208
Ind. Eng. C h e m . Res. 1987,26, 208-215
then the ammonia treatment would result in bond cleavage. The resultant amides undergo hydrolysis at high temperature to form C-H bonds with loss of C 0 2 amd ammonia. In contrast, the nitrogen heterocyclic solvents undergo condensation and coupling reactions with the activated aromatics in the coal, resulting in a high incorporation of the nitrogen solvent by formation of covalent linkages to coal structural units. These linkages are very stable and the incorporated solvent cannot be removed, unlike the ammonia conversion wherein the incorporated ammonia (as the amide) was readily eliminated at the higher temperature. Conclusions Aqueous ammonia (0.1-5 wt 7‘0) appears to be a promising solvent system for coal liquefaction. Conversions up to 42.5% MAF have been obtained. Ammonia is an inexpensive bulk chemical and does not suffer from the drawbacks of the nitrogen heterocyclics used in earlier studies. It produces a low ash, low molecular weight ( ~ 3 1 5 ) hydrogen-rich extract and a condensed residue which has very little oxygen and other heteroatom species. Ammonia
incorporation in the extract and residue was found to be very low. This residue should have a high calorific value and could readily be used as a solid fuel for combustion. On the basis of the preliminary results, addition of a pretreatment step might be beneficial. Acknowledgment The work was supported by the Electric Power Research Institute through research Project 2147-9. We thank Linda F. Atherton for helpful discussions. Registry No. NH3, 7664-41-7.
Literature Cited Atherton, L. F.; Kulik, C. J. “Advanced Coal Liquefaction“, presented a t the AIChE Annual Meeting, Los Angeles, CA, Nov 1982. Bienkowski, P. R.; Narayan, R.; Greenkorn, R. A.; Chao, K. C. Ind Eng. Chem. Prod. Res. Deu., preceding paper in this issue. Kershaw, J. R.; Bagnell, L. J. Prep. Pup.-Am. Chem. Soc., Dic. Fuel Chem. 1985, 30(3),101.
Received for review January 14, 1986 Accepted August 4, 1986
Aerosol Direct Fluorination: A Developing Synthesis Technology and an Entry Level Mechanistic Tool. A Short Review J a m e s L. Adcock* a n d M y r o n L. C h e r r y Department of Chemistry, T h e University of Tennessee-Knoxuille,
Knoxville, Tennessee 37996-1600
The aerosol direct fluorination process is unique (US Patent 4330 475) in that a heterogeneous reaction is carried out under gaslike conditions. Reactant molecules adsorbed onto airborne sodium fluoride particulates are subjected to attack by gaseous elemental fluorine under conditions such that the temperature and fluorine concentration in the reactor are continuously chaqged as the particulate traverses the length of the reactor. This allows optimal rates of reaction with minimal fragmentation of reactant structures to occur. Initial, reaction-limiting conditions necessary to control the high exothermicity of elemental fluorinations are gradually changed so as to provide the final, forcing conditions necessary to achieve perfluorination. Mechanistic interpretations of product distributions under controlled stoichiometric conditions are possible because-of the unusually clean reactions. Elemental fluorinations have a long and eventful history in which the great exothermicity of the reactions have generally defied attempts to simultaneously achieve useful reaction rates, safe operation, good yields, and minimal fragmentation of hydrocarbon skeletons. Many attempts have been made since the earliest work by Moissan (18911, some very ingenious, to achieve the above combined objectives. However, it was not until the advent of modern flow control technology that practical direct fluorinations were feasible. For example, Bockemuller (1933) bubbled a mixture of nitrogen and fluorine through a cold solution of the organic compound in an inert solvent arid successfully achieved mono- and difluorination, although not perfluorination. Bigelow (Calfee and Bigelow, 1937; Fukuhara and Bigelow, 1941) used a reactor that contained a “catalytic” metal such as copper gauze or shot and a fluorine stream diluted with nitrogen. This vapor-phase reaction produced perfluorinated compounds in yields as high as 62%. Although the catalytic action is questionable, the large amount of metallic surface area clearly functioned as a heat sink to disperse the heat of reaction. A later development by Bigelow (Tyczkowski and Bigelow, 1955) was the “jet” fluorination method which introduced diluted fluorine and 0888-5885/87/2626-0208$01.50/0
