, k6 = (8f 3) X X F + CH3C1 CH3F + C1, k4 = (3.7 f 1.3) X lo-“. In each case the abstraction of H from CH3X is very much faster than the substitution reactions.
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Homolytic bimolecular substitution (SH2) reactions at unstrained sp3-hybridized carbon atoms have long been of interest,l and many of the efforts to observe such reactions have been directed toward the study of halogen substitution reactions, including such examples as iodine atom substitution with 2-iodobutane2 and the interpretation of organic fluorination as involving the attack of F atoms on CF3R compounds to form CFb3t4 Ingold and Roberts summarized the existing data several years ago with the comment, “there would appear to be no unequivocal examples of such a reaction,” while pointing out that highly exothermic substitution reactions might offer better chances for success than thermoneutral reactions such as the iodine for iodine substitution.’ The substitution reactions of F atoms for other halogen atoms in methyl halides are all quite exothermic: 25 kcal/mol with F/C1, 38 kcal/mol with F/Br, and 52 kcal/mol with F / I substitution r e a ~ t i o n s . ~ Energetic l?F atoms formed by the fast neutron reaction 19F(n,2n)18Fin gaseous SF6are rapidly moderated toward thermal energies by collisions with SF6 if no other potentially reactive molecules are available during passage through the critical energy range below about 20-eV kinetic energy. With the exception of about 2% of these energetic 18Fatoms which undergo hot 18F/F substitution reactions to form SF518F,these well-moderated 18Fatoms continue to collide with SF6at near-thermal or thermal energies until reaction with a substrate in low concentration removes them from the (1)K. U. Ingold and B. P. Roberta, “Free Radical Substitution Reactions”, Wiley, New York, 1971. (2)R. A. Ogg, Jr., and M. Polanyi, Trans. Faraday SOC.,31,482(1935). (3)F. P. Avonda, J. A. Gervasi, and L. A. Bigelow,J.Am. Chem. SOC., 78,2798 (1956). (4)J. M. Tedder, Adu. Fluorine Chem., 2, 104 (1961). (5)Heats of formation (kcal/mol): CH3F,-55;CH3Cl,-19.6;CH3Br, -9.5; HF, -64.8;CHJ, 3.3;F,18.9; C1, 28.9;Br, 26.7;125.5;CH3,34.3;IF, -19.7;BrF, -14.0;ClF, -12.1. Data from S.W. Benson, “Thermochemcial Kinetics”, Wiley, New York, 1976;J. J. DeCorpo, D. A. Bafus, and J. L. Franklin,J.Chem. Thermodyn., 3,125(1971).The value for IF is based upon the experimenb of M. A. A. Clyne and I. S. McDermid, J. Chem. SOC.,Faraday Trans. 2, 74, 1644 (1978). (6)T. Smail, R. S.Iyer, and F. S. Rowland, J. Am. Chem. SOC.,94, 1041 (1972). (7)R. L. Williams and F. S. Rowland, J.Am. Chem. SOC.,94,1047 (1972).
Similar experiments can be performed with %C1atoms formed in CClF3 or CC12Fzby the 37Cl(n,y)38C1nuclear reaction, and moderated by multiple collisions with the parent halocarbon m~lecule.’~The chief conceptual problem in using this method of “moderated nuclear recoil” for determination of thermal reaction rate constants is the possibility for confusion with reactions initiated by such atoms prior to thermalization. In general, however, such nonthermal reactions are reduced to negligible yield levels when the moderator mole fraction exceeds 0.90 for 18Fin either SF6or C2F6,a12 or %l in CClF3or CC12F2.15-16 Thermalization for such 3sCl atoms is shown for the Habstraction reactions with CH414and CH3CP5by the observation of Arrhenius-type temperature-dependent reaction rate constants with activation energies consistent with those measured by other methods. Since many simple F-atom reactions have activation energies in the 0-2 kcal/mol range, the same temperature test for thermalization is not so widely applicable there. A second general thermalization procedure is the demonstration that further increase of the moderator/substrate mole ratio has no additional effect on the yields of the reactions under observation. One such example is the unchanging ratio of 18F reaction yields with CH4 vs. CF3CF=CF2 for mole fractions of C2F6 moderator between 0.95 and 0.9995.1° We have applied in our experiments a variant of this high-moderation procedure in which we extrapolate the observed product yields to zero mole fraction of substrate, and identify an observed nonzero intercept as the yield for the thermal reaction in the system. The kinetically hot (8) F. S. Rowland, F. Rust, and J. P. Frank, ACS Symp. Ser., 66,26 (1978). (9)S.-H. Mo, E.R. Grant, F. E. Little, R. G. Manning, C. A. Mathie, G.S.Werre, and J. W. Root, ACS Symp. Ser., 66,59 (1978). (10) C. A. Mathis, K. D. Knierim, and J. W. Root, Chem. Phys. Lett., 72,368 (1980). (11)R. S. Iyer and F. S. Rowland, J. Phys. Chem., submitted for publication. (12)F. S. Rowland and R. S. Iyer, Atomic Energy CommissionReport No. UCI-1973-1; R. S. Iyer, Ph.D. Thesis, University of California, Irvine, 1973. -- .-.
(13)F. 5.C. Lee and F. S. Rowland, J. Phys. Chem., 81,1229(1977). (14)F. S.C. Lee and F. S. Rowland, J. Phys. Chem., 81,86 (1977). (15)C. Yarbrough and F. S. Rowland, unpublished results; C. Yarbrough, Ph.D. Thesis, University of California-Irvine, 1981.
0022-3654/81/2085-2488$01.25/00 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 17, 198 1 2489
Thermal F/X Atomic Substitutlon Reactions
TABLE I: Absolute Yields o f Volatile I8F Products from Recoil I8F Atom Reactions in Mixtures of SF,, CH,Cl, and O,/HI component
SF6 CH,C1 0 2
HI
press., torr 2650 135 300 0
2590 261 272 0
2140 437 258 0
2060 663 339 0
I8F product SF.''F S0;F''F CH,1'F CH,"FCl
2790 122 0 14
2780 238 0 26
2590 450 0 45
2230 660 0 66
absolute yield, % total 18F 1.10 i. 0.03 0.98 f 0.03 0.94 0.18 * 0.02 0.07 f 0.02 0.10 0.27 f 0.02 0 . 4 3 f 0.03 0.68 a a a
I0.03 f
t
0.92 f 0.02 1.04 f 0.02 1.12 I0.02 1.04 i: 0.02 0.93 f 0.02
