Hydrocarbons, direct fluorination

The nonbonding electron clouds of the attached fluorine atoms tend to repel the oncoming fluorine molecules as they approach the carbon skeleton. This reduces the number of effective coUisions, making it possible to increase the total number of coUisions and stiU not accelerate the reaction rate as the reaction proceeds toward completion. This protective sheath of fluorine atoms provides the inertness of Teflon and other fluorocarbons. It also explains the fact that greater success in direct fluorination processes has been reported when the hydrocarbon to be fluorinated had already been partiaUy fluorinated by some other process or was prechlorinated, ie, the protective sheath of halogens reduced the number of reactive coUisions and aUowed reactions to occur without excessive cleavage of carbon—carbon bonds or mnaway exothermic processes. 

Simple and Complex Organic Molecules. Using modem direct fluorination technology, the synthesis of even the most complex perfluorocarbon stmctures from hydrocarbon precursors is now possible. For example, syntheses of the first perfluoro crown ethers, perfluoro 18-crown-6, perfluoro 15-crown-5, and perfluoro 12-crown-4 (54) have been reported. Perfluoro crown ethers (54,55) are becoming important as the molecules of choice for many F-nmr imaging appHcations (56) in humans and are particularly effective in brain and spinal diagnostics when... 

Hydrocarbon Polymers. It is difficult to produce perfluorocarbon polymers by the usual methods. Many monomers, such as hexafluoropropylene, polymerize only slowly because of the steric hindrance of fluorine. Furthermore, some monomers are not very stable and are difficult to synthesize. Direct fluorination can be used for the direct synthesis of fluorocarbon polymers (68—70) and for producing fluorocarbon coatings on the surfaces of hydrocarbon polymers (8,29,44—47,49,68—71). 

Direct fluorination involves the treatment of an appropriate hydrocarbon precursor dissolved in an inert Hquid with fluorine gas to yield a perfluorinated precursor to a long-chain carboxyflc acid. Equations 2 and 3 illustrate the process for perfluorooctadecanoic acid (17). 

Fluorinated ether-containing dicarboxyhc acids have been prepared by direct fluorination of the corresponding hydrocarbon (17), photooxidation of tetrafluoroethylene, or by fluoride ion-cataly2ed reaction of a diacid fluoride such as oxalyl or tetrafluorosuccinyl fluorides with hexafluoropropylene oxide (46,47). Equation 8 shows the reaction of oxalyl fluoride with HEPO. A difunctional ether-containing acid fluoride derived from HEPO contains regular repeat units of perfluoroisopropoxy group and is terminated by two alpha-branched carboxylates. 

Fluorides. Tantalum pentafluoride [7783-71-3] TaF, (mp = 96.8° C, bp = 229.5° C) is used in petrochemistry as an isomerization and alkalation catalyst. In addition, the fluoride can be utilized as a fluorination catalyst for the production of fluorinated hydrocarbons. The pentafluoride is produced by the direct fluorination of tantalum metal or by reacting anhydrous hydrogen fluoride with the corresponding pentoxide or oxychloride in the presence of a suitable dehydrating agent (71). The ability of TaF to act as a fluoride ion acceptor in anhydrous HF has been used in the preparation of salts of the AsH, H S, and PH ions (72). The oxyfluorides TaOF [20263-47-2] and Ta02F [13597-27-8] do not find any industrial appHcation. 

Peduoropolyethers, which constitute special class of fluoropolymer, are useful as lubricants,1 elastomers,2 and heat-transfer fluids under demanding conditions. Several commerical products are available, which are generally prepared by ring-opening polymerization of hexafluoropropylene oxide or by the random copolymerization oftetrafluoroethylene and hexafluoropropylene with oxygen under ultraviolet irradiation.3 Direct fluorination of hydrocarbon ethers has been reported4 but must be done very slowly under carefully controlled... 

The direct fluorination of inorganic,1,2 organometallic,3 5 and organic compounds,6-8 employing the LaMar9,10 and Exfluor-Lagow" methods, has impacted the synthesis of fluorinated compounds over the past 25 years. Among the most important applications of direct fluorination are the synthesis of fluoropolymers from hydrocarbon polymers and the conversion of the surface of the hydrocarbon polymers to fluoropolymer surfaces.12,13 The direct fluorination process is an excellent approach to the synthesis of fluoropolymers. 

The Lagow group first entered the perfluoropolyethers field in 1977, by reacting fluorine with inexpensive hydrocarbon polyethers to prepare perfluoropolyethers. In the simplest case (Figure 14.3) poly (ethylene oxide) is converted to perfluoroethylene oxide polymer, a simple reaction chemistry that we first reported in the literature.27 As will be seen later, this direct fluorination technology as well as many new patents from Exfluor Research Corporation have been non-exclusively licensed to the 3M Corporation by the Lagow research group.3 "39... 