hydrocarbon as separate, concentric streams in a hightemperature, turbulent reactor in which product was quickly removed from the reaction zone. Although ethanes and materials up to C4 could be successfully perfluorinated, the method essentially failed to produce higher molecular weight perfluorinated materials. The most significant advances in the understanding of direct fluorination occurred as a result of developments begun by Margrave and continued by Lagow (1979). The importance of solid-state stabilization and reaction-limiting initial conditions followed by reaction-forcing final conditions and the maximization of exposed surface area of hydrocarbon to elemental fluorine were recognized and integrated into a low-temperature-gradient (LTG) fluorination reactor. Although high yields were commonly obtained, the reactor was slow and of a “batch” type, which is not as easily scaled for commercial applications. The reactor did however possess the basic attributes which allowed achievement of all the subjective criteria except speed. The “LAMAR” methods were however lacking in the ability to handle compounds which had low solid-state volatility and which also did not maintain a large surface area exposure to elemental fluorine. It was not until the development of the aerosol fluorination device/process that 0 1987 American Chemical Society
Ind. Eng. Chem. Res. Vol. 26, No. 2, 1987 209 HYDROCARBON EVAPORATOR -
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these remaining problems were overcome.
Technical Explanation of Aerosol Direct Fluorination Aerosol direct fluorination (AF) is a radically new and different approach to the control and study of the reactions of elemental fluorine (Adcock et al., 1981). It is a valuable new approach from both the pure and the applied chemical points of view. It is, from the applied viewpoint, first and foremost a flow process which can be scaled up by presently available technology and is both fluorine efficient and capable of high yields of single products, especially perfluorinated products. The purities of many products collected directly from the reactor are as high as 98% (molar) by GLC integration. Aside from the practical value of the aerosol fluorination process, the versatility of the process and the unprecedented control of reaction parameters and conditions in a solventless reaction mixture have permitted for the first time a really controlled observation of the process called “direct fluorination”. Conceptually, the aerosol direct fluorination process is simplicity itself (see Figure 1). An organic vapor is ad-
sorbed or condensed onto the surface of a large number of microscopic (17 A) (Espenscheid et al., 1964) sodium fluoride preaerosol nucleating particles in a helium (or nitrogen) carrier gas a t low (-196 “C) temperatures. The particulates so formed are carried into a low-temperature (-78 to -40 “C) region of a tubular microporous walled (2 pm) reactor in which fluorine diffuses through the porous walls into the reactor stream. As the adsorbed molecules are fluorinated, they are simultaneously carried down the tubular reactor into regions of higher fluorine concentration and higher temperatures, where they undergo higher degrees of fluorination. The fluorine concentration in a typical reaction reaches 2-5% by volume which is usually calculated to be about twice the stoichiometrically required quantity. Molecules leaving the subambient temperature region of the reactor are usually from 40% to 60% fluorinated and are introduced into a photochemical reactor. The photochemical reaction completes the fluorination by removing the very unreactive residual hydrogens. The result is virtually 100% perfluorination and, surprisingly, almost no fragmentation of the fluorocarbon skeletons as long as the ambient, excess fluorine concentration in the photochemical stage is below about 5 To. The alternative
