Langmuir 1995,11,4433-4439
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P-Fluoride Elimination Reactions of Fluorinated Alkyl Groups on the Ag(ll1) Surface Anumita Pault and Andrew J. Gellman” Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received August 31, 199P The surface chemistry of CF&H2(ad) and CFsCFzCH2(ad) on the Ag(ll1) surface was studied by temperature-programmed-reaction (TPR)measurements. These species,which are generatedon the surface by thermal dissociation of the C-I bond in the corresponding fluoroalkyl iodides, undergo ,&fluoride elimination to give the corresponding fluoroalkenes and adsorbed fluorine. Kinetic parameters obtained from variable heating rate TPR studies show that while the pre-exponential factors for the P-fluoride s-l, the activation elimination reaction in CF&H2(ad) and CF&F&H2(ad) are the same, Le., 1014.7*0.8 energies are considerably different, Le., 18.1f 0.9 and 13.9 f0.7 kcavmol, respectively. Utilizing the fact that F has a greater electron-withdrawinginductive effect than CF3, we conclude that the charge separation in the transition state is of the form C6++...F6. The adsorbed F generated by the ,&fluoride elimination reaction desorbs as AgF at -850 K. Unlike adsorbed alkyl groups, which couple to produce alkanes, the hydrofluoroalkyl groups studied here react by the B-fluoride elimination pathway, probably due to the increase in the barrier to coupling caused by fluorination at the P-carbon.
1. Introduction Recently the surface chemistry of halocarbons has drawn considerable attention. Fluorocarbons and their derivatives find wide applications as lubricants in aerospace engineering’ and a s potential substitutes for chlorofluorocarbons (CFC) in the refrigeration and packing industries2 and play important roles in the etching of silicon-based semiconductor^.^-^ The wide range of applications of fluorine-based compounds are a result of their resistance to decomposition. For example, fluorocarbon ethers of various types are used as high-temperature lubricants. Another classic example is the high resistance of perfluoroalkanes and highly fluorinated alkanes to reagents which normally attack the carbon backbone of hydrocarbon alkane^.^ On metal surfaces the thermal stability of CF3CH20(ad)over CH3CH20(ad)is reflected in its decomposition temperature, which is 100 K greater onCu(ll1)and 155KgreateronAg(110).8 Asimilartrend is found in the reaction of CH3CH2CH2(ad)groups on C u ( l l l ) , where fluorination of the CH3 group (i.e., CF3CH2CH2(ad))leads to a decrease in the @-hydrideelimination reaction rate by -4 orders of m a g n i t ~ d e . ~ In this paper, we report on the reactions of primary alkyl groups generated by the thermal dissociation of the C-I bond in adsorbed CF3CH2I and CF3CFzCH21 on the Ag( 111)surface. Temperature-programmed-reaction(TPR) studies show that CF3CH2(ad)and CF3CF&Hz(ad) on Ag(ll1) do not couple, as the hydrocarbon alkyl groups do, but decompose via @-fluorideelimination, yielding
* Author to whom all correspondence should be addressed.
’ Present address: Molecular Physics Laboratories, SRI Inter-
national, 333 Ravenswood Avenue, Menlo Park, CA 94025. Abstractpublished inAdvance ACSAbstructs, October 15,1995. @
(1)Synder, C. E.; Gschwender, L. J.;Tamborski, C.Lubr.Eng. 1981, 37,344. (2) Manzer, L. E.; Rao, V. N. M. Adv. Catal. 1993,39, 329. (3)Robertson, R. M.; Golden, D. M.; Rossi, M. J. J . Vac.Sci. Technol. B 1988,6,1632. (4)Roop, B.; Joyce, S.; Scultz, J.; Steifeld, J. J . Chem. Phys. 1986, 83,6012. ( 5 ) Creasy, W. R.; McElvany, S. W. Surf. Sci. 1989,201,59. (6)McFeely, F. R.; Yarmoff, J. A,; Taleb-Ibrahimi, A,; Beach, P. B. Surf. Sci. 1989,210, 429. (7)Sheppard, W. A.;Sharts, C. M. Organic Fluorine Chemistry; W. A.Benjamin, Inc.: New York, 1969. (8) Gellman, A. J.; Dai, Q. J.Am. Chem. SOC. 1993,115, 714. (9) Forbes, J. G.; Gellman, A. J. J . Am. Chem. SOC. 1993,115,6277.
