Direct Fluorination of Hydrocarbons

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... 

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... 

Full details have been published of the direct fluorination of hydrocarbon polymers by the so-called LaMar procedure, - the principal feature of which is infinite dilution initially with helium or nitrogen followed by gradient changes of fluorine concentration with substrate particie sizes greater than 100 mesh a hydrocarbon core is retained, and large fabricated items such as polyethylene bottles can... 

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. 

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 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 ... 

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. 

Fluorination of Alkanes. Fluorination of alkanes is extremely difficult to control. The reaction usually results in substantial C—C bond rupture and can readily lead to explosion.136 However, several methods for controlled direct radical fluorination of hydrocarbons have been developed. The key and obvious observation was that the only reaction sufficiently exothermic to cause fragmentation is the termination step between a carbon radical and a fluorine atom. Consequently, if the atomic fluorine population and the mobility of hydrocarbon radicals are minimized, controlled fluorination becomes feasible. 

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. 

Scheme 2.1 Gas-phase perfluorination of a variety of hydrocarbons by the LaMar process. At the top the proposed mechanism of free radical direct fluorination of alkanes is shown [8].

Recent computational studies [23[ on the structure and charge distribution of hydrogen bonded F2 suggest a more differentiated view on the supposedly electrophilic mechanism of direct fluorination of aliphatic and aromatic hydrocarbons. Ab-initio calculations indicate that even for the complex of Fj with the extremely strong hydrogen-bond donor F3F as a model system, the energy of complex formation is very low - only 0.38 kcal mol (MP2/6-31+G //MP2/6-31+G level of theory, ZPE and BSSE correction) [24] (Scheme 2.9). Because of the low polarizabil-... 

Direct fluorination of zeolites has been reported as a method for increasing their acidity for hydrocarbon cracking reactions. Lok et al. treated various zeolites with fluorine gas and reported zeolite dealumination and stabilisation and in certain cases an increase in n-butane cracking activity.20 It appears that the optimum fluorine content for maximum cracking activity is about 1% m/m. 

Tfaditionally, elemental fluorine and hydrogen fluoride were used as flu-orinating agents. However, it was soon evident that fluorine was extremely reactive and too unselective to be synthetically useful. Over the years, many other fluorinating agents were developed, making the fluorination step a safe and reliable process. Through modifications of techniques, direct fluorination of the simplest hydrocarbons is, in some instances, possible. 

Even electrophilic fluorination of alkanes has been reported. F2 and fluoroxytrifluoro-methane have been used to fluorinate tertiary centers in steroids and adamantanes by Barton and coworkers ". The electrophilic nature of a reaction involving polarized but not cationic fluorine species has been invoked. Gal and Rozen have carried out direct electrophilic fluorination of hydrocarbons in the presence of chloroform. F2 appears to be strongly polarized in chloroform (hydrogen bonding with the acidic proton of CHCI3). However, so far no positively charged fluorine species (fluoronium ion) is known in solution chemistry. 

Potentially, this direct fluorination process is a new approach to the synthesis of fluorocarbon polymers. Polyethylene, polypropylene, polystyrene, polyacrylonitrile, polyacrylamide, resor phenol formaldehyde resin, and ethylene propylene copolymer have been fluorinated to produce perf1uorocarbon polymers which are structurally similar to the hydrocarbon starting materials and have physical properties similar to known structurally related fluorocarbon polymers obtained by polymerization of fluorocarbon monomers. High yield of fluorocarbon polymers approaching 100 have been obtained. This direct technique used for fluorination of hydrocarbons and polymers is called the LaMar process and has been previously described in connection with the direct fluorination of Lt ver molecular weight species ". ... 

The Direct Fluorination of Polyacrylonitrile. The finely ground polyacrylonitrile powder was placed in the boat. The initial flow rate was 60 cc/min helium and 1 cc/min fluorine for 6 hours followed by termination of the helium flow and a 2 cc/min fluorine flow for 30 hours. Polyacrylonitrile was more difficult to fluori-nate than the three previous hydrocarbon polymers but no visible decomposition occurred and the yield was approximately 95%. The product was a white powder which melted at 240°C and boiled at 250°C to 26o "c. 

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. 

---