Fluorinated species, thermochemical

The more correct statement is that while the early thermochemical literature cites a considerable number of enthalpies of combustion for fluorinated species [see, e.g., M. S. Kharasch, Bur. Stand. J. Res., 2, 359 (1929)], it is not obvious how to transform these data into the desired enthalpies of formation. (Difficulties in organofluorine calorimetry are discussed in section VII.)... 

We close this section with a brief discussion of the sole example of thermochemically comparable hexasubstituted species, hexafluorobenzene (48, X = F) and hexafluorocyclo-propane (49, X = F). Complications of vicinal and geminal fluorine substitution are inescapable85 although we are spared ambiguities of substitution pattern and of phase. The difference of gas-phase enthalpies of formation is 25 kJ mol 1 we remember the 29 kJ mol 1 for the parent, unsubstituted, hydrocarbons. This agreement is too good to be true86. 

This statement is not quite correct—in Reference 14, perfluorocyclohexane was used as a reference species for the thermochemical understanding of perfluorocyclopropane and perfluorocyclobutane. Enigmatically there are no enthalpy-of-formation data on perfluorinated (or even partially fluorinated) derivatives of any other cycloalkane. 

Taube and Bray noted in 1940 that the standard potential for the F(g)/F couple is 4.04 V (310). Berdnikov and Bazhin obtained a solution-phase potential by use of a thermochemical cycle that involves estimating the free energy of hydration of the fluorine atom (45) this led to a calculated E° of 3.6 V for the F/F couple and a corresponding AfG° of 68 kJ/mol for F. The chemistry of F2 and HOF has been discussed in Thompson s review (315), but there is no evidence of fluorine-containing radicals in the reactions of these species. There does not seem to be any reported evidence for the existence of F2 in aqueous solution, although the species has been detected in irradiated crystals. In the reversible reaction of H with F to give e q and HF, it has been argued that HF" does not exist as an intermediate but only as a transition state (16). 

By contrast, perf luoro/ir-f luoro effect reasoning correctly suggests B-fluorination of ethyl radical will increase the ionization potential because the species is nonplanar. For perfluoroethyl radical, the two fluorination effects on ionization potentials run counter to each other. Experiment does not help us decide which effect dominates because the heat of formation of C2F5+ is known no better than 15 kcal mol"1. That is, from appearance potentials of this ion from C2F5Z with Z = F, I, CF3 and C2F5 we obtain ionization potentials of 228, 218, 208 and 199 kcal mol"1 (ca. 9.88, 9.45, 9.02 and 8 63 eV). This is clearly too wide of a spread of values to be of thermochemical use to derive meaningful substituent effects. 

Figure 6 shows the uncorrected thermochemical excitation distributions corresponding to energetic substitution reactions at the fluorinated alpha carbon atom positions in CH3CHF2 and CH3CF3. From the left the respective fractional yield segments include the undecomposed primary species and the decomposition products from HF-eliminations, from secondary carbon-carbon scissions to alkyl radicals, and from tertiary radical dissociations. Following these alpha substitution processes product decomposition occurs principally via HF-elimination. 

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