210 Ind. Eng. Chem. Res. Vol. 26, No. 2 , 1987
to “photochemical finishing” of fluorocarbons is to greatly increase the concentration of fluorine which results in enormous (50X) stoichiometric excesses and greatly reduced efficiency of fluorine utilization if the fluorine is not recycled. For example, a t 30% fluorine (a 48X stoichiometric excess) up to 90% fluorination can be achieved in the subambient reactor. We were surprised to learn that the degree of fluorination could not be substantially improved by the introduction of long (30 m X 3 / s in.) coils a t temperatures up to 100 “C. Introduction of the photochemical stage a t these concentrations, however, results in good yields of perfluorinated product (65% or higher) but excessive (up to 30%) fragmentation. The entire fluorination process typically requires from 0.5 to 1.5 min for a given unit particle to traverse the reactor. The aerosol process can vary semiindependently: reactor temperatures (gradients), fluorine concentrations (gradients), reaction or residence times, reaction stoichiometry, hydrocarbon throughpbts, and whether or not photochemical finishing is employed. Since the process is a flow process, it can be maintained a t a steady state for any preset group of reaction parameters and conditions. The amounts of material produced thus depend only on the length of time the reactor is in operation. The aerosol fluorinator produces excellent yields of perfluorinated alkanes, ethers, ketals, alkyl chlorides, and acyl fluorides (from acyl chlorides and fluorides or esters) and moderate yields of perfluorinated esters and ketones, including the highly branched ketones F-pinacolone and F-tert-butyl F-isobutyl ketone (“F-provalone”) and the perfluorinated ortho esters, F-tetramethyl- and F-ethylene orthocarbonates (Adcock et al.. 1981-1985).
Theoretical Considerations and Empirical Rationalizations Relevant conditions contributing to a high-yield fluorination include the following. (1) Fluorination of Molecules Adsorbed on Sodium Fluoride and Held in a Crystalline State. Solid-state stabilization of molecules is a complex group of effects. The major effects which can conceivably result from adsorption of organic molecules onto sodium fluoride and which are relevant to aerosol direct fluorination are as follows: (a) a heat sink which allows the release of energy into the lattice by radiationless relaxation processes; (b) a template on which a molecule subjected to skeletal bond scission may recombine instead of fragment; (c) a means of immobilizing two radicals formed by hydrogen abstraction so that their interactions are minimized; (d) a shield protecting one hemisphere of the molecule from attack by gaseous fluorine; (e) a catalyst primarily involving the effect of fluoride ion on some substrates; (f) a possible interaction of elemental fluorine with sodium fluoride; (9) an interaction as a base which will absorb the cogenerated hydrogen fluoride produced by the fluorination; and (h) a base which will quench any carbocations produced by the fluorination or that would result from protonation. (2) Very High Surface Area Exposed to Gaseous Fluorine. The increased reactivity resulting from subdivision of bulk materials is well recognized. The explosive reactivity of seemingly innocuous materials such as grain dust and flour mixed with air is testimony to the startling change in perceived reactivity of a substance on the change of particle size and accessibility to oxidant. (3) Equal Exposure of All Molecules to Gaseous Fluorine at All Stages. This factor is as important as surface area in obtaining a high yield of unfragmented fluorocarbon in a gradient fluorination reactor of any type.
