Superacid

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. 

It was not fully realized until my breakthrough using superacids (vide infra) that, to suppress the deprotonation of alkyl cations to olefins and the subsequent formation of complex mixtures by reactions of olefins with alkyl cations, such as alkylation, oligomerization, polymerization, and cyclization, acids much stronger than those known and used in the past were needed. 

My research during the Cleveland years continued and extended the study of carbocations in varied superacidic systems as well as exploration of the broader chemistry of superacids, involving varied ionic systems and reagents. I had made the discovery of how to prepare and study long-lived cations of hydrocarbons while working for Dow in 1959-1960. After my return to academic life in Cleveland, a main... 

Over a decade of research, we were able to show that practically all conceivable carbocations could be prepared under what became known as stable ion conditions using various very strong acid systems (see discussion of superacids) and low nucleophilicity solvents (SO2, SO2CIF, SO2F2, etc.). A variety of precursors could be used under appropriate conditions, as shown, for example, in the preparation of the methylcyclopentyl cation. 

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... 

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. 

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. 

Acids are not limited to liquid (or gaseous) systems. Solid acids also play a significant role. Acidic oxides such as silica, silica-alumina, etc. are used extensively as solid acid catalysts. New solid acid systems that are stronger than those used conventionally are frequently called solid superacids. 

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. 

Dihydrogen (H2) is similarly protonated to by superacids, as was shown by studies using isotopic labeling. The structure of again involves 2e-3c bonding. 

Nonbonded electron pair donors (w-donors) are expectedly readily protonated (or coordinated) with superacids. Remarkably, this includes even xenon, long considered an inert gas. The protonation of some 7T-, (T- and -bases and their subsequent ionization to carbocations or onium ions is depicted as follows ... 

As expected, superacids were found to be extremely effective in bringing about protolytic transformations of hydrocarbons. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Many superacid-catalyzed reactions were found to be carried out advantageously not only using liquid superacids but also over solid superacids, including Nafion-H or certain zeolites. We extensively studied the catalytic activity of Nafion-H and related solid acid catalysts (including supported perfluorooctanesulfonic acid and its higher ho-... 

To solve some of the environmental problems of mixed-acid nitration, we were able to replaee sulfuric acid with solid superacid catalysts. This allowed us to develop a novel, clean, azeotropic nitration of aromatics with nitric acid over solid perfluorinated sulfonic acid catalysts (Nafion-H). The water formed is continuously azeotroped off by an excess of aromatics, thus preventing dilution of acid. Because the disposal of spent acids of nitration represents a serious environmental problem, the use of solid aeid eatalysts is a significant improvement. 

Processing heavy oils and bitumens represents a challenge for the current refinery processes, because heavy oils and bitumens poison the metal catalysts used m the refineries. In our research at the Loker Institute, we found the use of superacid catalysts, which are less sensitive to heavy oils, an attractive solution to their processing, particularly hydrocracking. 

Different types of other coal liquefaction processes have been also developed to convert coals to liqnid hydrocarbon fnels. These include high-temperature solvent extraction processes in which no catalyst is added. The solvent is usually a hydroaromatic hydrogen donor, whereas molecnlar hydrogen is added as a secondary source of hydrogen. Similar but catalytic liquefaction processes use zinc chloride and other catalysts, usually under forceful conditions (375-425°C, 100-200 atm). In our own research, superacidic HF-BFo-induced hydroliquefaction of coals, which involves depolymerization-ionic hydrogenation, was found to be highly effective at relatively modest temperatnres (150-170°C). 

The hydrocarbon research program of the Toker Institute was able in many ways to build on and utilize results of our fundamental work on superacid-catalyzed reactions and their mechanistic aspects (includ-... 

Under superacidic, low nucleophilicity so-called stable ion conditions, developing electron-deficient carbocations do not find reactive external nucleophiles to react with thus they stay persistent in solution stabilized by internal neighboring group interactions. 

Protonated methanes and their homologues and derivatives are experimentally indicated in superacidic chemistry by hydrogen-deuterium exchange experiments, as well as by core electron (ESCA) spectroscopy of their frozen matrixes. Some of their derivatives could even be isolated as crystalline compounds. In recent years, Schmidbaur has pre-... 

The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

The formation of the (X-delocalized norbornyl cation via ionization of 2-norbornyl precusors in low-nucleophilicity, superacidic media, as mentioned, can be considered an analog of an intramolecnlar Friedel-Crafts alkylation in a saturated system. Indeed, deprotonation gives nortricyclane,... 

Protoiiation (and protolysis) of alkanes is readily achieved with superacids. The protonation of methane itself to CH5, as discussed earlier, takes place readily. 

The isomerization of butane to iso butane in superacids is illustrative of a protolytic isomerization, where no intermediate olefins are present in equilibrium with carbocations. 

The superacid-catalyzed cracking of hydrocarbons (a significant practical application) involves not only formation of trivalent carbo-cationic sites leading to subsequent /3-cleavage but also direct C-C bond protolysis. 

Not only protolytic reactions but also a whole range of varied elee-trophilic reactions can be carried out on alkanes under superacidic conditions. 

A fundamental difference exists between conventional acid-catalyzed and superacidic hydrocarbon chemistry. In the former, trivalent car-benium ions are always in equilibrium with olefins, which play the key role, whereas in the latter, hydrocarbon transformation can take place without the involvement of olefins through the intermediacy of five-coordinate carbocations. 

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