Fluorine effect halogen

The halogenated hydrocarbons are generally of low acute toxicity, but several are associated with anesthetic effects and cardiac sensitization. Cardiac sensitization to halogenated alkanes appears related to the number of chlorine or fluorine substitutions. Halogenated alkanes in which >75% of the... 

Alkali metal hexafluoroantimonates arc used, like hexafluorophosphate or tetrafluoroborate salts, to effect halogen-exchange fluorination in organosilanes both in the presence and in the absence of solvents. Fluorotriphenylsilane and difluorodiphenylsilane are obtained in 87 and 95 % yield by heating the corresponding chlorides with sodium hexafluoroantimonate in tetra-ethylene glycol dimethyl ether (tetraglyme).102 Tetrafluoroborates react with chlorosilanes faster than hexafluorophosphate or hexafluoroantimonate salts. 

One clear tendency is apparent from the equilibrium product distribution of Letvis acid-induced transfluorination and halogen scrambling if fluorine can migrate it vill al vays tend to concentrate at one carbon atom. Most preferred reaction products are trifluoromethyl derivatives, followed by geminal difluoromethy] derivatives. This thermodynamic product control is often referred to as the general fluorine effect (Scheme 2.19). 

Scheme 2.19 The general" fluorine effect in Lewis acid-induced halogen scrambling.

Subsequently, many examples of this fluorine effect were discovered not only for more complex fluorocarbons, but also for the other similarly coordinated elements Si, B, P, etc. It was also demonstrated that whereas other hdogen derivatives show a similar progressive bond shortening with increasing halogenation, the magnitude of the effect is much smaller, as illustrated in Fig. 2. For Br and I, bond shortening is not discernible within the experimental uncertainties. The contrast between the fluorine and the other... 

The mix of inductive and resonance effects varies from one halogen to another but the net result is that fluorine chlorine bromine and iodine are weakly deactivating ortho para directing substituents... 

Heterogeneous vapor-phase fluorination of a chlorocarbon or chlorohydrocarbon with HP over a supported metal catalyst is an alternative to the hquid phase process. Salts of chromium, nickel, cobalt or iron on an A1P. support are considered viable catalysts in pellet or fluidized powder form. This process can be used to manufacture CPC-11 and CPC-12, but is hampered by the formation of over-fluorinated by-products with Httle to no commercial value. The most effective appHcation for vapor-phase fluorination is where all the halogens are to be replaced by fluorine, as in manufacture of 3,3,3-trifluoropropene [677-21 ] (14) for use in polyfluorosiHcones. 

Sulfur tetrafluoride [7783-60-0] SF, replaces halogen in haloalkanes, haloalkenes, and aryl chlorides, but is only effective (even at elevated temperatures) in the presence of a Lewis acid catalyst. The reagent is most often used in the replacement of carbonyl oxygen with fluorine (15,16). Aldehydes and ketones react readily, particularly if no alpha-hydrogen atoms are present (eg, benzal fluoride [455-31-2] from benzaldehyde), but acids, esters, acid chlorides, and anhydrides are very sluggish. However, these reactions can be catalyzed by Lewis acids (HP, BF, etc). 

The nonbonding electron clouds of the attached fluorine atoms tend to repel the oncoming fluorine molecules as they approach the carbon skeleton. This reduces the number of effective coUisions, making it possible to increase the total number of coUisions and stiU not accelerate the reaction rate as the reaction proceeds toward completion. This protective sheath of fluorine atoms provides the inertness of Teflon and other fluorocarbons. It also explains the fact that greater success in direct fluorination processes has been reported when the hydrocarbon to be fluorinated had already been partiaUy fluorinated by some other process or was prechlorinated, ie, the protective sheath of halogens reduced the number of reactive coUisions and aUowed reactions to occur without excessive cleavage of carbon—carbon bonds or mnaway exothermic processes. 

Health and Safety Factors. Fluorocarbons containing bromine or iodine are more toxic than the corresponding chloro compounds. When the ratio of the fluorine to other halogens is high, the toxicity can be quite low, especially for bromofluorocarbons. Perfluoro-l-bromooctane [423-55-2] has an LD q of greater than 64 mL/kg when adininistered into the gastrointestinal tract, and has Htde effect when instilled into the lungs (49). Other examples are included in Table 7. 

Chemical Properties. A combination of excellent chemical and mechanical properties at elevated temperatures result in high performance service in the chemical processing industry. Teflon PEA resins have been exposed to a variety of organic and inorganic compounds commonly encountered in chemical service (26). They are not attacked by inorganic acids, bases, halogens, metal salt solutions, organic acids, and anhydrides. Aromatic and ahphatic hydrocarbons, alcohols, aldehydes, ketones, ethers, amines, esters, chlorinated compounds, and other polymer solvents have Httle effect. However, like other perfluorinated polymers,they react with alkah metals and elemental fluorine. 

Qua.driva.Ient, Zirconium tetrafluoride is prepared by fluorination of zirconium metal, but this is hampered by the low volatility of the tetrafluoride which coats the surface of the metal. An effective method is the halogen exchange between flowing hydrogen fluoride gas and zirconium tetrachloride at 300°C. Large volumes are produced by the addition of concentrated hydrofluoric acid to a concentrated nitric acid solution of zirconium zirconium tetrafluoride monohydrate [14956-11-3] precipitates (69). The recovered crystals ate dried and treated with hydrogen fluoride gas at 450°C in a fluid-bed reactor. The thermal dissociation of fluorozirconates also yields zirconium tetrafluoride. 

Tetrasubstituted phosphonium halides are just as effective as their ammonium counterparts. A combination of tetraphenylphosphonium bromide and either 18-crown-6 or polyethylene glycol dimethyl ether with spray-dried potassium fluoride converts 4-chlorobenzaldehyde to 4-fluorobenzaldehyde in 74% yield [67] In addition, the halogen of a primary alkyl chloride or bromide is easily displaced by fluorine in aqueous saturated potassium fluoride and a catalytic amount of hexadecyltributylphosphonium bromide [68] (Table 7 Procedure 4, p 194)... 

This topic has been reviewed [2, pp 94, 100-111, 130-134] All of the standard approaches to the synthesis of a compound like methyl 2-fluorostearate from methyl 2-bromostearate result mall yield of the 2-fluoro ester and the unsaturated esters. Although silver fluoride is not a new reagent, its use moist in wet acetonitrile to convert methyl 2-bromostearate to its fluoro ester is a departure from the traditional set of anhydrous conditions (Procedure 6, p 194) [71] In contrast, silver tetrafluoroborate converts a-chloroketones to their respective fluoroketones under anhydrous conditions. The displacement of less activated halogen groups by silver tetrafluoroborate to form their respective fluorides is novel Although silver tetrafluoroborate could not be used to convert an aliphatic terminal dichloromethyl or trichloromethyl group to its corresponding fluoro derivative, it is an effective fluorine source in other situations [72] (Table 8)... 

Although both cuprous and cupric fluorides have been studied in the past, an active fluorine donor can be formed from cupric oxide and hydrogen fluonde. This donor, in combination with 2,2 -bipyridine, effectively displaces the halogen of... 

The chemical reactions of XY can be conveniently classified as (a) halogenation reactions, (b) donor-acceptor interactions and (c) use as solvent systems. Reactions frequently parallel those of the parent halogens but with subtle and revealing differences. CIF is an effective fluorinating agent (p. 820) and will react with many metals and non-metals either at room temperature or above, converting them to fluorides and liberating chlorine, e.g. ... 

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