Received: May, 2011

Fluorine Notes, 2011, 76, 9-10

Cross-dimerization of Perfluorovaleric and Perfluoro-2-propoxypropionic acids in the Presence of Butadiene

N.A.Mayorova ^a, N.D.Kagramanov ^b, V.A.Grinberg ^a, S.R.Sterlin ^b

^aThe establishment of Russian Academy of Sciences A.N.Frumkin Institute of Physical Chemistry and Electrochemistry RAS Russian Federation, 119991 Moscow, Leninskii pr. 31, Fax: (495) 9520846, E-mail: vgrinberg@phyche.ac.ru
^bThe establishment of Russian Academy of Sciences A.N.Nesmeyanov Institute of Organoelement Compounds RAS, Russian Federation, 119991 Moscow, Vavilova st. 28, Fax: (499) 135 6549, E-mail: lsg@ineos.ac.ru

Abstract:The electrochemical cross-dimerization of perfluorovaleric and perfluoro-2-propoxypropionic acids conducted in the presence of butadiene afforded a mixture of cross- and homodimers characterized by abnormally high percentage of the compounds containing perfluoro-2-propoxyethyl group. Presumably the result obtained reflects the competitive adsorption of electrolytes' components on the anode surface in the course of which butadiene preferably displaces perfluorovalerate-anion from anode.

Keywords:perfluorovaleric acid; perfluoro-2-propoxypropionic acid; Kolbe elrctrosynthesis; butadiene.

The electrochemical oxidation of two carboxylic acids that possess close physico-chemical characteristics – first of all the pK_a values – leads to the formation of homo- and cross Kolbe dimers in a ratio determined only by the concentrations of starting acids RCOOH and R'COOH and obeys the rule R-R:R-R':R'-R' = [RCOOH]²:2[RCOOH].[R'COOH]:[R'COOH]² [1]. Thus the anodic oxidation of equimolar mixture of perfluorovaleric (I) and perfluoro-2-propoxypropionic acid (II) resulted in the preparation of homo- and cross-dimers mixture in the ratio III:IV:V= 1:2,13:0,99 that is close to the theoretical ratio of these products [2].

Now we have established that this reaction [Pt-10%Ir anode; MeCN:H₂O = 9:1] conducted in the presence of butadiene after passing of 0.5 F electricity per 1 mole of acid afforded a mixture of compounds III-V in the ratio III:IV:V= 1:3,3:2,2 (The products of additive Kolbe electrosynthesis obtained from acid II and butadiene were described in [3]).

The electrolysis conducted at the incomplete conversion of the starting acids allows to estimate the current rates of perfluorovalerate-anion (Ia) and perfluoro-2-propoxypropionate-anion (IIa) electro-oxidation under given conditions by the ratio of products obtained by recombination of fluoroalkyl radicals generated in the course of oxidation.

The sharp decrease of compounds' IIIshare in a mixture of III-Vevidently indicates that in the presence of butadiene anion Ia oxidizes considerably slower than anion IIa.

Most probably the result obtained reflects the competitive adsorption of electrolytes' components on the anode surface in the course of which butadiene preferably displaces perfluorovalerate-anion from anode; the concentration of anion IIa hereat doesn't change virtually. Two-fold decrease of the current yield of additive Kolbe electrosynthesis products obtained from acid I and butadiene also witnesses the diminution in concentration of anion Iaat the anode in the presence of butadiene [4].

The nature of different adsorption energy of anions Ia-IIais not entirely clear. It can be assumed that the enhanced adsorption energy of anion IIais rather connected with the presence of oxygen atom in the main chain of the molecule that performs a role of an additional nucleophilic centre in spite of the electron withdrawing influence of fluoroalkyl substituents. To a certain extend such assumption ties up with our observations of the enhanced persistence and hence reduced reactivity of perfluorinated α-alkoxyalkyl radicals (VI) in comparison with their perfluoroalkyl analogues [5]. It was postulated that this phenomenon could be explained within the frame of the scheme reported in [6] according to which an oxygen atom at a-position towards paramagnetic centre reveals sufficiently high electron donating ability to form mesomeric structure (VII):

The study of the competitive adsorption of fluoroalkyl and oxafluoroalkyl carboxylic acids will be continued.

