3972
J. Phys. Chem. 1983, 87, 3972-3975
Methyl Fluoride Formation from Thermal Fluorine-I8 Reaction with Dimethylmercury F. P. McKeown, R. Subramonla Iyer, and F. S. Rowland" Department of Chemistry, University of California. Irvine, California 9271 7 (Received: February 10, 1983)
The attack of F on (CHJ2Hg with the formation of CH3Fhas been observed in the gas phase by using radioactive 18Fatoms thermalized by multiple collisions in SF6. Approximately 9.6 f 1.0% of thermal 18Fatoms are found as CH318Fby the reaction F + (CH3)2Hg CH3F + CH3Hg (5), with the remainder reacting with (CH3),Hg to form other products, chiefly H18F by the reaction F + (CH,),Hg HF + CH3HgCH2(6). The rate constant k5 for this substitution reaction has been measured at 287 K to be 0.68 times as fast for thermal 18F as hydrogen abstraction from CH,, and 1.67 times as fast as hydrogen abstraction from H,. These relative rates correspond to an absolute rate constant k5 = (4.6 f 0.4) X lo-" cm3molecule-'s-l. The total rate constant for all reactions with (CH3)2Hgis (4.7 f 0.5) X lo-'' cm3 molecule-' s-', including reactions 5 and 6 and the possible alternate pathway F + (CH3),Hg CH3HgF + CH, (7) leading to CH3HgF. Because our experiments provide no information about the existence of magnitude of pathway 7, the rate constant for reaction 6 is expressed as cm3 molecule-' s-'. an upper limit k6 I (4.2 0.5) x +
-
-
Thermalized 18Fatoms can easily be generated in the gas phase from the 19F(n,2n)18Fnuclear reaction with SF6, followed by multiple nonreactive collisions with the target When the mole fraction of SF, exceeds 0.9, approximately 2% of the 18Fatoms react while still kinetically hot to form SF,18F,and the remaining 18Fatoms are free to react thermally with minor substrates present in the SF,. The most abundant reaction of 18Fwith metal alkyl compounds is hydrogen abstraction, as illustrated in reaction 1 18F+ (CH,),Sn H18F + (CH3),SnCH2 (1) for tetrameth~ltin.~ The analogous reaction has also been reported to be the main process occurring for thermalized 38Clatoms reacting with (CH3)4Pb,as in reaction 2.5 In 38Cl+ (CH,),Pb H3%l + (CH3),PbCH2 (2) these systems the most abundant observed volatile organic products have been those formed from direct bimolecular homolytic substitution reactions to give CH318F and CH3W1, respectively, via reactions 3 and 4. 18F + (CH,),Sn CH318F + (CH3),Sn (3)
-
--
38Cl + (CH3),Pb CH3%1 + (CH3),Pb (4) Similar substitution reactions to form R18F have been found for the reactions of 18F atoms with several R4Sn compounds,6 as well as for (CH3)4Ge.7 We have now extended the exploration of such substitution reactions to gaseous compounds of divalent metals through the use of dimethylmercury as the substrate compound and have again found a substantial yield of the substitution product, as shown in reaction 5. We have estimated the absolute 18F+ (CH3)2Hg CH318F + (CHJHg (5) reaction rate constant k5 through measurement of the yield from reaction 5 in competition with the known rates of the hydrogen abstraction reaction of 18Fwith CH, and with H,.
-
(1) Rowland, F. S., Rust, F.; Frank, J. P. ACS Symp. Ser. 1978, No. 66, 26. (2) Smail, T.; Iyer, R. S.; Rowland, F. S. J . Am. Chem. SOC.1972, 94, 1041. (3) Williams, R. L.; Rowland, F. S. J . Phys. Chem. 1973, 77, 301. (4) Cramer, J.; Iyer, R. S.; Rowland, F. S. J. Am. Chem. SOC.1973,95, 643. (5) Kikuchi, M.; Lee, F. S. C.; Rowland, F. S. J. Phys. Chem. 1981,85, 84. (6) Kikuchi, M.; Cramer, J.; Iyer, R. S.; Frank, J. P.; Rowland, F. S. J. Phys. Chem. 1982,86, 2677. (7) Rogers, P. J.; Rowland, F. S., unpublished results.
