Fluorine 18 atom thermal reactions

Often the substitution of fluorine atoms for hydrogen atoms in a polymer chain markedly increases the thermal stabiUty of the base polymer this is tme for polyimides. A typical fluorinated polyimide is prepared from the reaction of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 2,2-bis-(4-amino phenyl)hexafluoropropane according to the following reaction (36) ... 

The fluoride [NS(0)F]3 is more stable thermally and towards nucleophilic reagents than the corresponding chloride. For example, 8.15a is hydrolyzed by water to NH(S02NH2)2, whereas sulfanuric fluoride is unaffected by cold water. In warm water, however, hydrolysis occurs to give the [N3S304F2] anion. All three fluorine atoms in [NS(0)F]3 can be replaced by primary or secondary amines at 80-90°C in the absence of a solvent. Mono- or diphenyl derivatives can be prepared by treatment of [NS(0)F]3 with PhLi in diethyl ether at -70°C, while the Friedel-Crafts reaction with benzene at reflux in the presence of AICI3 gives two isomers of [NS(0)Ph]3. ... 

The Balz-Schiemann and Wallach reactions The Balz-Schiemann reaction (the thermal decomposition of an aryl diazonium salt. Scheme 46) was for many years the only practical method for the introduction of a fluorine atom into an aromatic ring not bearing electron-withdrawing substituents. This reaction, first reported in the late 1800s, was studied in fluorine-18 chemistry as early as 1967 [214]. It involves the generation of an aryl cation by thermal decomposition, which then reacts with solvent, nucleophiles or other species present to produce a substituted aromatic compound. Use of fluorine-18-labelled... 

No thermal reactions were observed at temperatures up to 350 °C. and there was no evidence for the abstraction of fluorine atoms. 

In order to understand the thermal oxidation of C2F4, it is first necessary to understand the reactions in the absence of 02. Below about 280°C, C2F4 is thermally stable. Above this temperature, it dimerizes to c-C4Fe, but otherwise undergoes no other reactions below about 550°C. Between 550 and 800°C, the monomer-dimer mixture decomposes to other fluorocarbons via CF2 as an intermediate. At even more elevated temperatures the CF2-C2F4 equilibrium is achieved and ultimately forms carbon and CF4 via CF radicals and fluorine atoms. 

The formation of four-membered rings through 2 + 2 cycloaddition is a well-established reaction and the most generally effective synthetic approach to cyclobutanes. Most olefins cannot be induced to undergo this reaction thermally, a finding that is readily rationalized by the forbidden nature of the 2s + 2s addition and the steric difficulties associated with the allowed 2s + 2a pathway. There are nevertheless exceptions. Olefins substituted by two or more fluorine atoms undergo thermal 2 + 2 additions under relatively mild conditions,16 as do ketenes and allenes. 

The single-pulse shock tube method in the thermal decomposition of l,l,l-trifluoro-2-chloroethane gave parallel eliminations135. The major reaction involved HC1 elimination which was believed to be formed by an a,a-elimination (equation 45) with a concerted transfer of a fluorine atom, while to a lesser extent a slower a,/Felimination of HF also took place (equation 46). 

Introduction of a fluorine atom is achieved using the Schiemann reaction. Originally, the reaction involved gently heating the solid diazonium fluoroborate (Scheme 8.18), but improved yields result from the thermal decomposition the hexafluorophosphate, ArN2 PF, or hexa-fluoroantimonate, ArN2 SbFg, salts. 

In contrast to the cyclobutane systems, fluorinated cyclopropanes show a lesser thermal stability than the parent hydrocarbon. Trotman-Dickenson et al. (refs. ° )have studied thethermally induced unimolecular isomerisations of partly fluorinated cyclopropanes to mixtures of the corresponding fluoro-propenes. At 450 °C the rate of decomposition of monofluorocyclopropane is about three times that of cyclopropane products formed are 1-fluoropropene (79 %) cis-trans mixture), 2-fluoropropene (9 %) and 3-fluoropropene (11 %). The substitution of further fluorine atoms in the cyclopropane ring further increases the rate of isomerisation ° (see Table 2). Unlike the partly fluorinated cyclopropanes, perfluorocyclopropane does not isomerise to propene compounds but decomposes at 250-300 °C by a first-order reaction to perfluoroethylene . The rate coefficient is expressible as, k = 1.78 x 10 exp (-38,600//JjT) sec , and the decomposition is consistent with the mechanism... 

The substitution of Abu for leucine in the hydrophobic core position a9 shows that a significant decrease in spatial demand and hydrophobicity that thermally destabilizes the folding motif also reduces the rate of product formation. The y-di- and trifluorination of Abu further decelerates rather than accelerates the reaction. This trend correlates with the number of fluorine atoms within the side-chain. Interestingly, the same kind of... 

Results of ab initio calculations30,70,74 76 show that the reaction mechanism is quite complex and the reaction proceeds via the formation of intermediate complexes. The molecular structures of the transition states and intermediate complexes are very similar to those of the corresponding structures occurring in the CH3OH + F reaction system. However, in contrast to the reaction with fluorine atoms, all the molecular complexes are thermally stable structures. The profile of the potential energy surface obtained by Jodkowski et al,30 at the G2 level is shown in Fig. 4 (with intermediates labeled analogously to the CH3OH + F reaction system). 

The highly exothermic cesium fluoride catalyzed isomerization of hexafluorobutadiene (9) via 10 to hexafluoro-2-butyne (11) is another example [5] that shows that the fluorine atom is preferably bonded to the carbon atom through the sp3-hybridized orbital rather than through the sp2 orbital [6]. And here again, Bent s rule plays an important role in the isomerization [7]. The thermal reaction of hexafluoro-1,3-butadiene provides perfluorotricyclo[3.2.0.02,6]octane (15) as a stable final product via sequential intra-and/or intermolecular cycloaddition at 200°C [8]. In contrast, the parent hydrocarbon (16)... 

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