Acids, acid superacids

My work on long-lived (persistent) carbocations dates back to the late 1950s at Dow and resulted in the first direct observation of alkyl cations. Subsequently, a wide spectrum of carbocations as long-lived species was studied using antimony pentafluoride as an extremely strong Lewis acid and later using other highly acidic (superacidic) systems. 

I would like to credit especially the fundamental contributions of Ron Gillespie to strong acid (superacid) chemistry and also to recall his generous help while I was still working at the Dow Laboratories in Canada. 1 reestablished contact with him during this time. We first met in the winter of 1956 at University College in London, where he worked with Christopher Ingold. Subsequently, he moved to McMaster... 

In the 1960s Gillespie suggested calling protic acids stronger than 100% sulfuric acid superacids. This arbitrary but most useful definition is now generally accepted. It should be mentioned, however, that... 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

The high acidity of superacids makes them extremely effective pro-tonating agents and catalysts. They also can activate a wide variety of extremely weakly basic compounds (nucleophiles) that previously could not be considered reactive in any practical way. Superacids such as fluoroantimonic or magic acid are capable of protonating not only TT-donor systems (aromatics, olefins, and acetylenes) but also what are called (T-donors, such as saturated hydrocarbons, including methane (CH4), the simplest parent saturated hydrocarbon. 

The vastly increased acidity of superacidic systems resulted in the significant new field of superacid chemistry. I began to ask myself whether a similar but more general approach could be used to produce electrophiles of greatly enhanced electron deficiency and thus reactivity. Over the years, there were a number of unexpected results in my own research work, as well as some previously unexplained observations buried in the literature, that seemed worth pursuing. 

Evaporators have performed successfully in a number of industrial applications. Typical materials that are processed in evaporators include Caustic Soda, Caustic Potash, Sodium Carbonate, Sodium Dichromate, Sodium Nitrate, Ammonium Nitrate, Phosphoric Acid Superacid, Potash, Urea, Glue, Glycerine,... 

Strong acids or superacid systems generate stable fluorinated carbocations [40, 42] Treatment of tetrafluorobenzbarrelene with arenesulfonyl chlorides in nitro-methane-lithium perchlorate yields a crystalline salt with a rearranged benzo barrelene skeleton [43] Ionization of polycyclic adducts of difluorocarbene and derivatives of bornadiene with antimony pentafluonde in fluorosulfonyl chloride yields stable cations [44, 45]... 

Of these the most extensively studied is fluorosulfuric acid, made by direct reaction of SO3 and HF. Its importance derives from its use as a solvent system and from the fact that its mixtures with SbFs and SO3 are amongst the strongest known acids (superacids, p. 570). Anhydrous HSO3F is a colourless, dense, mobile liquid which fumes in moist air mp—89.0°, bp 162.7° J25 1.726gcm 1725 1.56centipoise, /C25 1.085 X 10 " ohm cm . ... 

Sulphated zirconia catalysts can be acidic or superacidic depending on the method of treatment. A variety of acid-catalysed reactions, referred to earlier in this section, can be carried out with sulphated zirconia. Yadav and Nair (1999) have given a state-of-the art review on this subject. Examples of benzylation of benzene with benzyl chloride / benzyl alcohol, alkylation of o-xylene with. styrene, alkylation of diphenyl oxide with 1-dodecene, isomerization of epoxides to aldehydes, acylation of benzene / chlorobenzene with p-chloro benzoylchloride, etc. are covered in the review. 

The hydroxylation of C-H bonds by radicals, in contrast to the case of electrophilic oxidants, leads to alcohols without retention of stereochemical configuration. H202, activated by strong acids (superacids (277), HF-BF3 (272), A1C13 (213), and CF3COOH (214)) have been used for the hydroxylation of aromatic compounds. These acid-catalyzed hydroxylations cannot be applied for aliphatic reactants because the hydroxylated products are more reactive than the starting compounds and, hence, they are oxidized further. 

Acyl-transfer reactions are some of the most important conversions in organic chemistry and biochemistry. Recent work has shown that adjacent cationic groups can also activate amides in acyl-transfer reactions. Friedel-Crafts acylations are known to proceed well with carboxylic acids, acid chlorides (and other halides), and acid anhydrides, but there are virtually no examples of acylations with simple amides.19 During studies related to unsaturated amides, we observed a cyclization reaction that is essentially an intramolecular acyl-transfer reaction involving an amide (eq 15). The indanone product is formed by a cyclization involving the dicationic species (40). To examine this further, the related amides 41 and 42 were studied in superacid promoted conversions (eqs 16-17). It was found that amide 42 leads to the indanone product while 41... 

Prior syntheses of long-lived perhalomethyl cations have been achieved by halide abstraction by use of either a strong Lewis acid (in superacidic or S02C1F solvent media) or Ag+ (vide supra), but no routes to such carbocations through oxidative removal of a halogen bound to carbon were known. The objectives of the current work have been to provide structural and spectroscopic data that, thus far, have been lacking for these systems, and to provide oxidative routes to... 

ACID AND SUPERACID SOLID MATERIALS AS NONCONTAMINANT ALTERNATIVE CATALYSTS IN REFINING... 

Figure 13.2 Acid strength of acid and superacid solids determined by Hammet method. For a comparable purpose, the acid strengths of some liquid acids are also included. (After Ref. 12.)...

In the case of C4-hydrocarbons, the use of acid or superacid solids will depend on both the acid strength required in each reaction and the reaction conditions required to optimize the thermodynamic equilibrium (Figure 13.3). For example, catalysts with very high acid strength could be substituted for a solid with a lower acidity by increasing reaction temperature. This has been proposed in both the isomerization of lineal alkanes and in the alkylation of isobutene with olefins, although the thermodynamic equilibrium should also be considered. 

Figure 13.3 Possible catalytic uses of acid and superacid solids in the selective transformation of C4-hydrocarbons by acid reactions.

In conclusion, more efficient and clean solid (acid and superacid) catalysts will be used in the coming years to reduce not only the emission of environmentally harmful products but also the use of noxious catalysts. The optimal catalytic systems will be determined from the nature of acid strength of its active sites, the nature of the reaction, and the reaction conditions. 

In addition, a new physicochemical method to characterize the acid strength of acid and superacid solids also will be required in order to evaluate and compare acid strengths with those obtained with liquid acids. 

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