Voltammetric studies of the Kolbe synthesis with perfluorocarboxylic acids prepared from hexafluoropropene oxide oligomers

Abstract

Carboxylic acids prepared from hexafluoropropene oxide CF 3 CF 2 CF 2 O[CF(CF 3 )CF 2 O] n CF .(CF 3 )COOH [n = 0, 2,5-bis(trifluoromethyl)-3,6-dioxaperfluorononanoic acid; n = 1, 2,5,8-tris(trifluoromethyl)-3,6,9-trioxaperfluorododecanoic acid], according to voltammetric data, enter the Kolbe reaction both at the Pt anode and at anodes from carbon materials in H 2 O3CH 3 CN and CH 3 OH3CH 3 CN solutions. The critical potential appreciably depends both on the anode material and on the solvent composition. Favorable effect of pyridine additions in H 2 O3CH 3 CN solutions is due to replacement of water molecules from the electrical double layer. The sodium ions exert a negative effect on the Kolbe synthesis.Anodic oxidation of solutions of aliphatic carboxylic acids in water or organic solvents involves decarboxylation of the acid accompanied by dimerization of the aliphatic residue:This reaction is known as the Kolbe synthesis. In electrolysis of a mixture of two acids RCOOH and R`COOH, the cross dimer R3R`can form along with the symmetrical hydrocarbons. Oxa acids prepared from oligomers of tetrafluoroethene and hexafluoropropene oxides form both symmetrical and cross Kolbe dimers in reasonable yields [133]. The acid molecules prepared from hexafluoropropene oxide oligomers contain side trifluoromethyl groups exhibiting a strong inductive effect. These groups neutralize the capability of the oxa group for specific interaction with the anode surface, which favors concentration of the acid molecules at the anode3electrolyte phase boundary. Such absorption layers are formed relatively readily, and in the electrochemical properties the acids derived from hexafluoropropene oxide resemble trifluoroacetic acid to a greater extent than do the previously studied acids derived from tetrafluoroethene oxide, [CF 3 O(CF 2 CF 2 O) n CF 2 COOH, n = 33 4] [2].To elucidate specific features of the mechanism of the Kolbe electrosynthesis with fluorocarboxylic acids derived from hexafluoropropene oxide, we studied how the anode material (platinum, glassy carbon, pyrographite, carbon3carbon composite), solvent, and pyridine additions affect the voltammetric characteristucs of the acids CF 3 CF 2 CF 2 O[CF(CF 3 )CF 2 O] n CF . (CF 3 )COOH [n = 0, 2,5-bis(trifluoromethyl)-3,6-dioxaperfluorononanoic acid I; n = 1, 2,5,8-tris(trifluoromethyl)-3,6,9-trioxaperfluorododecanoic acid II]. Acids I and II under definite conditions (mainly in nonaqueous media with Pt or glassy carbon anodes) give voltammetric curves characteristic of the occurrence of the Kolbe synthesis. The semilog plots have two linear portions. The first portion starts at 1.13 1.4 V depending on the experimental conditions and corresponds to the oxidation of the carboxylic acid molecules. Then, after reaching the so-called critical potential, the electrical double layer undergoes rearrangement, and the second Tafel portion starts (potential range 2.0 32.4 V), within which the Kolbe anodic condensation occurs.The best defined curves were obtained in an anhydrous acetonitrile...

Electrochemical conversion of valeric acid on carbon‐based materials

“…Carbon materials are beneficial to the formation of carbon cations, leading to the non‐Kolbe electrolysis. The third step is to form carbocations via one‐electron oxidation at about 2.4 V. The carbocations as intermediates undergo molecular rearrangement to produce more chemically stable carbocations (3) 62,63 . In aqueous solutions, nucleophilic reagents attack the carbocations to form alcohols (4).…”

“…The third step is to form carbocations via one-electron oxidation at about 2.4 V. The carbocations as intermediates undergo molecular rearrangement to produce more chemically stable carbocations (3). 62,63 In aqueous solutions, nucleophilic reagents attack the carbocations to form alcohols (4). Furthermore, esterification reactions at the anode can occur, leading to the formation of esters (5).…”

Anodic oxidation of alkane carboxylates and perfluoroalkane carboxylates at platinum and graphite anodes: product selectivity and mechanistic aspects

“…Baastad have reported the synthesis of half fluorinated hydrocarbons and half fluorinated ester by mixed Kolbe electrolysis of potassium salt of butyric and perfluorobutyric acid, potassium salt of nonanoic and perfluorooctanoic acid, and potassium salt of monoethyl sebacate and perfluorobutyric acid [19]. Electrochemical decarboxylation of higher homologs of perfluorocarboxylic acids are scanty [20][21][22][23]. Here, an attempt has been made to study the electrodecarboxylation of octanoic acid, butyric acid, perfluorobutyric acid, and perfluorooctanoic acid.…”