Fluorosulfurous acid

Perchloric acid (HCIO4 Ho —13.0), fluorosulfuric acid (HSO3F Ho — 15.1), and trifluoromethanesulfonic acid (CF3SO3H Ho —14.1) are considered to be superacids, as is truly anhydrous hydrogen fluoride. Complexing with Lewis acidic metal fluorides of higher valence, such as antimony, tantalum, or niobium pentafluoride, greatly enhances the acidity of all these acids. 

FLUORINECOMPOUNDS,INORGANIC - SULFUR - FLUOROSULFURIC ACID] (Volll)... 

Fluorosulfuric acid [7789-21-17, HSO F, is a colodess-to-light yellow liquid that fumes strongly in moist air and has a sharp odor. It may be regarded as a mixed anhydride of sulfuric and hydrofluoric acids. Fluorosulfuric acid was first identified and characterized in 1892 (1). It is a strong acid and is employed as a catalyst and chemical reagent in a number of chemical processes, such as alkylation (qv), acylation, polymerization, sulfonation, isomerization, and production of organic fluorosulfates (see Friedel-CRAFTSreactions). 

Table 1. Physical and Chemical Constants of Fluorosulfuric Acid...

Fluorosulfuric acid is stable to heat up to decomposition at about 900°C (13), where vapor-phase dissociation into hydrogen fluoride and sulfur trioxide probably occurs. Reviews of the chemistry and properties of fluorosulfuric acid have been pubUshed (14—16). 

Reactions. The reaction of fluorosulfuric acid and water is violent and exothermic it proceeds as follows ... 

Precipitated (hydrated) siUca reacts vigorously with fluorosulfuric acid to give siUcon tetrafluoride [7783-61-1] (21), but glass (qv) is not attacked in the absence of moisture (20). Alkali and alkaline-earth metal chlorides are readily converted to fluorosulfates by treatment with fluorosulfuric acid (7,13,22,23). 

Electrolysis of fluorosulfuric acid produces either S20 E2 [13709-32-5] (24) or SO2E2 [13036-75 ] plus OE2 (25), depending on specific conditions. Various reactions of fluorosulfuric acid with inorganic compounds are shown in Table 2, and with organic compounds in Table 3. 

Table 2. Reactions of Fluorosulfuric Acid and Inorganic Compounds...

Other studies which have been reported describe unusual chemistry such as HSO F—Nb(S02F) systems (42). Also the unique properties of fluorosulfuric acid have been found to provide unusual solvent systems, which can vary properties such as acidity, heats of solution, enthalpy, and heats of neutralization (43). 

Fluorosulfuric acid may be used to prepare diazonium fluorosulfates, ArN SO.F (44), which decompose on heating to give aryl (Ar) fluorosulfates (36,45). Aryl fluorosulfates are also obtained from arylsulfonyl chlorides and fluorosulfuric acid (35). Alkyl and other organofluorosulfates form during electrolysis of fluorosulfuric acid in the presence of organic species (46,47). 

Preparation and Manufacture. Fluorosulfuric acid, first prepared by combining anhydrous HF and cooled, anhydrous SO in a platinum container (1), has also been prepared from ionic fluorides or fluorosulfates and sulfuric acid (20,48). The reaction of chlorosulfuric acid (qv) with ionic fluorides also gives fluorosulfuric acid (49). 

Commercially, fluorosulfuric acid is made by processes utilizing the product as a solvent. Solutions of HF and SO in fluorosulfuric acid are mixed in stoichiometric quantities, or SO and HF are separately introduced into a stream of fluorosulfuric acid to produce essentially pure HSO F. Some of the product is then recycled (50,51). 

Fluorosulfuric acid can be very corrosive. A study of the corrosive properties of fluorosulfuric acid during preparation and use showed carbon steel to be acceptable up to 40°C, stainless steel up to 80°C, and aluminum alloys up to 130°C (52). 

Economic Aspects. U.S. manufacturers of fluorosulfuric acid are AUiedSignal and Du Pont. These companies have a combined aimual capacity estimated at 20,000 metric tons, most of which is used internally although some merchant sales exist. Fluorosulfuric acid is shipped in tank cars. 

Specifications and Analysis. Commercial fluorosulfuric acid contains approximately 98% HSO F and approximately 1% H2SO4 and lesser amounts of sulfur trioxide and dioxide. No free HF is present. 

Uses. Fluorosulfuric acid serves as catalyst in the alkylation (qv) of branched-chain paraffins (53—58) and aromatic compounds (59), and in the polymeriza tion of monoolefins (60) and rosin (61). Addition of strong Lewis acids, such as SbF, TaF, and NbF, to fluorosulfuric acid markedly increases... 

Derivatives. The nonmetaHic inorganic derivatives of fluorosulfuric acid are generally made indirectly, although complex fluorosulfates of the Group 15 (V) elements and of xenon can be made directly (85,86), as can the NO" and NO" 2 salts (26,27). 

Sodium fluorosulfate may be prepared by the action of fluorosulfuric acid on powdered, ignited sodium chloride (13) or of sulfur trioxide on sodium fluoride (48). In general, the alkah metal fluorosulfates may be prepared from the ammonium salt by evaporating a solution containing that salt and an alkah metal hydroxide (77). The solubiUties of some Group 1 and 2 fluorosulfates in fluorosulfuric acid have been deterrnined (93). 

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