Fluorination, direct

Cobalt trifluoride is readily prepared by reaction of fluorine (qv) and C0CI2 at 250°C or C0F2 at 150—180°C. Direct fluorination of C0F2 leads to quantitative yields of 99.9% pure CoF (4). 

Manufacture and Economics. Nitrogen tritiuoride can be formed from a wide variety of chemical reactions. Only two processes have been technically and economically feasible for large-scale production the electrolysis of molten ammonium acid fluoride and the direct fluorination of the ammonia in the presence of molten ammonium fluoride. In the electrolytic process, NF is produced at the anode and H2 is produced at the cathode. In a divided cell of 4 kA having nickel anodes, extensive dilution of the gas streams with N2 was used to prevent explosive reactions between NF and H2 (17). 

Replacement of Hydrogen. Three methods of substitution of a hydrogen atom by fluorine are (/) reaction of a G—H bond with elemental fluorine (direct fluorination, (2) reaction of a G—H bond with a high valence state metal fluoride like Agp2 or GoF, and (J) electrochemical fluorination in which the reaction occurs at the anode of a cell containing a source of fluoride, usually HF. 

The fluorination reaction is best described as a radical-chain process involving fluorine atoms (19) and hydrogen abstraction as the initiation step. If the molecule contains unsaturation, addition of fluorine also takes place (17). Gomplete fluorination of complex molecules can be conducted using this method (see Fluorine compounds, organic-direct fluorination). 

The principal advantage to this method is that the heat evolved for each carbon—fluorine bond formed, 192.5 kj/mol (46 kcal/mol), is much less than that obtained in direct fluorination, 435.3 kJ/mol (104 kcal/mol). The reaction yields are therefore much higher and less carbon—carbon bond scisson occurs. Only two metal fluorides are of practical use, Agp2 and GoF. ... 

Kinetic as weU as thermodynamic problems are encountered in fluorination. The rate of reaction must be decelerated so that the energy Hberated may be absorbed or carried away without degrading the molecular stmcture. The most recent advances in direct fluorination ate the LaMar process (18—20) and the Exfluot process (21—24), which is practiced commercially by 3M. 

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. 

Thus, for a successful fluorination process involving elemental fluorine, the number of coUisions must be drasticaUy reduced in the initial stages the rate of fluorination must be slow enough to aUow relaxation processes to occur and a heat sink must be provided to remove the reaction heat. Most direct fluorination reactions with organic compounds are performed at or near room temperature unless reaction rates are so fast that excessive fragmentation, charring, or decomposition occurs and a much lower temperature is desirable. 

Aerosol-Based Direct Fluorination. A technology that works on Hter and half-Hter quantities has been introduced (40—42). This new aerosol technique, which functions on principles similar to LaMar direct fluorination (Fig. 5), uses fine aerosol particle surfaces rather than copper filings to maintain a high surface area for direct fluorination. The aerosol direct fluorination technique has been shown to be effective for the synthesis of bicycHc perfluorocarbon such as perfluoroadamantane, perfluoroketones, perfluoroethers, and highly branched perfluorocarbons. 

In 1954 the surface fluorination of polyethylene sheets by using a soHd CO2 cooled heat sink was patented (44). Later patents covered the fluorination of PVC (45) and polyethylene bottles (46). Studies of surface fluorination of polymer films have been reported (47). The fluorination of polyethylene powder was described (48) as a fiery intense reaction, which was finally controlled by dilution with an inert gas at reduced pressures. Direct fluorination of polymers was achieved in 1970 (8,49). More recently, surface fluorinations of poly(vinyl fluoride), polycarbonates, polystyrene, and poly(methyl methacrylate), and the surface fluorination of containers have been described (50,51). Partially fluorinated poly(ethylene terephthalate) and polyamides such as nylon have excellent soil release properties as well as high wettabiUty (52,53). The most advanced direct fluorination technology in the area of single-compound synthesis and synthesis of high performance fluids is currently practiced by 3M Co. of St. Paul, Minnesota, and by Exfluor Research Corp. of Austin, Texas. 

The following companies manufacture organic fluorine compounds by direct fluorination techniques 3M Exfluor Air Products and Chemicals,... 

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). 

Many perfluoroaUphatic ethers and tertiary amines have been prepared by electrochemical fluorination (1 6), direct fluorination using elemental fluorine (7—9), or, in a few cases, by fluorination using cobalt trifluoride (10). Examples of lower molecular weight materials are shown in Table 1. In addition to these, there are three commercial classes of perfluoropolyethers prepared by anionic polymerization of hexafluoropropene oxide [428-59-1] (11,12), photooxidation of hexafluoropropene [116-15-4] or tetrafluoroethene [116-14-3] (13,14), or by anionic ring-opening polymeriza tion of tetrafluorooxetane [765-63-9] followed by direct fluorination (15). 

Direct Fluorination. This is a more recently developed method for the synthesis of perfluorinated compounds. In this process, fluorine gas is passed through a solution or suspension of the reactant in a nonreactive solvent such as trichlorotrifluoroethane (CFC-113). Sodium fluoride may also be present in the reaction medium to remove the coproduct hydrogen fluoride. There has been enormous interest in this area since the early 1980s resulting in numerous journal pubHcations and patents (7—9) (see Fluorine compounds, organic-direct fluorination). Direct fluorination is especially useful for the preparation of perfluoroethers. 

Multiple ether oxygen atoms can be present in the molecule. Cleavage and coupling reactions occur with direct fluorination although to a lesser extent than with ECF. This allows the direct fluorination of acid-sensitive materials, such as the formal shown below, which would not survive ECF (8). 

As opposed to ECF, direct fluorination affords a much lower degree of isomerization so that the carbon skeleton of the reactant remains intact in the perfluorinated product. Direct fluorination is also complementary to ECF in the significantly higher yields observed for the direct fluorination of ethers. As with ECF the products are purified by treatment with base and subsequent distillation. 

Perfluoropolyethers with the linear perfluoropropoxy repeat unit have been commercialized (28). They are prepared by the anionic oligomerization of tetrafluorooxetane followed by direct fluorination to remove the acyl fluoride end group as well as to fluorinate the remaining CH2 groups n can vary widely. 

Prepa.ra.tlon There are five methods for the preparation of long-chain perfluorinated carboxyUc acids and derivatives electrochemical fluorination, direct fluorination, telomerization of tetrafluoroethylene, oligomerization of hexafluoropropylene oxide, and photooxidation of tetrafluoroethylene and hexafluoropropylene. 

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). 

Unlike ECF, direct fluorination does not alter the carbon backbone preparation of isomerically pure acids is possible (18). Both direct fluorination and ECF permit a great variety of stmctures to be made, but each method is better at certain types of stmctures than the other. Ether acids are produced in good yields, by direct fluorination (17), while ECF of ether-containing acids is fair to poor depending on the substrate. Despite much industrial interest, the costs and hazards of handling fluorine gas have prevented commercial application of this process. 

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