| 3.
Synthesis of perfluorinated compounds from internal perfluoroolefins and elementary
fluorine.
The significant part of works is devoted to processes of double bond fluorination. This way is vary important for producing of perfluorinated paraffins and freons [197-203]. The expected highly exothermic nature of the reaction of F2 with olefins discouraged many experimental workers. Reactions of organic compounds with fluorine gas are dangerous and require special equipment. Often the yields of the desired product in these reactions are poor and low selectivity is observed because the fluorine gas has high reactivity. Merritt [204-207] showed that it is not impossible to add the fluorine to certain simple olefins. His technique was, however, quite unusual and rather inconvenient. It seems that the major obstacle for realization of this reaction is the fact that the F-F bond is weak and can be readily separated to very reactive fluorine radicals. A number of indirect methods have been developed for producing of vicinal difluoro compounds to circumvent the direct fluorination. Some success was achieved with adding F2 to perfluoroalkenes. The result was the formation of the corresponding perfluoroalkanes. So, tetrafluoroethylene reacts with element fluorine both in gas-phase and in liquid-phase (Freon 114) conditions at 80 oC, and gives the hexafluoroethane with yield 87.8 % [208-210]. Taking into account, that hexafluoroethane use as quality propellants, working bodies for refrigerating machines and for dry etching of semiconductors, this method is perspective for industrial scale [211]. Similarly, perfluoro-2-methylpent-2-ene and perfluoro-4-methylpent-2-ene or their mixture react with elemental fluorine both in liquid phase at -120 oC - -30 oC and without solvent resulting the perfluoro-2-methyl-pentane with a quantitative yield [212]. Author of [213] have developed effective techniques for a production of polyfluorinated compound from polyfluoroolefins and fluorine gas in liquid phase. According to this method, 100 % fluorine gas react in mild condition (reaction temperature: -64 - +58 oC; reaction pressure: atmospheric) without explosion or combustion of the initial substances. Preparation of the branched perfluoroalkanes was carried out with mixture of 20 % F2 and 80% N2 [214] or 100 % F2 [215] in liquid-phase absorption column system (Table 3). Table 3. Fluorination of perfluoroolefins (slight excess of F2 )[215].
Perfluoro-2-methylpentane can be used as a hydraulic liquid, dielectric and heat-carrier. This method has appeared convenient for reception of others perfluoroparaffins from various perfluoroolefins. So, trimer of a hexafluoropropylene react with elemental fluorine and perfluoro-3-isopropyl-4-methylpentane is produced with 86 % yield [216]. Fluorination by elemental fluorine at -40 - -120 oC in inert solvents (CFCl3, CF2Cl2) also gives perfluorinatered products with high yields [79,217-220]. Perfluoro-3-isopropyl-2-methylpentane was produced at 70 oC with quantitative yield [221].
Others perfluoroolefins and their derivative also give perfluorinatered products, for example, such perfluoroolefins as : (iso-C3F7) 2C=CFCF3, (CF3)2C=C(C2F5)CF(CF3)2, CF3CF=C(CF3)C(CF3)2(C2F5) 2, (CF3)2C=C[CF(CF3)2]CF(CF3)OCH2C3F7-n, (CF3) 2CFC(C2F5)=CFCF3 , perfluoro-2,2-propanebicyclo [5.2.0.]non-1(7)-ene [212,221], (CF3)2CFCF(C2F5)C(CF3)=CF2, CF(CF3)2C[C(CF3)3]=CF2 [218]. Thus, fluorine react at -78 oC in alcohol only with multiple bond and vicinal products are formed, while the epoxy compounds is observed at 0oC in acetonitrile. The CF3. readily detach and new perfluoroolefins is formed. They can, firstly, yield radicals themselves and, secondly, lead further to perfluoroparaffins with altered carbon skeleton. These results have led to processes of destruction and isomerization of perfluoroparaffins. The reaction of perfluoro-3-ethyl-2,4-dimethylpent-2-ene and perfluoro-3-isopropyl-4-methyl-pent-2-ene with fluorine in Fomblin Y06/6 in the presence of UV light was studying by the authors of [220]. Perfluoro-2,3,3,4-tetramethylpentane and perfluoro-2,3,4-trimethyl-pentane were the main products of reaction together with small amounts of other known compounds 60-62. Therefore perfluoro-2,3,3,4-tetramethylpentane has a potential as a new initiator for vinylic polymerization. But photochemical isomerization of perfluoropent-2-ene isomer yield two terminal olefins: perfluoro-3-ethyl-2,4-dimethyl-pent-1-ene 63 and perfluoro-2-iso-propyl-3,3-dimethylbut-1-ene 64, which are not the unsaturated counterparts of photo induced fluorination of main products. Fluorination of 63 and 64 gives, respectively, 60 and perfluoro-2,2,3,4-tetramethylpentane 65, together with other decomposition products.
The terminal olefins (CF3)2CFCF(C2F5)C(CF3)=CF2 and (CF3)2CFC[C(CF3)3]]=CF2 were fluorinated and highly branched perfluoroalkanes was obtained. They provide the perfluoroalkyl radicals, which are the initiators for monomers polymerization [218]. Other perfluoroolefins: (CF3)2CFC(C2F5)=C(CF3)2 and [(CF3)2CF]2C=CFCF3 were fluorinated under UV radiation with producing (CF3)2CCF(CF3)CF(CF3)2, (CF3)2CFC(CF3)2CF2CF2CF3 and (CF3)2CFCF2CF2CF3 (yields 26, 50, 24 %, respectively). Direct gas-phase fluorination of hexafluoropropene trimers, perfluoro-3-ethyl-2,4-dimethyl-pent-2-ene and perfluoro-3-isopropyl-4-methylpent-2-ene, at 130 -150 oC in the presence of copper and nickel yields the first stable perfluororadical, which was found by Scherer [222]. So the stable radical can undergo n
Perfluoro-3-isopropyl-2-methylpent-3-ene was fluorinated and 98 % yield of perfluoro-3-isopropyl-2-methylpentane was obtained [201,223,224]. When the temperature of fluorination is higher than 170oC the process proceeds non-selectively with formation mainly the carbon and tetrafluoromethan. An obvious change in the products type is observed when using the special catalyst Ni/support: perfluoro-2-methyl-3-isopropylpentane as the main product at 130 oC is obtained [224].
The basic idea is the localization of fluorination acts on the catalyst surface. In the direct gas-phase catalytic fluorination of incompletely fluorinated organic compounds the adsorbed excited intermediate adducts have the possibility to leave the excessive energy to the solid without destruction, resulting the increasing of the selectivity of fluorination. This idea is illustrated by the results of gas-phase direct fluorination of fluoroolefins (dimer and trimer of hexafluoropropylylene). New catalysts allow to obtain excellent results not only in laboratory, but also in industry. The process of fluorination proceeds by the radical mechanism. The average lifetime of intermediate fluorocarbon radicals in various radical reactions in a liquid at room temperature is much more (102 - 103 h) than their hydrocarbon analogues.
|
* transition of the unpaired
electron with forming a very unstable species which can rearrange by two ways:
-elimination of CF3. radical from the CF3CF2
group with terminal alkene formation (perfluoro-2-isopropyl-3-methylbut-1-ene), which
react with F2 and intermediate perfluoro-2,3,4-trimethylpentan-3-yl is formed.
The last one can react with F. radical or CF3. radical
with formation of perfluoro-2,3,4-trimethylpentane and
perfluoro-2,3,3,4-tetramethylpentane; 
