Fluorination, direct thermodynamics

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. 

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. 

The basic problem of direct fluorination involves both kinetics and thermodynamics. The rate of the reaction must be slowed down so that the energy liberated from the reaction may be absorbed or carried away. The most significant and crucial innovations in the evolution of the direct fluorination process recently developed by Lagow and Margrave (6) have been kinetic considerations. Their technique has been named the La-Mar direct fluorination process (7). Most of the kinetic considerations involve fluorine dilution schemes and probability considerations. 

It should also be noted that while an increase in the number of fluorine substituents leaves the rate of reaction of the cation with water unaffected, the reverse reaction is profoundly affected. In the latter direction the full equilibrium effect of the substituent is felt on the rate. This is because now the effects of changes in thermodynamic driving force and intrinsic barrier complement each other. 

For exchange of tertiary alcohols direct fluorination without activation can be achieved by use of aHF or other sources of acidic fluoride (Scheme 2.61). This type of reaction in an acidic medium proceeds via a stabilized carbocation by an Sjjl mechanism. Fluoride addition is often reversible, and the stereochemistry of the reaction is controlled thermodynamically only by the relative free enthalpies of the possible product isomers. 

Difluorine combines directly with all elements except O2, N2 and the lighter noble gases reactions tend to be very violent. Combustion in compressed F2 (fluorine bomb calorimetry) is a suitable method for determining values of Af77° for many binary metal fluorides. However, many metals are passivated by the formation of a layer of nonvolatile metal fluoride. Silica is thermodynamically unstable with respect to reaction 16.5, but, unless the Si02 is powdered, the reaction is slow provided that HF is absent the latter sets up the chain reaction 16.6. 

For the tetravalent cations of the first transition series only five MF anions are known. These are the complexes VFe, CrFe, MnFe, CoFe, and NiFe" , corresponding to the configurations d, d, d, d, and With the solitary exception of MnFe , which has been known since 1899 81), aU these salts have been prepared only within the leist twenty years or so, by direct fluorination techniques, and their extreme reactivity has severely hampered spectroscopic studies. Recently however, some spectroscopic data has become available, mostly by reflectance techniques, for all of these anions, but repeated attempts, in the authors laboratory and elsewhere, to prepare the elusive d FeFe ion have been unsuccessful, possibly because it may simply be thermodynamically unstable with respect to the high-spin d FeFe ion. 

It is felt that the analytical method presented here is distinctly superior to any of the empirical methods for estimating gas consumption, such as the so-called "saturation rule." Application of the analytical method, however, is somewhat more difficult. The method should be suitable for all tank systems that use cryogens (such as hydrogen, oxygen, nitrogen, fluorine, carbon monoxide and neon) and that are equipped with a gas diffuser to prevent direct jetting of the gas into the liquid. In addition, the method is set up so that large variations in system operation may be incorporated by a person reasonably well versed in the principles of thermodynamics and heat transfer. 

Determination of the basic thermodynamic properties of the rare earth trifluorides remains incomplete. This is in part due to experimental difficulties, and only one direct calorimetric measurement has been reported. The enthalpy of formation, AH 29s, of YF3 (-410.7 0.8 kcal/mole) has been measured by fluorine bomb calorimetry (Rudzitis et al., 1965). In an expansion of earlier work, Polyachenok (1967) has obtained values for several trifluorides (La, Pr, Nd, Gd, Er) by an equilibrium exchange reaction RCbi -f-A1F3( ->RF3(0 +AICl3(g). Solid state emf data have been reported by Skelton and Patterson (1973) for the trifluorides of Nd, Gd, Dy and Er. Similar measurements have been described by Rezukhina et al. (1974) for the trifluorides of La, Pr and Y. The AHt-m values are 5-10 kcal/mole more negative than values reported earlier. 

The current, critical evaluation of the ideal gas thermodynamic data on the f1uoromethanes (8) and f1uoroethanes 9) will be particularly appropriate to the reduction of C-H bond dissociation energy data. Additional data on the enthalpy of formation of fluorine containing compounds will be recalculated (if necessary) to reflect the recent, direct determination of AH2(HF-nH20, 298) of Johnson, Smith and Hubbard (10). The data for the reference states, C(graphite), H2(g) and p2(g) are the same as those used by Rodgers et al. (8). 

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