Perfluoroalkanesulfonic acid fluorides

Perfluoroacylbenzene sulfonates, 12 174 Perfluoroacyl fluorides, 11 865 alkylation of, 11 883 Perfluoroalkanesulfonic acids, 12 191 Perfluoroalkoxy (PFA) fluorocarbon resins, 18 329... 

Perfluoroalkanesulfonic acids are mainly prepared by two methods. The first method based on electrochemical fluorination (ECF) in anhydrous hydrogen fluoride (AHF) is the most practical. Although fluorination of the alkanesulfonic acids cannot be carried out directly by ECF9,10, the alkanesulfonyl fluorides can be successfully fluorinated with fairly high yields. The prefluorinated sulfonyl fluoride produced may then be converted to the corresponding salt by alkaline hydrolysis. The free acids were obtained either by their distillation from a solution of the alkaline metal sulfonate in concentrated sulfuric acid or by treatment of the salt with strong acid-type ion exchange resin (equation 1). 

Numerous perfluoroalkane sulfonyl fluorides, R/S02F, are prepared by the electrolysis of sulfonyl chlorides in liquid hydrogen fluoride. From these, the corresponding perfluoroalkanesulfonic acids are obtained by (1) reaction of the sulfonyl fluoride with a base to give a salt, (2) distillation of the acid from a mixture of the salt with concentrated sulfuric acid (29, 36,115). 

The alkanesulfonic acids themselves are not suitable starting materials for electrochemical fluorination because the yields of perfluoroalkanesulfonic acids obtained from the free acids are much lower compared to the alkanesulfonyl fluorides and chlorides. Another major disadvantage of this approach is the formation of water, which may result in the formation of explosive oxygen difluoride (OF2). 

Perfluoroalkanesulfonic acid salts (e.g., potassium, lithium, ammonium, and diethanol ammonium salts) are useful surfactants and are prepared by the hydrolysis of the fluoride as shown in Scheme 18.1. Long-chain perfluoroalkanesulfonyl fluorides are only slowly hydrolyzed by water. For example, perfluorooctanesulfonyl fluoride is minimally hydrolyzed by water at 180°C, even... 

Similar to perfluoroalkanesulfonic acids, the yield of the fluorination reaction decreases with the increasing chain length of the starting material. For example, the electrochemical fluorination of acetyl fluoride yields 76% of perfluoroacetyl fluoride, whereas the fluorination of octanoyl fluoride yields only 10% of perfluorooctanoyl fluoride. The yield is also lower when the readily available acid chlorides, and not the fluorides, are employed as starting materials. A major side reaction of the fluorination of acid chlorides such as octanoyl chloride is the formation of cyclic products. Other by-products and impurities present in PFCA fluorides obtained by electrochemical fluorination include short-chain PFCA fluorides, perfluoroalkanes, COFj, and HF. Perfluorooctanoyl fluoride, the most important PFCA, was industrially synthesized from the corresponding chloride despite the lower yield and formation of cyclic by-products. ... 

Fluorination of alkanesulfonyl fluorides or chlorides gives higher yields of perfluoroalkanesulfonic acids than fluorination of alkanesulfonic acids [49,51,52,54]. Another disadvantage associated with the fluorination of free alkanesulfonic acids is the potential explosion hazard caused by excessive amounts of oxygen difluoride and hypofluorites formed as by-products [55]. 

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