part2 - Perfluoroalkanesulfonic acids syntesis.

2. Perfluoroalkanesulfonic acids syntesis.

2.1. Electrochemical fluorination of hexavalent sulfur derivatives.

Electrochemical fluorination method of sulfur organic derivatives was the main one, used for obtaining of perfluorinated alkanesulfonic acids syntesis [3]. As a rule in this case the source material is sulfur hydrocarbon derivatives. Among important practical applications of electrochemical fluorination method there is working-off of obtaining method of perfluoroalkanesulfofluorides and perfluoroalkanesulfonic acids, starting from derivatives of alkanesulfonic acids, because the derivatives of perfluoroalkanesulfonic acids have special interest for their industrial applications. This approach became the main method of perfluoroalkanesulfonic acids synthesis in the present time [7-11]. Perfluoroalkanesulfonic acids fluoroanhydrides formed during this are hydrolyzed by alkali up to alkaline salts of perfluoroalkanesulfonic acids, which can be used directly as surface active materials. Electrolysis is held using nickel electrodes in anhydrous hydrogen fluoride at voltage below the one of gas fluorine evolving (4,5-6,0 V) and current density about 0,02 A/m² .

Reviews on electrochemical fluorination of sulfur containing compounds, including the ones containing two functional groups (CH₃CHClSO₂Cl, FSO₂(CH₂)_nSO₂F, n = 3, 4) and their application are listed in [3,9].

Electrochemical fluorination of alkanesulfonic acids passes through preliminary formation of fluoroanhydrides. At that the length of carbon backbone chain influences the yield of base perfluorinated products and when this length increases the yield decreases. Thus trifluoromethanesulfofluoride is obtained with the 96% yield, C₂F₅SO₂F - 61% and C₈F₁₇SO₂F -25% [10a].
Chloronhydrides of alkanesulfonic acids can be used as start substartes too [10b].

n	1	2	3	4	5	6	7	8
Yield,%	97	79	68	58	45	36	31	25

The main by-product of this reaction is perfluoroparaffin with the same number of carbon atoms. Along with shortening of carbon backbone chain, leading to formation of perfluoroalkanesulfonic acids fluoroanhydrides with lesser number of carbon atoms the sulfurylfluoride and sulfur hexafluoride are formed. Base product yield varies from 5 to 40%. This explains the influence of functional groups on transformation of carbon skeleton and the formation of partly fluorinated products at closing stages of fluorination.

If there are two SO₂F groups in molecule, for example CH₂(SO₂F)₂ [11-16], 1,2- and 1,3-propanedisulfonylfluorides, electrochemical fluorination in anhydrous fluoride hydrogen (current density 0.2 A/m² , voltage 3.75-6 V) results in formation of base products with yields high enough. Perfluoroalkanesulfonic acids fluoroanhydride is obtained by implication, subjecting the appropriate acid to electrochemical fluorination in electrolyzer with electrodes made from nickel in anhydrous HF. Thus fluoroanhydride of difluoromethanesulfonic acid is obtained with the yield 75-82% (effectiveness of using electric energy is 33%). Fluoroanhydrides of 1,3-propane-, 1,3-butane- and 1,4-butanedisulfo-acids are obtained analogously to this [17,18]. It is determined that during the electrochemical fluorination process anode corrode 40 times faster than cathode, at that the corrosion is growing arcwise with the growth of anode tension. Because of this the development of convenient method of synthesis of dialkanesulfonic acids fluoroanhydrides is an important industrial task.

At that the character of alkyl row doesn't matter.

Nitrogen-containing functional group in linear part of alkanesulfonic acid doesn't influence in principle the passing of the electrochemical fluorination process [17-20].

During the electrochemical fluorination of sultones the uncovering of cycle and fluoroanhydrides of sulfonic and carboxylic acids formation take place. Thus 1,3-propanesultone at electrochemical fluorination is turned into fluoroanhydride of -fluorosulfonylbutanoic acid [21].

In the case of 2,5-dihydrothiophene-1,1-dioxide the uncovering of the cycle with the formation of perfluorobutansulfonic acid fluoroanhydride but with preserving of heterocyclic system takes place [22].

,-Difluoro--sultones, which are obtained from commercial available perfluoroolefines, during electrochemical fluorination yield appropriate perfluoroalkanesulfonyl fluorides [23].

In the case of sultone on the basis of tetrafluoroethylene perfluoromethanesulfonyl fluoride, COF₂, CH₄ and SO₂F₂ are obtained. This method is used for obtaining of perfluoroethanesulfonyl fluoride and perfluorobutansulfonyl fluoride.