Molecules protected from attack by fluorine in the initial reaction-limiting stages will undergo extensive fragmentation a t later stages where conditions are much more vigorous. Once a molecule is approximately 40% fluorinated, its skeletal integrity seems to be markedly enhanced and can withstand vigorous attack. This requirement is the reason for the inability of the LTG reactor and LAMAR techniques in general to fluorinate molecules which do not maintain a highly open structure and which cannot be caused to renew their surface by volatilization/ condensation. This requirement is also the main factor limiting the degree of hydrocarbon loading of the preaerosol sodium fluoride particulates. This factor also determines the amount of sodium fluoride which must be introduced relative to the amount of hydrocarbon. (4) High Initial Dilution of Gaseous Fluorine. Dilution of the fluorine results in several beneficial effects: (a) a reduction in the overall rate of reaction resulting in a lower heat flux; (b) a reduction in the chance of multiple simultaneous reactions on a single molecule resulting in skeletal fragmentation; and (c) a reduction in the chance of adjacent molecules forming radicals in proximity close enough for coupling to occur. (5) Initial Low Temperatures. Low temperatures have the effect of lowering the overall kinetic energy of molecules which results in a reduction in the frequency of molecular collisions as well as a reduction in the percentage of effective collisions. Additionally lower temperatures aid in the removal of excess heat of reaction. (6) Highly Efficient Mechanism of Heat Dissipation. Two major mechanisms of heat dissipation are operational in the aerosol system. Adsorbed molecules may lose heat directly to the aerosol particulate, and both molecule and particulate may lose energy to the reactor walls and thus to the cooling system by collisions with the main carrier gas (helium or nitrogen). For a complete fluorination to occur, however, the above conditions are not sufficient to maintain a reasonable reaction rate as the organic molecule becomes more highly fluorinated. Several effects are a t work here to reduce the reactivity of organic molecules to molecular fluorine and thus slow the rate of reaction. First as the molecule becomes more highly fluorinated the number of reaction sites (hydrogen atoms) is reduced. There cannot be as high a percentage of effective collisions to total collisions because some molecular fluorine collisions with the substrate will be with fluorine substituents. Second the activation energy for effective collisions may change due to the strong inductive effect of the fluorine substituents. Third, the steric bulk of these fluorine substituents will limit the angles of attack of an incoming molecule of fluorine. Relevant conditions contributing to perfluorination include the following. (1) Higher Concentration of Gaseous Fluorine. Increasing the concentration of fluorine will increase the number of collisions and thus the number of effective collisions. This will compensate for the reduction in reactivity and the reduced number of reactive sites (hydrogens). There is, however, a practical limit to increasing the concentration of fluorine by addition of gaseous fluorine without means of removing the carrier gas. However, even a t the lower fluorine concentrations 40-6070 fluorination of neopentane is easily achieved. Also it should be noted that a t 50% stoichiometries or less, the photochemical stage has no observable effect (see Figure 2 ) . This indicates that all readily available fluorine is consumed in the initial stage a t stoichiometries of 50% and lower. It is also important to note that since the reaction
Ind. Eng. Chem. Res. Vol. 26, No. 2, 1987 211 31%
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is kinetically driven and is a flow process, the single-pass fluorine utilization efficiency will probably never exceed 80% (40% of total F atoms if HF is not recovered). (2) Higher Temperatures. Increasing temperatures will increase the collision frequency and the collision energy of gaseous fluorine on the organic molecule. Thus, the number of effective collisions and thus the rate of fluorination will increase. We have found that the fluorination reaction is broadly affected by temperature changes, but it is generally insensitive to small changes. We are also unable to measure the true vibrational temperature of the organic molecules but can only maintain temperature gradients in steady-state fashion. (3) Photochemical Dissociation-Activation of Molecular Fluorine. The practical limitations on the increase in concentration of fluorine gas in the reactor imposed by the minimum carrier flow in the aerosol process caused us to look for means of activating the reactant gas and thus increasing the rate of fluorination. The introduction of the photochemical stage allowed/required the reduction of the concentration of fluorine to levels comparable to the stoichiometry. In earlier runs a t 30% fluorine, extensive fragmentation occurred, giving only 67 % unfragmented product (neopentane) (Adcock et al., 1981). The reduction of fluorine to about 6% (4X stoichiometry) resulted in 94% unfragmented product in isolated yields approaching 90%. This development increased the utilization of fluorine in a single pass to about 25% (see Figure 3). In our most recent work single-pass efficiencies of 50% have been achieved by reducing total carrier flow and achieving the necessary fluorine concentration at lower stoichiometric excess. (4) Increased Vibrational Temperature of Molecules Leading to Increased Reaction with Molecular Fluorine. This effect occurs as a result either of photochemical activation of the organic molecule or of its activation as a result of the reaction with atomic fluorine (Adcock et al., 1981). The effect is manifested in the anomalous product distributions which occur when a slight deficiency of fluorine is introduced into the reactor when the photochemical stage is operating (see Figure 3). The result is seen as perfluorination of a fraction of the organic molecules with the remainder expressed as a distribution similar to that originating from the subambient reactor with the photochemical stage not operating. In our most recent work, which is still unpublished, the use of a "Rayonet" photochemical reactor of the same radiated power as our earlier medium-pressure mercury lamps did not result in the same anomalous perfluorination effect.
