Direct Fluorination Reactions

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. 

It is clear that many new fluoropolymers can be synthesized by direct fluorination technology that cannot be obtained through other routes. The information in this chapter should serve as a strong indication that perhaps the best and ultimate synthetic method for fluoropolymers on both laboratory and manufacturing scales in the future will be direct fluorination reactions. 

Electrochemical fluorination of dialkyl sulphides leads to sulphur(VI) derivatives (see Chapter 2, Section III), and oxidation of sulphur also occurs [246] in cobalt fluoride or direct fluorination reactions (Figure 8.89). 

The theoretical maximum temperature of the H2S-F2 flame has been calculated as 3469 this was confirmed by measuring the rotational temperatures of the 1-0 and 3-0 bands of HF. The interaction of fluorine and graphite powder has been studied in the temperature range —78 to +200 C at pressures up to 160 mmHg. Watanabe and co-workers demonstrated that the reaction is irreversible however, they found that the reaction product would oxidize aqueous KI solution. A room-temperature solid-state direct fluorination reaction has been reported to yield 85% conversions of m-carbaborane, m-BioHn,C2H2, after a reaction time of 10 days. Evidence was also presented to show that decafluoro-m-carbaborane, m-C2H2BioFio, was an intermediate in the reaction. Kennedy and Cady have described their systematic investigation of the influence of metal fluorides on the outcome of the two main reactions between fluorine and carbonyl fluoride ... 

N. de Mas, et cd.. Scalable microfabricated reactors for direct fluorination reactions. In Proceedings of IEEE Transducers, Boston, 2003. 

The directed fluorination reaction of diethyl malonate can be performed in a three-channel microreactor (Scheme 8.1) [8]. The synthesis offluoro- and difluoromalonate esters by the fluorination of Meldrum s acid is readily achieved in a microflow reactor without the use of added catalysts or malonate salts as substrates that are required in a conventional macrobatch reaction. 

Direct fluorination reactions of para-disubstituted aromatic systems bearing an electron-withdrawing and -releasing group using a multichannel microreactor can proceed in either acetonitrile or formic acid reaction media to give a high selectivity and yield of monofluorinated products (Table 8.4) [10]. 

N. de Mas A. Gunther, M.A. Schmidt, K.F. Jensen, Scalable microfabricated multiphase reactors for direct fluorination reactions, TRANSD UCERS, Solid-State Sensors, Actuators... 

Scheme 9 Synthesis of triazolotriazin-7(4H)-ones 10 3.2.2 Direct Fluorination Reactions...

A rare example of the incorporation of a fluorine atom into azaaromatic compounds is the direct fluorination reaction of 6-azauracyl 13a and 2-(2,3,5-tri-0-acetyl-p-D-ribofuranozyl-l,2,4-triazin)-3,5(2H,4H)-dione 13b which takes place on passing of fluorine through a solution of azauracils 13a,b in acetic acid, thus giving 6-fluoro-l,2,4-triazin-3,5(2H,4H)-diones 14 in 20-55 % yields (Scheme 10) [17,18]. 

A complete set of trihalides for arsenic, antimony and bismuth can be prepared by the direct combination of the elements although other methods of preparation can sometimes be used. The vigour of the direct combination reaction for a given metal decreases from fluorine to iodine (except in the case of bismuth which does not react readily with fluorine) and for a given halogen, from arsenic to bismuth. 

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. 

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. 

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). 

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. 

Titanium Tetrafluoride. Titanium tetrafluoride [7783-63-3] is a white hygroscopic soHd, density 2798 kg/m, that sublimes at 284°C. The properties suggest that it is a fluorine-bridged polymer in which the titanium is six-coordinate. The preferred method of preparation is by direct fluorination of titanium sponge at 200°C in a flow system. At this temperature, the product is sufficiently volatile that it does not protect the unreacted sponge and the reaction proceeds to completion. The reaction of titanium tetrachloride with cooled, anhydrous, Hquid hydrogen fluoride may be used if pure hydrogen fluoride is available. 

Table 1. Direct Fluorination of Organic Compounds Using Batch Reaction Techniques...

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