2. Synthesis of Perfluorinated Carboxylic Acids

2.1. Electrochemical Fluorination of Carboxylic Acids and
Some of Their Derivatives in Anhydrous Hydrogen Fluoride.

Perfluorinated carboxylic acids are the most important semi-products and components of fluoroorganic compounds chemistry, that is expressed in great field of their practical application [4-9]. Just the working out of this class compounds synthesis methods became a stimulus at the first stage of electrochemical fluorination method development.

Different compounds, containing C=O group, were introduced into electrochemical fluorination process. Thus, electrochemical fluorination of aldehydes results in formation of fluoroanhydrydes of corresponding perfluorinated carboxylic acids [10].

The use of carboxylic acids themselves at electrolysis in anhydrous hydrogen fluoride results in their total decomposition, the reaction products are partly or totally fluorinated parafines, while dicarboxylic acids and their derivatives are being fluorinated smoothly, but intramolecular cyclization takes place, which results in formation of penta- and hexa-component heterocyclic compounds [11,12]. It is interesting, that the presence of two groups - SO₂F and COF in the molecule is accompanied by cyclization along with affecting of COF group and forming of fluoroanhydride of corresponding sulpho-acid.

The formation of cyclic esters at electrochemical fluorination of dibasic acid derivatives was an undesirable process, because it lowered the yield of commercial carboxylic acids. However, the fields of application were found for them later. Now they are considered as most perspective compounds for compositions of artificial blood and for storage of transplantation organs, for dielectrics and coolers for wide class of transformers, high-temperature heat transfers. That's why the interest to perfluorinated cyclic esters synthesis according to this method doesn't surprise, because direct fluorination method is limited due to low availability of source hydrocarbons that is cyclic ethers.

In case of electrochemical fluorination of dibasic acid chloroanhydrides [13,14] and lactones [15] fluoroanhydrides of dicarboxylic acids are formed.

The formation of new heterocyclic compounds is observed at electrochemical fluorination of dicarboxylic acid ethyl esters, that takes place due to intramolecular cyclization, passing with the participation of oxygen atom C=O group. Analogously perfluorodioxolan is obtained out of EtO₂C(CF₂)nCO₂Et (n = 1-6) [16] also.

Contrary to this, the yield of perfluorocarboxylic acid fluoroanhydrides greatly increases and becomes defining product, if using as source substrate not the acids themselves, but fluoroanhydrides or chloroanhydrides of corresponding acids (table. 1) [17,18].

Table 1. Electrochemical fluorination of carboxylic acid derivatives [22,23].

At electrochemical fluorination of acetic acid fluoroanhydride the trifluoroacetate fluoride is obtained, it's yield is 36-45% (according to current the yield is 32-50%) [17,19,20]. The electrochemical fluorination of carboxylic acid chloroanhydrides, for example, chloroanhydrides of caproic, enanthic, caprylic and capric acids results in formation of fluoroanhydrides of corresponding perfluorinated acids with small yield (11-15%), the main products of this process are perfluorinated cyclic ethers (24-35 %) [5].

Esters of carboxylic acids at electrochemical fluorination in anhydrous hydrogen fluoride usually saponify and in the beginning fluoranhydrides of corresponding acids are formed, which further are subject to fluorination. However, perfluorinated heterocyclic compounds are obtained in this case too. [13,21].

Detailed studying of this process showed, that substrates, containing cycloalkyl groups as substituents produce bicyclic ethers  with satisfactory yields (table 1) [20]. Such cyclic ethers contain two penta- component cycles or one penta-component and one hexa-component. The yield of products is not very high. Besides, in small quantities the corresponding perfluorinated esters are formed out of acids and the products of their transformation - perfluoroparafines and perfluoroacyclic derivatives are formed also [11].

At electrochemical fluorination of methyl  2-cyclohexylacetate  and some other derivatives of carboxylic acids the authors of works [18,24,25] found new cyclization, passing with participation of multiply bond, resulting in formation of heterocyclic compounds. This process is general for carbonyl  compounds, having multiply bonds in -position.

Perfluoro(alkoxycycloalkane) derivatives of carbonyl fluoride

where R is perfluoroalkyl C1-C4;
 n = 2-5, is obtained by electrochemical fluorination of corresponding acid chloroanhydrides, for example, 3,4,5-(MeO)₃C₆H₂COCl in the presence of 10% dimethylsulphide in boiling anhydrous hydrogen fluoride at 2000 tor and at 50 ^oC (nickel anode) gave 3,4,5-(CF₃O)₃C₆F₈COF [26]  with the yield of 30 %. These compounds have surfactants properties and they are used as film forming foams.