Fluorinated solvents hexafluorobenzene

An alternative hexafluorobenzene process features exchange fluorination (KF) of hexachlorobenzene in the presence of polar solvents (226,249) or under solvent-free conditions (450—540°C, autoclave) (250). Intermediates such as chloropentafluorobenzene can be further fluorinated to hexafluorobenzene (42—51% yield) by cesium fluoride in sulfolane (226,249). 

The choice of solvent may have a critical impact on efficiency too. In metathesis, dichloromethane, 1,2-dichloroethane and toluene are the solvents most commonly used. There are examples that show much higher yields in ring closing metathesis (RCM) when using fluorinated solvents [150]. An impressive effect of hexafluorobenzene as a solvent for CM is the modification of the steroid 93 the use of 1,2-dichloroethane leads to a very low yield and significant amounts of dimerisa-tion while the same reaction proceeds in 90% yield in C F (Scheme 3.14) [151]. 

Interestingly, in fluorinated solvents where the solubility of the reagents is not compromised, for example hexafluorobenzene, the rate of reaction is not increased (Figure 7.2). This implies that there is an inverse relationship between solubility of reagents and the rate of reaction, and that this relationship may be exploited by the use of either water or suitable perfluorinated solvents. 

The physical properties of these perfluoro polymers are summarized in Table 16.2. These perfluoropolymers are completely amorphous and contain no hydrogen atoms, show excellent chemical and thermal stability, and are soluble in fluorinated solvents such as hexafluorobenzene (HFB) and perfluorohexane. The dielectric constants of these fluoropolymers are considerably low, and they are almost unaffected by humidity. 

Addition, substitution, and telomerization can occur when solutions or suspensions of chlorinated aromatic substrates in halogenoalkane solvents are subjected to direct low-temperature (<25 C) fluorination, and fluoro-aromatics may be obtained by dehydrohalogenation of appropriate products ie.g., see Elcheme 1). Fluorination of hexafluorobenzene in 1,1,2-trichlorotri-... 

The para-fluorine atoms on highly fluorinated aromatic compounds such as hexafluorobenzene or decafluorobiphenyl are activated and thus can go through aromatic nucleophilic substitution with bisphenols in an aprotic solvent at low temperatures (<80°C).121 123... 

A mechanism in which ketyl radicals play a role was also postulated for the formation of cyclohexylpentafluorobenzene upon irradiation of a cyclohexane solution of hexafluo-robenzene in the presence of benzophenone756. In this case, however, the excited benzophenone abstracts a hydrogen atom from the solvent and the cyclohexyl radical attacks hexafluorobenzene. The resulting radical is transformed further into the substitution product by loss of fluorine and into an addition product by abstraction of a hydrogen atom. Irradiation of a cyclohexane solution of pentafluoropyridine in the presence of benzophenone resulted in the formation of 4-cyclohexyltetrafluoropyridine, while no addition product was observed. 

The activity coefficients of benzene in higher normal hydrocarbons at 323 K vary from 0.993 in hexadecane to 0.644 in dotriacontane [65] and those of fluorobenzenes rise with increasing numbers of fluorine atoms in n-octadecane at 323 K as solvent it is 1.130 for fluorobenzene while for hexafluorobenzene it is 1.945 [66]. 

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