CF2=CH2 at -253 K and CF&F=CH2 a t -206 K, respectively, as gas phase products and adsorbed fluorine on the Ag(111)surface. The effect of different electron-withdrawing groups, F and CF3, on the rate of the /3-fluoride elimination reaction, allows us to draw important conclusions about the electronic nature of the transition state. Using substituent groups of variable electronegativity it is possible to probe the electronic nature of a reaction center. This approach, called the substituent effect, is widely employed in linear free energy relationship (LFER)studies ofgas and solution phase reactions and equilibria, to shed light on differences in electronic structures between the transition state (in rate measurements) or the final state (in equilibrium measurements) and the initial state.1° Previous research in our group has demonstrated that such methods can also be used successfully to probe the transition state for a number of elementary reactions on single crystal surfaces. As examples, the charge separation in the transition state for the @-hydrideelimination reaction in adsorbed alkyls and alkoxides on Cu(ll1) and Ag(ll0) was shown to be of the type C6f”’Hb-.s,g A surprising result was obtained in similar studies on a supposedly symmetric reaction: the coupling of alkyl groups on Ag( lll).ll Fluorine substitution past the /3-carbon position in adsorbed alkyl groups decreased the coupling reaction rate. Fluorine substitution a t the P-carbon position results in the observation of the @-fluorideelimination reaction reported in this paper. Variable heating rate TPR measurements yielded kinetic parameters for the P-fluoride elimination reaction. They are E , = 18.1f0.9 kcall mol and v = 1014.7*0.s for CF&Hz(ad) and E , = 13.9 f 0.7 kcal/mol and v = 1014.7*0.8 for CF3CF2CH2(ad).The -4 kcallmol larger activation barrier for CF3CH2(ad) is consistent with a charge separation of the form Cb+Fbin the transition state, justifylng the name “P-fluoride elimination”reaction. The stronger electron withdrawing F atom destabilizes the positive charge on the P-carbon in the transition state to a greater extent than does the weaker electron-withdrawing CF3 group. This accounts (10)Wells, P.R. Linear Free Energy Relationships; Academic Press Inc.: New York, 1968. (11)Paul, A,; Gellman, A. J. J . Am. Chem. SOC. 1995,117,9056.
0743-746319512411-4433$09.00/0 0 1995 American Chemical Society
4434 Langmuir, Vol. 11, No. 11, 1995 for the larger barrier for the /3- fluoride elimination reaction in CF3CHz(ad) in comparison to CF3CFzCHz(ad). The /3-fluoride elimination reaction observed onAg(ll1) has also recently been shown to occur on Si(100) a t temperatures between -200 and 600 K.12 Although to date, this reaction has been studied only on Ag(ll1) and Si(lOO),the facility of this reaction on these two surfaces has important implications in catalytic synthesis of CFC alternatives. CFCs contain chlorine, which is undesirable. In the stratosphere U V light generates C1 radicals, initiating a cycle of radical chain reactions which deplete the ozone layer and consequently cause global warming.13 As a result a total phase out of the use of CFCs by the year 2000 was called for in 1990. These CFCs are to be replaced by the hydrofluorocarbons (HFCs),the synthesis ofwhich poses a major challenge to the catalyst industry.2 Catalytic synthesis of HFCs often involves fluorination of chloroalkenes, the first step of which is the reverse of the facile /?-fluorideelimination reaction. Therefore a fundamental understanding of the mechanism of the /3-fluoride elimination reaction would be useful to tailor synthetic routes which optimize the rate and yield of HFC synthesis. 2. Experimental Section Experiments were performed in two ultrahigh vacuum (UHV) chamberswhich have been described ear1ier.l' The experimental procedure is exactly the same as described in ref 11. Briefly, most of the TPR studies were performed in a chamber equipped with a Dycor mass spectrometer (Ametek, M200M). This chamber, pumped with a cryopump, had a base pressure of -4 x lo-" Torr. The secondchamber was equippedwith a sensitive mass spectrometer (Extrel)and was used to detect the desorption of AgF, which was below the detection limits of the Dycor mass spectrometer in the first chamber. The base pressure of this second chamber was maintained at -3 x Torr with the use of an ion pump. Reactants were dosed onto the surface using leak valves fitted with capillary-array dosers. Exposures are reported in units of monolayer (ML),where 1 ML corresponds to the coverage required to observe the onset of multilayer desorption in TPR experiments. On the basis of the calibration studies of linear alkyl iodides on the Au(ll1)surface, and on the fact that Au and Ag have similar van der Waal radii, we estimate that 1ML coveragecorrespondsapproximatelyto one physisorbed alkyl iodide molecule for every 12 surface silver atoms.ll The Ag(ll1)surfacewas cleanedbetween TPRexperimentsby heating to 950 K, to desorb atomic iodine14and AgF (see section 3.1.3.). Reagents and Their Sources. CF~CHZI (99%purity) was from Aldrich, and CF3CF2CHzI (purity not reported) was from Lancaster Synthesis.
3. Results and Discussion 3.1. Reaction Products. 3.1.1. Reaction of CF3CHz(ad). Adsorbed CF3CHz(ad),generated on the Ag(111) surface by the thermal dissociation of the C-I bond in CF~CHZI, undergoes /3 C-F bond dissociation a t -253 K to form CFz=CHz. The dissociation temperature of the C-I bond in adsorbed CFsCHzI on Ag(ll1) has not been determined. However, we believe that it is similar to that for C H ~ C H Zwhich I occurs at