Experimental

The mass spectra were recorded on VG ANALYTICAL 70-70E (70 eV) and Finnigan Polaris/GCQ Plus (70 eV) spectrometers.

The electrolysis of acids I and II in the presence of buradiene

A mixture of 4.76 g (18 mmol) of acid I, 6.0 g (18 mmol) of acid II, 30 ml of aq. MeCN (MeCN:H₂O = 9:1) and 0,48 g (7.2 mmol) 85% KOH was loaded into a glass undivided electrochemical cell equipped with a water-cooled jacket, reflux condenser, inlet tube and magnetic stirrer. Anode – Pt-10%Ir (15.5 cm²); the anode current density 64.5 mA/cm²; cathode – stainless steel. The butadiene was bubbled through electrolyte at a rate 30 ml/min. After passing 0.48 A-h of electricity (0.5 F/mole of acid) at 20°C the lower layer of electrolyte was separated, washed with aq. NaHCO₃ solution, 10% aq. HCl, dried over MgSO₄to give 1.5 g of a mixture that contained 80% compounds III-V(III:IV:V= 1:3.3:2.2) (according to GLC and CMS data).

Mass-spectrum for C₈F₁₈ (III) (m/z, reference): 362 [C₈F₁₄]⁺; 331 [C₇F₁₃]⁺; 319 [C₆F₁₃]⁺; 281 [C₆F₁₁]⁺; 243 [C₆F₉]⁺; 231 [C₅F₉]⁺; 219 [C₄F₉]⁺; 181 [C₄F₇]⁺; 169 [C₃F₇]⁺; 131 [C₃F₅]⁺; 119 [C2F₅]⁺; 112 [C₃F₄]⁺; 100 [C₂F₄]⁺; 81 [C₂F₃]⁺; 69 [CF₃]⁺ (100%); 62 [C₂F₂]⁺; 50 [CF₂]⁺.

Mass-spectrum for C₄F₉-CF(CF₃)OC₃F₇ (IV) (m/z, reference): 504 [M-F]⁺; 319 [C₆F₁₃]⁺; 297 [C₆F₁₁O]⁺; 231 [C₅F₉]⁺; 181 [C₄F₇]⁺; 169 [C₃F₇]⁺; 147 [C₃F₅O]⁺; 131 [C₃F₅]⁺; 119 [C₂F₅]⁺; 100 [C₂F₄]⁺; 69 [CF₃]⁺ (100%); 59 [C₂FO]⁺.

Mass-spectrum for [C₃F₇OCF(CF₃)]₂ (V) (m/z, reference): 385 [C₇F₁₅O]⁺; 363 [C₇F₁₃O₂]⁺; 285 [C₅F₁₁O]⁺; 263 [C₅F₉O₂]⁺; 219 [C₄F₉]⁺; 197 [C₄F₇O]⁺; 169 [C₃F₇]⁺; 147 [C₃F₅O]⁺; 131 [C₃F₅]⁺; 119 [C₂F₅]⁺; 100 [C₂F₄]⁺; 69 [CF₃]⁺ (100%); 62 [C₂F₂]⁺; 50 [CF]⁺.

References

1. H.J.Schafer "Recent Contributions of Kolbe Electrolysis to Organic Synthesis" // Electrochemistry IV, Akademie-Verlag, Berlin, Topics Current Chemistry vol.152. p.105 (1990)
2. V.F.Cherstkov, V.A.Grinberg, S.R.Sterlin, Yu.B.Vasil’ev, L.S.German, E.I.Mysov, Izv. Akad. Nauk SSSR, Ser. Khim., 1991, 1141 (Russ.Transl.).
3. N.A. Mayorova, N.D.Kagramanov, V.A.Grinberg, S.R.Sterlin, Fluorine Notes N 4(65), 2009 July-August
4. N.A. Mayorova, N.D.Kagramanov, V.A.Grinberg, S.R.Sterlin, Russ.J.Electrochem., in press.
5. V.A.Grinberg, S.R.Sterlin, Russ.Chem.Bull., 2005, 54, N 8, 1942 (Engl. Transl.).
6. Nonhebel D.C., Walton J.C.Free-Radical Chemistry. University Press. Cambridge. 1974. P. 62

Fluorine Notes, 2011, 76, 9-10