We found the perfluorination effect was more related to light flux than overall power. This indicates that the fluorine atom concentration/proportion may have a threshold value. Therefore, by extensively dissociating molecular fluorine and producing some threshold concentration of fluorine atoms, we may be generating activated hydrylfluorocarbon molecules by atomic fluorine attack, which scavenge the available molecular fluorine.
Typical Aerosol Fluorination Procedure Aerosol Fluorination of Methyl 2,2-Dimethylpropanoate. Methyl 2,2-dimethylpropanoate was prepared by the reaction of trimethylacetyl chloride (Aldrich) and an excess of anhydrous methanol in pyridine. The ester was purified by distillation (bp 101 "C). The yield is near quantitative. The density of the ester is 0.891 g/mL. A Precision Scientific Pressure-Lok syringe was filled with approximately 5.0 mL of the purified ester and was placed in a Sage Instruments Model 341A syringe pump. The needle of the syringe was attached to a length of 1/16-in.stainless steel tubing leading to the hydrocarbon inlet of the heated (110 "C) evaporator unit (see Figure 1). Previously two nickel combustion boats filled with approximately 10.0 g of anhydrous sodium fluoride were placed inside the tube/furnace preaerosol particulate generator. Also, approximately 200 g of 'I8-in. sodium fluoride pellets was placed in the product trap to absorb excess hydrogen fluoride not absorbed by the aerosol particulates. The main helium carrier gas flow through the preaerosol furnaceltube was set to 500 mL/min and the furnace heated to a 950 "C thermocouple reading for the furnace effluent. (Note: the NaF melt is at least 100 "C hotter than this reading.) The secondary hydrocarbon carrier (helium) flow entering the top of the reactor was set to 450 mL/min, and the primary hydrocarbon carrier entering along side the hydrocarbon liquid inlet was set to 50 mL/min. The circulating coolant (R-11 at -78 "C) was fed into the bottom of each module until the thermocouple temperature controller at the top of each module shut off the flow at the present temperaturs of -20 and -10 "C for the top (1)and bottom (2) modules, respectively. Fluorine gas was then introduced into the first three (of four) inlets in the following amounts diluted by helium (in parentheses): Module 1 inlet 1 (top), 5 mL/min of F2 (150 mL/min of He); module 1 inlet 2 (bottom), 18 mL/min of F, (150 mL/min of He); module 2 inlet 1, 20 mL/min of F, (150 mL/min of He). The second fluorine inlet on the second module is sealed off. When all flows and temperatures
212 Ind. Eng. Chem. Res. Vol. 26, No. 2, 1987
were stabilized, the photochemical stage was energized (two stacked 550-W Hanovia medium-pressure mercury arcs in water-cooled quartz immersion wells produced approximately 250 W of ultraviolet energy or 500 W total UVvis-IR, over 10 in.). When the system had stabilized, the Dewar flask surrounding the product trap was filled with liquid nitrogen and the syringe pump was started. The ester (M, 116) was introduced at 0.5 mL/h (0.446 g/h, 3.84 mmol/h). A total of 3.0 mL was introduced over a 6-h period. The fluorinelester stoichiometry was 2.251, a 27:l mole ratio of F2to ester. When 3.00 mL of ester had been delivered by the syringe pump, it was shut off. The reactor was allowed to run an additional 15 min before the fluorine and the mercury arcs were shut off. The preaerosol furnace, the evaporator heater unit, and the coolant pump were shut off. Once the system approached ambient conditions, all the helium carriers were shut off and the product trap valves were closed. The product trap and its liquid-nitrogen-filled Dewar flask were removed to the vacuum line where the trap was evacuated. The contents of the trap were transferred to the glass vacuum line by vacuum distillation, and the products were fractionated through -45, -78, -131, and -196 “C traps under active pumping (3 pm of Hg). The product (6.6 g) was collected in the -78 “ C trap. The remaining vacuum line traps were virtually empty. The product assayed by gas chromatography (Fluorosilicone QF-1, 13% on 60-80-mesh Chromosorb P packed column 7-m X 3/8-in. conditioned a t 225 “C) proved to be essentially pure perfluorinated methyl 2,2-dimethylpropanoate a t an 86% yield.
