Fluorocarbon isomerism

In a search for fluorocarbons having anesthetic properties 1 2 dichloro 1 1 difluoropropane was subjected to photochemical chlorination Two isomeric products were obtained one of which was identified as 1 2 3 tnchloro 1 1 difluoropropane What is the structure of the second com pound" ... 

Pelosi and Miller have reported the results of an elegant study of the photolytic and thermal isomerizations of C CFj) isomers.42 The anti-dimer of perfluoro(tetramethylcyclobutadiene). anti-21, gives a mixture of perlluoro(octamethylcubane) (22) and perfluoro(octamethylcuneane) (23) in 20% and 15 % yield, respectively, when irradiated for 36 hours in a fluorocarbon solvent [a mixture of perfluoro(1,3- and perfluoro(1,4-dimethylcyclohexane)]. Cubane 22 is presumably formed via xyn-21, whereas 23 is postulated to be formed via intermediates per-lluoro(oclamcthyleyclooctatetraene) (24) and the corresponding scmibullvalene. 

Apart from these synthetically impractical examples of hydrolysis of chloro-fluorocarbons, there are useful applications converting some chlorofluorocarbons to fluonnated carboxylic acids. As an alternative to the use of the highly corrosive fuming sulfuric acid, normally used in batch processes, a continuous hydrolytic process for converting 1,1,1-trichlorotrifluoroethane (CFC-113a), available by isomerization of CFC-113 [44], to trifluoroacetic acid has been developed [45] (equation 45). It uses metal chloride catalysts deposited on high-surface-area supports Unreacted CFC-113a can be recycled. 

In addition, highly perfluoroalkyl-substituted molecules are kinetically and thermodynamically stable due to sterical protection by the bulky perfluoroalkyl groups from any external attacks. Comparison of the half-lives of thermal isomerization of the perfluoroalkylated strained molecules (28 and 30) with those of the parent hydrocarbons (29 and 31) reveals an exceptionally high stability of these fluorocarbons (Scheme 1.38) [10]. Both the strengthening C—C bond and the steric protection of the inner carbon skeleton by perfluoroalkyl groups are responsible. 

Van Dyke Tiers G (1960) Fluorine nudear spin resonance. Part III. The slow conformational isomerism of perfluorocyclohexane and the nature of rotational barriers in fluorocarbons. Proc. Chem. Soc. London, 389... 

Studies of the kinetics and steric course of the first-order intramolecular stereoisomerizations of the Al /3-diketonates tris(2,6-dimethylheptane-3,5-dionato(aluminium(iii) and bis( 1,1,1,5,5,5 -hexafluoropentane-2,4-dionato)alu-minium(m) have been made. It appears that chelates having alkyl or aryl substituents on the /3-diketonate rings isomerize by a rhombic twist mechanism. If fluorocarbon substituents are present, however, a bond-rupture mechanism is more likely, with formation of a square-pyramidal apical five-co-ordinate inter-mediate. ... 

Saturated fluorocarbon production via cobalt trifluoride fluorination has featured in work on (i) a new synthesis of polyperfluorocyclobutene (2) (see Scheme 1), (ii) compounds derived from photo-isomerization of perfluoro-cyclohexene (see Scheme 2 and p. 55),(iii) polyfluoro-n-octanes and -n-hexadecanes (see Scheme 3), (iv) the fluorination of alkyladamantanes, °... 

Copolymerization of TFE with perfluoroalkyl vinyl ethers proved to be quite facile, either by dispersion techniques similar to that employed with FEP [7] or by a newly developed process ploying a fluorocarbon solvent. However, from the earliest studies it was evident that some complications had to be overcome. The most significant of these was a tendency for the alkyl vinyl ethers to rearrange when exposed to free radicals. In the extreme case a chain reaction could be initiated which would result in incomplete rearrangement to the isomeric acid fluoride. During polymerization at temperatures low enough to prevent excessive reaction by this route, the process, nevertheless, competes effectively with free radical coupling as a termination mechanism. 

The photochemical and photophysical behavior of azobenzene adsorbed on polymer or silica films has been reported recently " azobenzene adsorbed in water-swollen Nafion-H+ (Nafion is a polymer consisting of a perfluorinated backbone and short pendant chains terminated by sulfonic acid groups) exhibited strong fluorescence. The irradiation resulted in the formation of benzo[c]cinnoline and benzidine in quantitative yield, while azobenzene incorporated into a methanol-swollen Nafion-H+ membrane did not emit fluorescence but merely underwent E,Z-isomerization. Thus, solvent-swollen Nafion is useful in the selective phototransformation of azobenzene. According to the authors, protonated azobenzene molecules are located at the fluorocarbon/water interface in water-swollen Nafion-H+ Z-Azobenzene formed on these membranes resulted in cychzation to give the observed products (Scheme 52). 

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