We discovered another synthetic technique that involves the conversion by direct fluorination of hydrocarbon polyesters to perfluoropolyesters followed by treatment with sulfur tetrafluoride to produce new perfluoropolyethers.42 The first paper in this area ofreasearch reported that conversion of poly(2,2-dimethyl-1,3-propylene succinate) and poly( 1,4-butylene adipate) by using the direct fluorination to produce novel branched and linear perfluoropolyethers, respectively. The structures are shown in Figure 14.6. The second paper concerns the application of the direct fluorination technology base directed toward oligomers, diacids, diesters, and surfactants.43... 

The principal laws for the fluorination of polymeric hydrocarbons are the same as those described above for the simple case. Direct fluorination has been used extensively in organic chemistry (but only since the early 1970s) in low-temperature methods, where the fluorine is strongly diluted with some inert gas (helium, argon, nitrogen, krypton). One can note the La Mar, aerosol-based, and liquid-phase fluorination methods. 

The violent nature ofreactions between fluorine and hydrocarbon compounds has already been noted here, and the direct fluorination of organic polymers is not a exception it is so exothermic that if the reaction is not controlled, it generally leads to fragmentation and charring of the substrate. Moderation of the reaction rate can be effected by ... 

Surface fluorination changes the polymer surface drastically, the most commercially significant use of polymer surface direct fluorination is the creation of barriers against hydrocarbon permeation. The effectiveness of such barriers is enormous, with reductions in permeation rates of two orders of magnitude. Applications that exploit the enhanced barrier properties of surface-fluorinated polymers include (1) Polymer containers, e.g., gas tanks in cars and trucks, which are produced mostly from high-density polyethylene, where surface fluorination is used to decrease the permeation of fuel to the atmosphere and perfume bottles. (2) Polymeric membranes, to improve selectivity commercial production of surface-fluorinated membranes has already started.13... 

The preparation of perfluorinated compounds is largely based on the exhaustive fluorination of the corresponding hydrocarbon species and three synthetic procedures have been widely used. Two of these processes, electrochemical fluorination [27] (ECF), successfully used for the preparation of perfluoroacids (3M), and fluorination by high valent metal fluorides [28] such as cobalt trifluoride (itself prepared from cobalt difluoride and fluorine), used for the preparation of perfluorocarbons (Flutec fluids, BNFL), have been reviewed elsewhere. The third major process for the preparation of perfluorinated compounds involves direct fluorination. 

Many saturated linear, branched [47], cyclic [47, 48] and cage [49, 50] hydrocarbons have been transformed into the corresponding perfluorocarbons by direct fluorination (Figs. 5 and 6). 

There arc a number of other factors that become very important to consider aside from the simple differences in bond strength. The earliest approach to direct fluorination of solid hydrocarbons operated on the principle of very gradual addition of fluorine over a period of time stretching in length from 4 hours to several days as seen in Figure 1. The thermodynamic strategy, the kinetic strategy and the Lamar Process dilution make it much easier to produce a myriad of new fluorocarbon materials which were not accessible by any other fluorination technique. 

In the next few years it will become clear that traditional problems with the physical properties, such as strength and durability of polymers produced by direct fluorination, are being solved largely by specific design of precursor hydrocarbon polymer morphology for the fluorination process. Polymers produced by direct fluorination have previously been satisfactory or outstanding with regard to their thermal and chemical properties. Stimulated by new efforts this field will expand rapidly. 

Fluorination of normal hydrocarbons is not difficult with the La-Mar fluorination process however, fluorination studies also have been successful with structurally unusual hydrocarbon compounds (65-67a) (see Fig. 13). These studies were undertaken to establish that direct fluorination was useful even with some of the most sensitive hydrocarbon structures. While the initial studies of the successful direct fluorination of these species often resulted in yields as low as 10%, these same experiments, after additional technical developments in our laboratory, routinely give yields of 70—95%. Such syntheses have often been repeated in our laboratories to satisfy scientific needs for such compounds in other laboratories. The sterically crowded fluorocarbon compounds prepared in... 

Another series of compounds, the fluorine-substituted 2,2,4,4-tetra-methylpentanes, have proven to be a good artificial blood candidate (69). The compounds are prepared by the direct fluorination of the hydrocarbon precursor (70a) (see Fig. 15). The overall yield of the reaction can be seen from the figure to be in excess of 99%. The central protons are retained preferentially during the fluorination, a result that is attributed to steric rather than electronic factors (70a). 

CF3)3CF, for which cobalt trifluoride and electrochemical-fluorination methods have not been successful, a yield of over 50% has been obtained. The direct fluorination of ethyl acetate (75), shown in Fig. 17, was the first example of conversion of a hydrocarbon ester to a perfluorocarbon ester by any fluorination technique. 

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