Chemical Considerations and Observations The aerosol direct fluorination process exercises control of the fluorination reaction on both the macrosystems level and on the molecular level. The immobilization of molecules subject to reaction is not a new concept and has been used extensively in the biological sciences to control protein synthesis as well as building other molecules of biological importance. This is the first instance in which molecular control has been attempted in a fluorination reaction. It is this aspect of the process which has made the study of fluorine interactions with organic molecules possible. This aspect of the process also provides for the exceptionally clean reactions, which from a synthetic point of view means that products of the reactions will in many cases need very little purification. The reduction in the need for complex separations technologies makes aerosol direct fluorination a viable synthetic technology despite the expense of elemental fluorine relative to hydrogen fluoride or other non-fluorine-derived fluorinating agents. It is therefore essential to view the entire process in order to evaluate its economic feasibility. The aerosol fluorination of functional hydrocarbons to their perfluorocarbon analogues is successful provided several conditions are met. It should be kept in mind that elemental fluorine is not only a radical generator but is also a strong oxidizing agent as well. Functional groups subject to oxidation will of course be oxidized. In fact all atoms subjected to fluorination are oxidized in the formal sense. For example, the oxidation state of carbon in methane is near zero while in tetrafluoromethane it has the maximum value of four and is actually more highly oxidized than carbon dioxide. Since this is generally the case in any fluorination of organic molecules, it should not be surprising that the behavior of even formally unchanged functionality is considerably altered not only in reactivity but often in behavior and stability as well. The aerosol fluorination process has proved successful in the fluorination of a large number of different classes of functional
organic compounds. These include alkanes, ethers, ketals, ortho esters, acid chlorides and fluorides, esters, ketones, and alkyl chlorides while preserving the functional group without loss. Alkanes. The fluorination of alkanes generally proceeds to the analogous perfluorinated compounds. Neopentane, an exceptionally easy compound to fluorinate by this process, can be fluorinated in virtually quantitative yield (95%) with high fluorine efficiency (Adcock et al., 1981). It therefore served as an excellent model compound for the initial explorations of the aerosol process. The reaction of neopentane under stoichiometrically controlled conditions provided us with the first evidence that fluorine attack on a molecule is not purely statistical. The separation and quantization of isomers a t each substitution stage showed that fluorine preferred to substitute the least highly fluorinated methyl group, and in later work on other molecules, given a choice, fluorine will choose the methyl group most distant from a previously substituted group. This set of effects works to produce an approximately 4-fold preponderance of the symmetrical isomers over their predicted amounts based on purely statistical considerations. Steric effects are important as had been previously reported by Lagow using the “LTG” method (Shimp and Lagow, 1977). Similarly, aerosol fluorination of 2,4-dimethylpentane showed that only small quantities of the perfluorinated analogue could be produced and that residual hydrogen atoms were at the highly protected tertiary positions (Cherry, 1986). Attempts to selectively monofluorinate 2,4-dimethylpentane at the tertiary positions failed, showing that fluorine is not sufficiently selective to significantly differentiate between primary, secondary, and tertiary hydrogens (Cherry, 1986). The much larger electronic and steric effects of fluorine substitution do produce selective responses since the preponderant difluoro isomer to 2,4-dimethylpentane is the 1,5-isomer. The statistically normalized generalization of these observations can be summarized as CH, > CH,F > CHF2 >> CF,