Experimental Section Detailed discussions of our experimental procedures involved in using thermalized 18Fatoms from nuclear recoil reactions initiated in a fast neutron generator have been given in previous publication^.'-^^"^^ The contents of the samples were frozen into approximately 10-mL cylindrical ampules made from Corning Pyrex 7740 glass. Gas pressures were measured on a calibrated grease-free vacuum line with a spiral gauge recorder. The indicated bulb pressures are those measured during the gas-filling process a t room temperature (297 K) and can be converted into the appropriate gas densities for the experiments a t 287 K. The samples were irradiated with 14-MeV fast neutrons from a Kaman A-711 generator whose target area is water cooled to a temperature of 287 f 2 K. The absolute calibration of the 18F radioactivity yields was carried out through monitoring with a sodium iodide scintillation counter of the 18Factivity induced in a concentric teflon sleeve placed around the glass ampule during irradiation. Sulfur hexafluoride (Matheson) was degassed a t 77 K, stored in a reservoir on the vacuum line, and used without further purification. Dimethylmercury (Alpha Products) was stated to have an initial purity of >98% and was thoroughly degassed at both 77 and 195 K prior to storage on the vacuum line in a cold trap held a t 273 K. Further degassing of (CH3)2Hgwas carried out prior to the filling of each sample on the vacuum line. A portion of the dimethylmercury was further purified by cryogenic trapping after separation into components on a 3-ft silicone oil gas-chromatographic column operated a t 297 K. The experiments with this chromatographically purifed (CH3),Hg showed product yields essentially the same as those found with the material purified only by repetitive degassing on the vacuum line. As shown later, (CH,),Hg reacts very rapidly with thermal fluorine atoms, and it is quite unlikely for impurity materials in the initial (CHJ2Hg to interfere appreciably in such reactions. Methane and hydrogen were obtained from the Matheson Gas Co. and were of purity >99.97% (UHP grade) and >99.95% (Prepurified), respectively. (8) Smail, T.; Iyer, R. S.; Rowland, F. S. J . Phys. Chem. 1971, 75, 1324. (9) Williams, R. L.; Rowland, F. S. J . Am. Chem. SOC.1972,94,1047. (10) Milstein, R.; Williams, R. L.; Rowland, F. S. J . Phys. Chem. 1974, 78, 857. (11) Smail, T.; Miller, G. E.; Rowland, F. S. J . Phys. Chem. 1970, 74, 3464. (12) Williams, R. L.; Rowland, F. S. J . Phys. Chem. 1972, 76, 3509.
0022-3654/83/2087-3972$01.50/00 1983 American Chemical Society
Methyl Fluoride Formation
The Journal of Physical Chemistry, Vol. 87, No. 20, 1983 3973
TABLE I: Yields of CH,I8F from Gas-Phase Reactions of Thermal 18Fwith (CH,),Hg a t Approximately 3000-torr Total Pressure
I
press., torr
% YIELD
product yield, % 18F
SF,
(CH,),Hg
SFjI8F
CH,18F
2980 2990 3000 2990 3000 3020 3010 3570 3020 3000 2900 3020 2390 2960
0.29 1.47 1.51 3.2 3.1 4.6 4.9 9.5 14.9 19.9 25.6 29.4 38.4 45.7
1.29 1.23 1.21 1.23 1.33 1.16 1.41 1.33 1.16 1.34 1.31 0.99 0.90 1.18
7.7 8.3 9.4
0 0
lo
-0
0
0
0
al
O
"
0
10.0 10.0 10.6 11.0 10.0 9.7 11.1 10.6 9.4 8.7 10.7
The lsF-labeled reaction products were analyzed by radio gas chromatography with an external flow proportional counter.13J4 A 50-ft dimethylsulfolane (DMS) column at 297 K was used for the separation of these products from (CH3),Hg/CH4 mixtures or from (CH,),Hg as the sole substrate. The only two radioactive peaks which were observed in these experiments were those corresponding to the hot product SF,18F and the thermal product CH28F. Approximately 1.2% of the total 18Fyield was observed as the former, consistent with the observations of the hot yield from SFs found previously with a wide variety of substrate molecules present in the system. The observed variations in yields of CH318Fare considerably larger than the error expected from the random statistical error of radioactive decay and are probably caused by inhomogeneities in the neutron flux as it intersects the Teflon monitor and the irradiated ampule itself. T h e (CH3),Hg/H2 mixtures were analyzed with a 75-ft dibutyl phthalate (DBP) column a t 297 K, the choice between DMS and DBP columns having no experimental significance. In our experiments only those species containing lsF radioactivity are measured, and no information is obtained about the eventual fate of the residual CH,Hg radical from reaction 5. No specific separation procedure was devised to search for the possible CH,Hg18F product. The remaining 18Fradioactivity is assumed also to have reacted with (CH,),Hg, and to have been deposited on the ampule walls, largely in the form of H18F. No measurements were made in these experiments to monitor the formation of such H18F, but numerous previous experiments have confirmed its presence in approximately the expected amounts in other systems.
