Properties of Perfluorinated Solvents

Perfluorinated alkanes and cycloalkanes are prepared from the corresponding hydrocarbons, either by electrochemical fluorination or by cobalt trifluoride fluo-rination [3], Many perfluorinated solvents are available commercially covering a wide selection of boiling points and densities. Some examples of perfluorinated solvents are listed in Table 3.1 together with their key physical properties. 

Perfluorocarbons are chemically inert, are nonflammable and have low toxici-ties. Unlike the halofluorocarbons, perfluorocarbons are not ozone-depleting and have been used as CFC replacements [7], However, perfluorocarbons have very long lifetimes in the atmosphere ( 2000 years) [8] and this has consequences for their use as solvents since any process has to take into account the fact that any loss of solvent to the atmosphere is extremely undesirable. 

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Some of the disadvantages of the Stille reaction, e. g. the low reactivity of some substrates, separation difficulties in chromatography, and the toxicity of tin compounds, have been ameliorated by recent efforts to improve the procedure. Curran has, in a series of papers, reported the development of the concept of fluorous chemistry, in which the special solubility properties of perfluorinated or partly fluorinated reagents and solvents are put to good use [45]. In short, fluorinated solvents are well known for their insolubility in standard organic solvents or water. If a compound contains a sufficient number of fluorine atoms it will partition to the fluorous phase, if such a phase is present. An extraction procedure would thus give rise to a three-phase solution enabling ready separation of fluorinated from nonfluorinated compounds. 

Unsaturated fluoroethers form an extremely significant class of monomers which are used for improving the properties of perfluorinated polymers. Most of the dehalogenations have been carried out using zinc in protic or dipolar aprotic solvents (see Table 8). 

Physical and chemical properties of perfluorinated organic solvents... 

Another approach to isolate the catalyst from the products is the application of perfluorinated catalytic systems, dissolved in fluorinated media [63], which are not non-miscible with the products and some commonly used solvents for catalysis like THE or toluene at ambient temperature. Typical fluorinated media include perfluorinated alkanes, trialkylamines and dialkylethers. These systems are able to switch their solubility properties for organic and organometallic compounds based on changes of the solvation ability of the solvent by moving to higher temperatures. This behavior is similar to the above-mentioned thermomorphic multiphasic PEG-modified systems [65-67]. 

Recently, efforts have been initiated to examine intrinsic host-guest chemistry in the solvent-free environment of a mass spectrometer. Of present interest are preliminary reports on perfluorinated hosts, crown ethers and cryptands, which are physiologically compatible and may possess important biological properties, such as the ability to carry oxygen and transport ions through membranes. Specifically, the oxygen-binding properties of... 

The unique properties of highly fluorinated and perfluorinated ( fluorous ) solvents and reagents open several routes to a solution of these problems and to a sustainable green chemistry [1-5]. These properties include their very temperature-dependent miscibility with typical hydrocarbons, their non-toxicity, and their extreme chemical inertness. 

Although perfluorinated hydrocarbons are well known, they have only recently found application in organic chemistry as a useful class of solvents. Their physical properties mice them unique for their use as reaction media. Since perfluorocarbons are immiscible with many common organic solvents, they are suitable for the formation of biphasic systems. This overview describes the utilization of perfluorocarbons as reaction media for various kinds of reaction such as oxidation, bromination, etc. In addition, a novel biphase reaction system based on perfluorinated solvents as well as its application in organic synthesis is presented. 

Table 1. Physical properties of some typical perfluorinated solvents...

The technique of aqueous catalytic reactions has had such an impact on the field of more general two-phase reactions that scientists have now also proposed and tested other solutions. Fluorous systems (FBS, perfluorinated solvents cf. Section 7.2) and nonaqueous ionic liquids (NAILs, molten salts cf. Section 7.3) meet the demand for rapid separation of catalyst and product phases and, owing to the thermoreversibility of their phase behavior, have advantages in the homogeneous reaction and the heterogeneous separation. However, it is safe to predict that the specially tailored ligands necessary for these technologies will be too expensive for normal applications. Compared to the cheap and ubiquitous solvent water, with its unique combination of properties (cf. Table 1), other solvents may well remain of little importance, at least for industrial applications. Other ideas are mentioned in Section 7.6. 

The level of scientific work on supercritical COz (scC02) has increased and overlaps with the properties of media for FBSs [28]. However, new and specific process variants with disadvantageous properties arising from perfluorinated compounds are no longer introduced by the solvents (eg., scC02), but only by the ligands present in catalytic amounts. In these developments too, simultaneous catalyst regeneration with the aid of membrane separation is conceivable. 

In the previous section it was suggested that the parent polymer structure considerably influence the physical properties of the derived polysulfonates, imparting to them some of the mechanical and thermal properties of the precursors. This trend is particularly evident in the case of the perfluorinated hydrocarbon polymers. Polymers of this kind, such as e.g., poly(tetrafluoroethylene) (PTFE) are exceptional in their inertness to offensive environment, solvent resistance and high-temperature stability. These considerations led in the sixties to the development of unique sulfonic-acid derivatives of fluorocarbon copolymers by the DuPont Company. While several compositions were disclosed in the patent literature51, the preferred composition, which is the basis for the commercial Nafion ion-exchange membrane, is a copolymer of tetrafluoroethylene with a perfluorinated vinyl ether/sulfonyl fluoride52 ... 

All three commercial amorphous fluoropolymers. Teflon AF, Hyflon AD, and Cytop posses a unique set of properties. All dissolve in fluorinated solvents and thus may be spin coated to produce thin hlms and coatings. The polymers may also be extruded and molded using traditional polymer processing techniques. Note that the polymers are not soluble in hydrocarbon solvents or water and retain the chemical and thermal stability of perfluorinated polymers such as Teflon . These polymers have lower density than the well-known semicrystalline perfluorinated polymers such as pTFE that results in lower refractive index, lower thermal conductivity, higher gas permeability, and lower dielectric constant. The polymers are transparent and have excellent mechanical properties below their Tg due to their amorphous character. The presence of a heterocyclic ring in the polymer backbone of these materials is key... 

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