Ethers. Ethers are among the most stable and easily fluorinated groups of molecules. A large variety of ethers have been perfluorinated by the method ranging from simple methoxyalkanes and cyclic ethers (Adcock et al., 1981; Adcock and Robin, 1984) to the “glyme” polyethers up to tetraglyme (Adcock and Cherry, 1985). For example, 1,4-dioxane produces its perfluoro analogue in 90% effluent purity and 70% yield with less than 5% fragmentation or ring cleavage. Ketals. Ketals fluorinate well without solvolysis despite their sensitivity to acid. This finding gave evidence for the conclusion that the sodium fluoride aerosol particles were active in the process not only as a nucleating particle but chemically as a base as well. Ethylene glycol ketals are exceptionally stable to fluorination and can be used to “protect” carbonyl groups on ketones sensitive to cleavage during fluorination such as cyclic ketones. The difficulty with utilization of this technique lies in the high stability of the perfluorinated ketals to removal of the protecting group. For example the ethylene glycol ketal of cyclopentanone, F-1,4-dioxaspiro[4.4]nonane, requires a temperature of 450 “C in 100% sulfuric acid for 24 h to convert 23% of the fluorinated ketal to the cyclic ketone in 45% yield (Adcock and Robin, 1984). This route requires harsher conditions than those required for the hydrolysis of perfluorinated methoxycyclopentane which at 360 “C in 100% sulfuric acid produces the same ketone in 89% yield and 61% conversion after only 14 h. Ortho Esters. Ortho esters which are as sensitive to
Ind. Eng. Chem. Res. Vol. 26, No. 2, 1987 213 acid as the ketals are also fluorinated in high effluent purity and reasonable yields (Adcock and Robin, 1986). Tetramethyl orthocarbonate is prone to loss of a methoxy group, forming trimethyl orthoformate in a 1:3 mixture. Bis(ethy1ene) orthocarbonate is however very resistant to ring cleavage and is produced in 98% effluent purity. Acid Chlorides and Fluorides. Acid fluorides produce the analogue perfluorinated acid fluorides in good yields and high purities (Evans, 1983). The aerosol fluorination of acid chlorides, on the other hand, does not produce the perfluorinated acid chlorides but does produce solely the perfluorinated acid fluorides. This observation also suggests that the sodium fluoride particles also function as a source of fluoride ion which displaces a chloride ion nucleophilically. This result is supported by the inability of the relatively fluoride-ion-free LTG process to fluorinate acid chlorides without substantial decomposition, while acid fluorides fluorinate uneventfully (Adcock and Lagow, 1974; Adcock et al., 1975). Esters. The presence of fluoride ion is not always advantageous; methyl esters undergo varying amounts of cleavage of the fluorinated ester linkage due to the presence of fluoride ion (Cherry, 1986). The major byproducts are the perfluorinated acid fluorides formed from the acid parts of the perfluorinated esters and carbonyl fluoride from decomposition of the trifluoromethoxide displaced. Surprisingly the esters of higher primary alcohols do not seem to undergo cleavage nearly as easily nor do the esters of secondary and tertiary alcohols. Steric hindrance at the acid also appears to play a role in cleavage of the ester linkage. Ketones. The aerosol fluorination process reliably and reproducibly fluorinates ketones in moderate to good yields (Adcock and Robin, 1983,1984). The presence of fluoride ion does not, somewhat surprisingly, promote the oxidation of the carbonyl group to a fluoroxy group. This indicated that the fluoride ion is not as catalytically active in sodium fluoride as, for example, it might be in potassium or cesium fluorides which are known to catalyze the oxidation of carbonyl groups (Ruff et al., 1966, 1967; Hohorst and Shreeve, 1968). We hope to investigate whether substituting potassium fluoride for sodium fluoride for the nucleating particles would change the observed behavior in the reactor. Since no other direct fluorination process has reported much success in the fluorination of ketones, we speculate that the reduced acidity