Results and Discussion A series of experiments were carried out near 3000-torr total pressure with varying mole fractions of (CH3),Hg available for reaction with thermalized 18F. The results of these experiments over the range from lo4 to 1.6 X lo-, mole fraction of (CH,),Hg are shown in Table I and Figure 1 and exhibit a consistent absolute yield of CH318Fat 9.6 f 1.0%. The independence of the yield vs. mole fraction illustrates that the reaction leading to the formation of CH28F is not affected by increasing moderation of the 18F atoms and is therefore initiated throughout by thermalized 18Fatoms. As with all such studies, the absolute yields do not furnish any direct rate constant information, but rather (13) Iyer, R. S. Ph.D. Thesis, University of Califomia, Irvine, CA, 1973. (14) Lee, F. S. C. Ph.D. Thesis, Univeristy of California, Irvine, CA, 1975.
IO-^
~ X I O - ~
I . 5 x 10-2
10-2
(CH3 12 Hg / SFG
Flgure 1. Percentage yields of CH318F vs. (CH3),Hg/SF, 3000-torr total pressure.
ratio at
TABLE 11: Yields of CH,18F from Gas-Phase Reactions of Thermal 18Fwith (CH,),Hg at Varying Total Pressures product yield, % I8F
press., torr SF,
(CH,),Hg
SF,18F
920 1520 1990 3990
6.8 11.1 14.9 30.3
1.23 i 0.05 1.14 i 0.04 1.29 i 0.04 1.03 i 0.03
CH,18F 9.45 10.10 10.93 9.80
i i i i
0.15 0.12 0.11 0.07
reflect the relative rates of the various reactions available to these thermalized atoms. The conversion of relative yields to absolute rate constants requires the inclusion of some competitive pathway whose rate constant has been established by some other technique. A second series of reactions was carried out in which the thermalized lsF atoms were permitted to react with (CH3),Hg at various total pressures, as shown in Table 11. Despite a fourfold change in total pressure, the observed yield of CH31sF remained constant at 10 f 170,The independence of the yield vs. pressure is consistent with the results obtained for 18Fplus a variety of R,Sn compounds6 and indicates that no intermediates for reaction 5 exist which have lifetimes competitive with the collision frequency range appropriate for the pressures of Table 11, i.e., intermediates with lifetimes of s. The absence of such pressure-dependent yields suggests that reaction 5 occurs through a direct bimolecular substitution rather than through an addition-decomposition mechanism. Extensive experiments by a variety of techniques have shown that hydrogen abstraction by thermal fluorine atoms is usually a very fast reaction, often approaching gas collision rates.1k18 Without any specific knowledge of the C-H bonding in (CH3),Hg, the qualitative expectation is that hydrogen abstraction, as in reaction 6, should also be 18F + (CH,),Hg
-
H18F + CH3HgCH2
(6)
a very rapid reaction. Still another substitution reaction is also possible in this system, with the displacement of CH, by F, as in reaction 7. Little information is available 18F + (CH,),Hg
-
CH,Hg18F + CH,
(7)
(15) Mo, S.-H.; Grant, E. R.; Little, F. E.; Manning, R. G.; Mathis, C. A,; Werre, G. S.; Root, J. W. ACS Symp. Ser. 1978, No. 66, 59. (16) Root, J. W.; Mathis, C. A.; Gurvis, R.; Knierim, K. D.; Mo, S.-H. Adu. Chem. Ser. 1981, No. 197, 207. (17) Foon, R.; Kaufman, M. Prog. React. Kinet. 1975, 8 , 81. (18) Jones, W. E.; Skolnik, E. G. Chem. Reu. 1976, 76, 563.
3974
McKeown et al.
