Perfluoroalkane sulfonic acids

The perfluoroalkane sulfonic acids were fkst reported ki 1954. Trifluoromethanesulfonic acid was obtained by the oxidation of bis(ttifluoromethyl thio) mercury with aqueous hydrogen peroxide (1). The preparation of a series of perfluoroalkanesulfonic acids derived from electrochemical fluotination (ECF) of alkane sulfonyl haUdes was also disclosed ki the same year (2). The synthetic operations employed when the perfluoroalkanesulfonic acid is derived from electrochemical fluotination, which is the best method of preparation, are shown ki equations 1—3. 

This article focuses on the commercial, ethylene-based ionomers and includes information on industrial uses and manufacture. The fluorinated polymers used as membranes are frequently included in ionomer reviews. Owing to the high concentration of polar groups, these polymers are generally not melt processible and are specially designed for specific membrane uses (see Fluorine compounds, organic—perfluoroalkane sulfonic acids Membrane technology). 

Germain and Commeyras have found that perfluoro sulfonic esters and fluorosulfates are formed in high yields by direct anodic oxidation of R,I in perfluoroalkane sulfonic acids and fluorosulfuric acid (Eqs. 12 and 13) [36]. 

Perfluoroalcoholates, 16 126 Perfluoro alcohols, 16 124 Perfluoroalkanes, 16 124 Perfluoroalkane sulfonic acids, 2 140-142, 3 421 24... 

Strong Brpnsted acids are also available to induce acylations.3,8,9 Perfluoroalkane-sulfonic acids were shown to be highly effective. Certain metal powders, such as Zn, Cu, Al, and Fe, were also found to effect acylations with acyl chlorides. The de facto catalysts are the in situ formed corresponding metal halides.3,8 A number of other catalysts were developed over the years however, many of these are effective only for the acylation of highly reactive aromatics, such as heterocycles.9... 

The production of industrially important perfluoroalkane sulfonic acids is generally accomplished by electrochemical fluorination. This method of preparation remains expensive and proceeds in good yields only for short hydrocarbon chains.30 Recently however, Wakselman and Tordeux have described a chemical method for the preparation of trifluoromethane sulfonic acid.31 The procedure involves reaction of a metal selected from zinc, cadmium, manganese, and aluminum with sulfur dioxide in DMF, followed by the introduction of trifluoromethyl bromide under slight pressure. The intermediate sulfinate is subsequently oxidized by hydrogen peroxide, and then hydrolyzed which leads to formation of the trifluoromethane sulfonic acid. Successful extension of the sulfination process to the modification of PCTFE should result in the formation of a sulfinated polymer which can ultimately be oxidized to give a sulfonic-acid modified polymer. 

The copolymerization between trioxane and suitable comonomers (ethylene oxide, 1,3-dioxolane, diethylene glycol formal, 1,4-butane diol formal in amounts of 2-5% by weight) is performed using cationic initiators. The cationic initiators could be Lewis acids, such as BF3 or its etherate BF3Bu20 which was used, for example by Celanese (the mechanism of this reaction was studied in detail [163,164]) or protic acids such as perchloric acid, perfluoroalkane sulfonic acids and their esters and anhydrides. Heteropoly acids were used and also a series of carbenium, oxocarbenium salts, onium compounds, and metal chelates. To regulate the molecular weight chain-transfer agents, such as methylal and butylal, are added. 

A second factor which is responsible for the discovery of many new compounds is the electrochemical technique of Simons 281, 282) for the synthesis of fluorides by the electrolysis of hydrogen fluoride containing various solutes. By this method many sulfonyl fluorides and perfluoroalkane sulfonic acids have been prepared for the first time. 

One of the driving forces behind recent developments in the synthesis and characterization of new acids is the requirement to replace mineral acids by reagents and catalysts that will lead to reduction in waste, in other words to an improved atom economy. Thus, HCl used in the synthesis of diamino diphenyl methane, an intermediate in polyurethane production, can be replaced by a silico-aluminate catalyst in which the acid sites of the zeolite are accessible through the external surface. Less conventionally, simple aromatic hydrocarbons can be nitrated using 60-70% nitric acid in the presence of the strong Lewis acids, lanthanide-perfluoroalkane sulfonic acid salts, such as the ytterbium(lll) compounds (VI) and... 

The perfluorosulfonyl fluorides obtained by the electrochemical fluorination process can be readily converted into the corresponding acids and salts used as surfactants (Fig. 2.4). Hydrolysis yields the perfluoroalkane sulfonic acid (I) or its salt (II). Primary or secondary amines yield sulfonamides (III), which can be converted to a carboxylic acid (IV) or an alcohol (V). The alcohol is an intermediate for nonionic surfactants (VI), phosphates (VII), or sulfates (VIII). The tertiary aminosulfonamide (IX) can form a cationic surfactant (X), a amphoteric surfactant (XI), or an amine salt (XII). 

Quaternary anunonium salts of perfluoroalkanesulfonic acids can be prepared by neutralizing perfluoroalkanesulfonic acid with tetraalkylanmionium hydroxide. This procedure is not economical, however, for industrial use. A discovery [134—136] that quaternary ammonium salts of perfluoroalkane sulfonic acids can be obtained directly by reacting the perfluoroalkanesulfofluoride with a tertiary amine and an alkoxysilane is the basis for a commercial... 

Table 3.12 Boiling Points of Perfluoroalkane-sulfonic Acids...

Trimethylsilyl trifluoromethanesulfonate (trimethylsilyl triflate) is the most synthetically useful representative of the family of trialkylsilyl perfluoroalkane-sulfonates (for a review, see reference 101) This reagent is commercially available or can be prepared easily by the reaction of chlorotrimethylsilane and triflic acid [101] It has wide application in organic synthesis as an excellent silylating reagent... 

Some [hydroxy(tosyloxy)iodo]perfluoroalkanes, RFI(OH)OTs, were prepared from RfI(02CCF3)2 by reaction with either a sulfonic acid, e.g.p-TsOH, or a silyl sulfonate, e.g. Me3SiOTf [30,31]. Several organic acids derived from phosphorus... 

In the preparation of mixed carboxylic-perfluoroalkane sulfonic anhydrides from the corresponding acids, water formahon is responsible for the low yields of the acylation product. By removing water from the reaction mixture by azeotropic distillation and adsorption onto molecular sieves, the yield of benzoylahon of para-xylene with BAC in the presence of perfluorobutanesulfonic acid (10% mol) is improved to 90%. 

Many acid-catalyzed reactions can be advantageously carried out using solid superacids instead of conventional acid systems. The reactions can be carried out in either the gaseous or the liquid phase. Using the example Nafion-H (a perfluoroalkane resin sulfonic acid, developed by DuPont) solid acid, several simple procedures were reported to carry out alkylation, transbromination, nitration, acetalization, hydration, and so on. 

Perfluoroalkane carboxylic and sulfonic acids and their derivatives have been synthesized by Zn-Barbier reactions using iodoperfluoroalkanes in dimethyl-sulfoxide (DMSO) as solvent [71]. A Zn/Cu (100 1) couple was used for most of the reactions but other couples such as Zn-Pb, Zn-Cd or Zn-Hg led to the same results ... 

Perfluoroalkane ionomers consist of a perfluorinated backbone and pendant chains terminated with an anionic group, for example with sulfonic acid groups (Nafion H) [181-185] ... 

Isolation of perfluoroalkane-sulfonate from acid etch bath... 

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