3. Characteristics of perfluorocarboxylic acids and practical application of fluoromaterials obtained on their base.

3.1. Decarboxylation and decarbonylation of perfluorocarboxylic acids.

Among characteristics of perfluorocarboxylic acids we should first of all mention the transformation of carboxylic group. Depending on conditions during these processes different products are obtained. At that perfluoroalkyl fragment can also participate in these processes. Thus, polyfluorinated carboxylic acids H(CF₂CF₂)_n COOH (n = 1-5) at alkali action transform into terminal perfluoroolefines with different length of perfluoroalkyl substituent. The obtaining method of terminal perfluoroolefines is based on this.

Perfluorocarboxylic acids natrium salts can be used, at that the yield is stable and almost quantitive [122].

Best results are obtained at carrying out decarbonylation of perfluorocarboxylic acids halogenanhydrides and their alkyl ethers in the presence of catalysts (oxides of metals Mg, Cr, Ba, Zn, Al, Ni, Si), promoted by using  of 20-50 % alkali metals halogenides at 100-300 ^oC. The yield of terminal perfluoroolefines reaches 95 % [108].

The reactions of  -unsaturated perfluorocarboxylic acids, going with their decarboxylation, open the way to synthesis of different compounds of ethylene and acetylene series [123]. For example, thermal decomposition of Na perfluoro-,-unsaturated perfluorocarboxylic acids carboxylates in ethylene glycol results in formation of 1-hydroperfluoroalkenes-1, while the one (decompositon) of carboxylates Ag of these acids results in formation of perfluorinated -diens [123,124].

Thermal decomposition of copper perfluoroalkenylcarboxylates Cu²⁺ in high-boiling organic solvents, for example in N- methylpirrolidone or sulfolane, results in formation of mixture of 1,3-diens and 1-hydroperfluoroalkens-1 [124].

Halogenides and esters of perfluorocarboxylic acids are subject to decarboxylation at 100-300 ^oC in the presence of catalysts (oxides of magnesium, calcium, barium, zinc, nickel) resulting in formation of perfluoroolefines or perfluoroalkylvinyl ethers with the yield close to 95% [125].

Terminal perfluorolefines can be semi-products to obtain:
1. polyfluoroalkansulpho-acids and on their base the surface active materials and electrolytes for lithium batteries and rechargeable accumulators,
2. new chelation for salts of rare elements,
3. creation of high-temperature liquid dielectrics, heat-transfers and hydraulic liquids.

At the same time at perfluorocarboxylic acids fluoroanhydrides passage over Al₂O₃ decarbonylation with perfluoroalkanes formation is going. It should be noted, that at these conditions chloro- and bromoanhydrides of corresponding acids do not change using of activated carbon is required to carry out this process, at that chloro-and bromperfluoroparafines are formed [126].

If in the system there are sources of halogenide-ions (for example, alkali metals halogenides, haloids etc.) then the thermolysis of perfluocarboxylic acids chloroanhydrides, conducted in flow reactor at atmospheric pressure and 250-350 0C in the presence of potassium iodide, will result in formation of iodoperfluoroalkanes with the yield of 80-85%, and heating of mixture of perfluorinated acids fluoroanhydrides and bromine at 300-450 ^oC at activated carbon will result in formation of bromoperfluoroalkanes [126].

Thermal reactions of sodium and potassium salts of mono- and di-perfluorocarboxylic acids  (temperatures 150-250 0C) result in processes of decarboxylation and formation of hydrocarbons with end group CF₂H. By the example of thermal decarboxylation of perfluoropolyetherdicarboxylic acids with the structure ROOCCF₂-R_F- CF₂COO, which was produced at oxidizing polymerization tetrafluoroethylene with O₂ at UV-irradiation, were studied the kinetics and reaction products.

Perfluorinated carboxylic acids are presented as reagents for introduction of R_FCO or R_FCOO fragments and they are rather deeply and thoroughly studied [131]. We will point out, for example, acylation of compounds with active hydrogen atom, that are used for temporarily blocking of OH and NH₂ groups in carbohydrateû and peptides, acetylation of aromatic compounds according to Friedel-Crafts with formation of perfluoroalkylarylketones, reactions with metalo-organic compounds with formation of perfluoroalkyl-containing tertiary alcohols, addition to olefins and acetylene reactions, including different unsaturated fluorinated systems in the presence of fluoride- ions for synthesis of perfluorinated dialkylketones. This important synthetic achievement, using fluorine carb-anion intermediate products, for example:

In connection with high need of partly fluorinated alcohols their obtaining approach based on their esters of perfluorocarboxylic acids is developing. Thus, reduction of esters of carboxylic acids like R_FCOOR [ R_F = CF₃(CF₂)_m (CH₂)_n (m = 0-20, n = 0-5), H(CF₂)_m(CH₂)_n (m = 1-20, n = 0-5), (CF₃)₂CF, CF₂=CF, CF₂=CFCF₃; R = Me, Et, n-Pr,
i-Pr] by NaBF₄ action in teterahydrofurane produce the corresponding alcohols [132].

For example, CF₃(CF₂)₆CH₂OH alcohol is obtained out of methyl  perfluorooctanoate with the yield equal to 98.3 % (100 % conversion). Like this the reduction of perfluoro(hexyldecylacetate) is carried out by action of NaBF₄ in the special solvent up to perfluoro-1H,1H-2-hexyldecanol [133].

Thereby, there are no special difficulties at target setting of carbonyl-containing compound synthesis based on perfluorocarboxylic acids. We consider discussion of these questions inexpedient, and we'll dwell on practical application of these fluorine materials.

To be continued