The Journal of Physical Chemistry, Vol. 87, No. 20, 1983
TABLE 111: Measured Yields of CH,18F from Gas-Phase Reactions of Thermal 18F in Mixtures of ( C H , ) , H g and C H , at Approximately 3000-torr Total Pressure 35
press., torr SF,
(CH,),Hg
CH,
product yield (CH:8F), % 18F
2990 2940 2910 291 0 2830 2900
9.9 10.2 8.2 9.7 9.9 5.5
10.7 49.2 80.7 102 150 104
9.02 6.34 4.05 3.81 3.03 2.74
30
25
pertaining to reaction 7 , but similar reactions with Br or I have been postulated for some systems in ~olution.'~The observation that about 10% of the total thermal 18F is found as CH3I8F indicates that reaction 5 is slower than reactions 6 + 7 by only about a factor of 9 and therefore must also be very rapid in comparison to most gas-phase reactions. Niki et aLZ0have recently reported the observation of CH,HgCI from photolytic reactions in Clz/ (CH,),Hg mixtures and have assigned this yield to the C1 analogue of reaction 7 . They have also observed CH3C1, which they assign to the reaction of CH, with CIz. However, the same pair of products is expected for the C1 analogue of reaction 5, followed by reaction of CH3Hg with Clz. Such ambiguity is typical for reactions in which one molecular species serves as both the photolytic source and the scavenger. Separate experiments with different source and scavenger molecules are needed to establish the relative importance of the C1 analogues of reaction 5 and 7 in the C1 reaction with (CH,),Hg. However, the CH318F observed in the present radiotracer experiments cannot be a product of reactions of CH, formed in reaction 7 because 18Fis not incorporated into any scavenger molecule in this system. Quantitative estimates of the rate constants for k5 and k6 + k , can be obtained through observations of the yield from the former in competition with some other wellknown reaction of thermal fluorine atoms. The comparison reactions used in our experiments have been the abstraction of H from CHI, as in reaction 8, or from Hz, as in I8F + CH,
-
H18F + CH,
(8)
reaction 9. The accepted rate constants for these reactions
18F+ H2
H18F
+H
(9)
at 300 K are k , = (7.3 f 0.5) X 10-l' cm3 molecule-' s-' and k , = (2.5 f 0.1) X lo-" cm3 molecule-l s-'.16 The fate of thermal lSFatoms in reaction with (CH3)2Hg in excess of SF6is to divide among the three products from reactions 5-7 in proportion to their reaction rate constants. In mixtures of (CH,),Hg with CHI in excess SF6, the thermal yield is divided among the four reaction paths 5-8. The fraction of thermal I8F found as CH31sFis then given by k,[(CH,),Hgl divided by [ k , + k6 + k71[(CH3)2Hgl+ k,[CH,]. The reciprocal yield of CH318Fcan be equated to the inverse of this ratio and then normalized for the 98% of the total 18Fwhich is available as thermal atoms, resulting in eq 10. The measured CH,lsF yields for var0.98
ying concentration ratios of CH4 vs. (CH,),Hg are shown (19)Ingold, K . U.; Roberts, B. P . "Free-Radical Substitution Reactions"; Wiiey-Interscience: New York, 1971. (20) Niki, H.; Maker, P. S.; Savage, C. M.; Breitenbach, L. P. J. Phys. Chem., in press.
20
15
io
5
a 0
I
I
I
I
5
IO
15
20
Flgure 2. Reciprocal yields of CH,'*F
in (CH3)pHg/CH, mixtures.
in Table 111, and graphed by eq 10 in Figure 2. The error bars in Figures 2 and 3 represent the standard deviation estimated for these individual experiments on the basis of the standard deviation found for the replicate experiments in Table I. The slope of the linear plot in Figure 2 is a direct measure of the relative rates of k , and k,, while the intercept is a measure of the magnitude of ks + k, vs. k,. The least-squares intercept from Figure 2 is 9.4 f 0.8, and the slope is 1.46 f 0.07. The measured slope indicates that the rate constant k , is 46% larger than k5 and, therefore, that the absolute constant for k , is (5.0 f 0.4) x lo-" cm3 molecule-'^-^. The relative value for k6 + k , vs. k , is 8.4, giving an absolute rate constant of k6 + k7 = (4.2 f 0.5) x 10-l' cm3 molecule-l s-'. There is no evidence that k , is large, or even that it is not zero, while there is abundant evidence that reactions such as reaction 6 occur very r e a d i l ~ . ' J ~ -We ' ~ therefore hypothesize that reaction 6 is the major contributor to the sum of reactions 6 and 7 and that the upper limit indicated, k , I (4.2 f 0.5) X cm3 molecule-' s-', is probably close to the actual value for k,. On a molecule-for-molecule basis, the combination of reactions 6 plus 7 is about 6 times more likely than abstraction of H from CHI, or, if reaction 6 is dominant over reaction 7, as much as a factor of 4 more reactive per C-H bond. In comparison, the rate constant for hydrogen abstraction from CzH6 is 2.1 x cm3 molecule-' or about 1.9 times as likely as from CH, on a per-bond basis.16 The relative ease of hydrogen abstraction by fluorine atoms shows some correlation with the C-H bond dissociation energies, and one possible implication of our current results is that the C-H bond in (CH&Hg is probably considerably weaker than in CZH6, i.e.,
