Fluorination with elemental fluorine

For example, direct fluorinations with elemental fluorine are kept imder control in this way, at very low conversion and by entrapping the molecules in a molecular-sieve reactor. As with some other aromatic substitutions they can proceed by either radical or electrophilic paths, if not even more mechanisms. The products are dif ferent then this may involve position isomerism, arising from different substitution patterns, when the aromatic core already has a primary substituent further, there may be changed selectivity for imdefined addition and polymeric side products (Figure 1.31). It is justified to term this and other similar reactions new , as the reaction follows new elemental paths and creates new products or at least new... 

Direct fluorinations with elemental fluorine still are not feasible on an industrial scale today they are even problematic when carried out on a laboratory-scale [49-53]. This is caused by the difficulty of sustaining the electrophilic substitution path as the latter demands process conditions, in particular isothermal operation, which can hardly be realized using conventional equipment. As a consequence, uncontrolled additions and polymerizations usually dominate over substitution, in many cases causing large heat release which may even lead to explosions. 

Free-radical halogenation of hydrocarbons induced thermally or photochemically can be performed with all four halogens, each exhibiting certain specificities. Because of the thermodynamics of the process, however, only chlorination (and bromination) are of practical importance.31,106-108 Fluorination with elemental fluorine is also possible. This reaction, as discussed above (see Section 10.1.1), follows an electrophilic mechanism in the solution phase.109,110 Under specific conditions, however, free-radical fluorination can be performed. 

Perfluorinated organic bromides can be oxidatively fluorinated with elemental fluorine to derivatives containing tn- [124] and pentavalent [725 126 127] bromine in yields up to 42% Perfluoroheptylbromine tetrafluoride has been used to fluonnate double bonds in halogenated alkenes [127]... 

Figure 4.33 Schematic of the single-channel microreactor used for fluorinations with elemental fluorine (by courtesy of the Royal Society of Chemistry) [272].

Very dilute fluorine was reacted with aromatic hydrazones to produce a benzylic CF2 moiety (equation 174)294. Aromatic fluorination with elemental fluorine had also been attempted. Purrington and Woodard found that ortho fluorination usually took place with systems carrying either electron-donating or electron-withdrawing substituent, although neither the yields nor the conversions were very high. They studied the influence of strong... 

Fluorination with elemental fluorine diluted 1 20 with nitrogen was used to prepare pentafluorophenyl tellurium trifluoride from bis[pentafluorophenyl] ditellurium at — 60° with fluorotrichloromethane as the reaction medium. The yield in this reaction was 80% 4-Methoxyphenyl tellurium trifluoride was obtained by electrochemical oxidation of the diaryl ditellurium in 0.5 M hydrofluoric acid at platinum electrodes2. Bisfpentafluoroethyl] ditellurium and chlorine fluoride (1 6) reacted at — 78° to give pentafluoroethyl tellurium trifluoride, a white solid melting at 95°3. The same compound was obtained with xenon difluoride as the fluorinating agent and melted at 143°4. 

Although the formation of fully fluorinated fullerene C6o was firstly reported in 1991 based on the sharp singlet of 19F NMR spectrum of the product in tetrahydrofuran solution [29], the preparation in macroamount has not been confirmed in the late 1990s. The fully fluorinated C6oF60 has been detected only in mass spectra of the products obtained by direct fluorination with elemental fluorine [30,31]. 

By the direct fluorination with elemental fluorine, it is still difficult to prepare pure compounds C6oF.v and C70F of specific fluorine content because the reaction between fluorine gas and solid state C6o and C70 proceeds under heterogeneous conditions and, therefore, lacks control. Attempts to prepare stoichiometrically pure compounds have become successful using solid phase reactions between fullerenes and metal fluorides. These are described in the next section. It has been unsuccessful so far, but efforts are being made to produce fluorinated fullerenes C6oF v of lower fluorine content (x < 18) [24],... 

The direct fluorination with elemental fluorine at — 78 "C of trimethylsilyl enol ethers derived from diketones results in the formation of the corresponding monofluoro diketones 11 in moderate yield. The trimethylsilyl ethers from cyclic diketones undergo smooth fluorination to give the enol forms, c.g. 12, and not the keto forms.Higher yields are generally observed for the analogous reactions of silyl derivatives of esters, carboxylic acids, malonates, dimethyl amides and lactones (Table 4). ... 

Table 3.2 Selective fluorinations with elemental fluorine...

Earlier syntheses of 8-fluoropurines are limited to a few reports involving nucleophilic displacement, Schiemann reactions, halogen-exchange reactions, and electrochemical oxidations. Recently, the first direct fluorination with elemental fluorine has been reported for the synthesis of 8-fluoroguanines, e.g. 

Direct fluorinations with elemental fluorine and other powerful fluorinating agents can be carried out, but they are, to say the least, inconvenient. They are also potentially very hazardous for the inexperienced chemist and are best left to a fluorine specialist. Such reactions are, of course, carried out industrially, but using dedicated special equipment with rigorous safety control.)... 

The most important use of XV is its telomerization with fluoride ion to give a linear hydrofluoroacyl fluoride (XVI), that is fluorinated with elemental fluorine to give a linear perfluorinated polyether (XVIII), which is sold by Daikin as Demnum (Eq 13.18). Note that Demnum and Krytox are isomeric materials. In general, their properties are quite similar, but there are subtle differences in physical properties and in chemical stability in tribological applications. 

Fluorination is thus accompanied by degradation of hydrocarbon chains. Because of these problems, fluorination with elemental fluorine is rarely attempted in laboratory syntheses. 

Hung and Rozen have described a process for the preparation of PAVE by fluorination with elemental fluorine of selected novel partially fluorinated di-chloroethyl ethers, followed by dehalogenation to the corresponding PAVE. PAVE have been found to be useflil as monomers for molding resins and elastomers [28]. 

Direct fluorination with elemental fluorine is not practical for commercial synthesis of fluorinated surfactants. Elemental fluorine is extremely reactive and difficult to handle. The heat of formation of the C—bond (about 460 kJ/mol or 110 kcal/mol) and the H—F bond (566 kJ/mol or 135 kcal/mol) exceeds that of the C—C bond (about 348 kJ/mol or 83 kcal/mol) [1]. Hence, the fluorination with elemental fluorine leads to a violent fragmentation of the substrate unless the reaction is carefully controlled and the reaction heat effectively dissipated [2,3]. Commercially important pathways to fluorinated surfactants are electrochemical fluorination, telomerization, and oligomerization of tetrafluoroethylene [4-6]. 

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