Hydrogen fluoride, as solvent

Acetone can be converted into 2,2-difluoropropane by reaction with bromotrifluoromethane using hydrogen fluoride as solvent (yield 93%). ° Other ketones suffer fragmentation or polymerization reactions. 

The Prins reaction (electrophilic a-hydroxyalkylation of alkenes) is practicable with hydrogen fluoride as solvent and offers a possibility for the synthesis of fluorinated alcohols. 

A number of reactions using anhydrous hydrogen fluoride as solvent have been formulated as involving electrophilic addition of X+F. Hexafluoropropene is uiu eactive towards anhydrous hydrogen fluoride, even at 200° C, but silver fluoride in anhydrous hydrogen fluoride reacts at 125° C and it has been suggested, therefore, that an initial electrophilic addition of silver fluoride occurs [156] (Figure 7.48). 

An effective route to a number of bis(perfluoroalkyl)- and bis(perfluorocycloalkyl)-mercurials involves fluoride-ion-induced reactions of fluoroalkenes [31] this follows an earlier method involving addition of mercury(ll) fluoride to fluoroalkenes, e.g. using anhydrous hydrogen fluoride as solvent [62] (Figure 10.18). 

The reaction of HCN with 2,4,4-trimethylpent-2-ene in hydrogen fluoride as solvent gives t-octylaminocyanoketen-N-t-octylimine (37) as the major product (62%) together with a small quantity of N-t-octylamino-malononitrile (38) and 4-N-t-octylamino-5-cyanoimidazole (39) [116]. 

Trifluoromethyl hypofluorite displays continuous absorption of light below 370 nm, the primary photolytic process being CFj-OF CFa-O- + F. Thus, in first approximation, photofluorination, as above, is considered as a radical chain reaction with either F- or CFj-O- as chain carriers. " The selectivities observed in liquid hydrogen fluoride as solvent [in addition to the example shown above, 1-aminoadamantane (antiviral drug Symmetrel) yields a mixture of (69) and (70), and a high proportion of 8-fluorination occurs with L-isoleucine] presumably arise through protonation of the substrate. "... 

In the case of 2,3,5-tri-O-benzoyl-D-ribofuranose (27) pyridinium poly(hydrogen fluoride) was added to a solution of in anhydrous dichloromethane and the solution was shaken at room temperature for 10 h in an atmosphere of dry argon. Anhydrous acetone was found to be equally effective in most reactions. The reaction of compound with Olah s reagent required the use of c.nhydrous acetone or anhydrous dichloromethane—collidine [1 1 (v/v)] in the case of compound 30 the addition of collidine was disadvantageous, whereas in the case of compound best results were obtained using anhydrous acetone—collidine [1 1 (v/v)] as the solvent. Compounds 32 and 33 were treated using pyridinium poly(hydrogen fluoride) as the only solvent. Reaction times varied from 2 h for compound 31 to more than 12 h for 32 and 33. 

The electrofluorination of acetophenone and benzophenone takes place in anhydrous HF and in the presence of solvents such as chloroform and acetonitrile [38]. The fluorination of the aromatic rings occurred to various extent. Further uses of anhydrous hydrogen fluoride as a liquid environment for electrofluorination processes have been reported, for example, by Matalin etal. [39]. In particular, systems with low conductivity in liquid hydrogen fluoride and nonselective processes have been studied and optimized. The fluorination of benzene and halobenzenes in the presence of Et4NF—(HF) in an undivided cell has been studied by Horio et al. [40] Cathodic dehalogenation is observed to accompany the anodic fluorination process. 

Fluoroolefins add mercuric fluoride at 50- 100 °C to give bis(polyfluoroalkyl)-mercury derivatives. Reaction is usually carried out in hydrogen fluoride as a solvent. This process was reviewed and the electrophilic nature of this process established [6]. Addition to olefin starts by the attack of cation 3 (Eq. 5) on the double bond of fluoroolefin. 

Katz, J. J. Anhydrous hydrogen fluoride as a solvent for proteins and other biologically important substances. Arch. Biochem. Biophys. 51, 293 (1954) Anhydrous trifluoroacetic acid as a solvent for proteins. Nature 174, 509 (1954) A. L. Koch, W. A. Lamont and J. J. Katz The effect of anhydrous strong acids on ribonuclease and lysozyme. Arch. Biochem. Biophys. 63, 106 (1956). 

Although no attempt will be made to describe the chemistry of all of the nonaqueous solvents listed in Table 5.4, the survey to this point has included ammonia as a basic solvent and liquid hydrogen fluoride as an acidic solvent. Another solvent that has been extensively utilized in both inorganic and organic chemistry is sulfur dioxide. Accordingly, we will give a brief survey of the chemistry of liquid sulfur dioxide for which the physical properties are presented in Table 5.8. 

Good yields can be achieved by using anhydrous hydrogen fluoride as the solvent. Thus, the fluorination of acetone with sulfur tetrafluoride in hydrogen fluoride gives 2,2-difluoropropane in ON" > yield 14 without hydrogen fluoride, only a 60% yield is obtained. 

Our thinking in terms of acid strength is usually confined to aqueous solutions where we have considerable quantitative data, but in strongly basic or strongly acid nonaqueous solvents differences in acid or basic strength of weak acids or bases will be marked. As a specific example, let us consider the interaction of the methylbenzenes with the solvent anhydrous hydrogen fluoride. As this solvent has a high dielectric constant, the equilibrium constant for the reaction... 

Xenon dtfluoride dissolves in the halogen fluorides BrFa, BrFs, and IF5 and in AHF (anhydrous hydrogen fluoride) as monomolecular species geometrically similar to the gas-phase species. Xep2 sometimes forms complexes with the solvents (e.g. Brp3 and IF5). Xep2 is also soluble in MeCN and other organic solvents. 

In the case of cysteine (5) the product obtained is contaminated by cystine and also by thiazolidine carboxylic acid (obtained by reaction of cysteine and formaldehyde, the latter arising from hydrolytic decomposition of acetamidomethanol, see section VI). However, the product could easily be purified by using ion exchange columns. On the other hand, anhydrous conditions should avoid the decomposition of the acetamidomethanol and indeed a reaction using hydrogen fluoride as a solvent results in quantitative yield of the pure product". 

Fluorocomplexes may be prepared in various covalent fluorides as solvents, such as liquid hydrogen fluoride, bromine(III) fluoride, chlorine(III) fluoride, iodine(V) fluoride, arsenic(III) fluoride or even the pentafluorides of vanadium, niobium and tantalum. All of them seem to be associated in the liquid states due to fluorine-bonding and splitting of such bonds may lead to fluoride ion transfer processes between solvent molecules (autofluoridolysis) . 

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