of the aerosol process plays a role in the successful fluorinations. One major complication in fluorinating methyl ketones appears to be their ability to dissolve sodium fluoride to a slight extent. Solubilization promotes the phenomenon of “wet” aerosols which tend to agglomerate and wet the walls of the reactor. The materials thus deposited are recovered at the end of the reaction. Yields based on input are thus very low. Alkyl Chlorides. Aerosol fluorination of alkyl chlorides occur in high yields with respect to chlorine loss and fragmentation (Adcock et al., 1983; Evans, 1983; Adcock and Evans, 1984). A complication, r$arrangement, occurs partially in secondary alkyl chlorides and completely in tertiary alkyl chlorides. This rearrangement occurs very early in the reaction when few or no hydrogens have been replaced by fluorine. Primary alkyl chlorides can be fluorinated to their perfluorinated analogues in generally good to excellent yields. Secondary alkyl chlorides generally produce a mixture of primary and secondary perfluorinated alkyl chlorides due to 1,Zshifts of the chloro group which may occur more than once during the fluorination (Adcock and Evans,
Figure 4. Complete radical rearrangements of chloroalkanes; small arrow indicates cis or trans isomers.
1984). The composition of the mixture is a function of the relative number of primary hydrogens adjacent to or once removed from the chloro group and other subtle factors not well understood. The 1,2-shift occurs with very low activation energy since shifts can occur between virtually equivalent secondary positions. Tertiary alkyl chlorides undergo complete rearrangement to primary alkyl chlorides and occasionally some secondary products especially when the secondary products are necessary intermediate products (Evans, 1983). The necessity of intermediate products, not radicals, is a consequence of the observation that in cases where 1,2-shifts have been blocked no shifts have occurred. Thus, it is assumed that in all cases where shifts greater than 1,2shifts are observed that these are sequential, multiple 1,Z-shifts.
Radical Rearrangements Occurring during Direct Fluorination The occurrence of partial or complete rearrangement is a consequence of the relative stability of the radical formed by hydrogen abstraction to the radical formed by rearrangement. The only other consideration is the activation energy of the rearrangement, the energy of the transition state separating the radicals. In general the 1,2-shift of the halogens except fluorine is a very low activation energy process and so the major consideration is the relative energies of the initial and rearranged radical. In attempting to categorize the rearrangements observed in alkyl chlorides during fluorination, it was obvious that tertiary to primary chloride rearrangements which go to completion are the result of primary initial radicals rearranging to tertiary radicals by 1,2-chloride shifts (Figure 4). On the other hand, secondary to primary chloride rearrangements which do not go to completion involve primary initial radicals rearranging to secondary radicals (Figure 5). This is in line with the expected stabilities of the radicals, Le., tertiary > secondary > primary. Since it is known that a-halogens stabilize radicals in the order I > Br > C1> F and that the heavier halogens stabilize radicals better than carbon which is much better than hydrogen, we decided to look at the rearrangements occurring during the fluorination of alkyl dichlorides, trichlorides, and tetrachlorides. The results are entirely consistent with the order of stability summarized in Figures 4-6. The effects of 0-halogens on radicals is less clear; it is clear that 0-halogen effects are much smaller in magnitude than a effects. The crucial assumptions implicit in the above interpretation of the experimentally observed results is that rearrangement is much faster than fluorine attack on the intermediate radicals, that the mixture produced is related to the statistical probability of attack, and that any equilibrium between intermediate radicals has been
214
Ind. Eng. Chem. Res. Vol. 26, No. 2, 1987
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