Olah G.A. - A Life of Magic Chemistry (2001)(en)

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The finding that highly deactivated aromatics do not react with NO2 salts is in accord with the finding that their greatly diminished-donor ability no longer suffices to polarize NO2 . Similarly, -donor hydrocarbons such as methane (CH4) are not able to affect such polarization. Instead, the linear nitronium ion is activated by the superacid. Despite the fact that NO2 is a small, triatomic cation, the nonbonded electron pairs on oxygen can further interact with the acid. This interaction can be considered to be protosolvation (the electrophilic equivalent of usual nucleophilic solvation), which tends to bend the linear NO2 ion. In the limiting case, de facto protonation forms NO2H2 . The protionitronium (which has even been observed by Helmut Schwarz by mass spectrometry) is a highly reactive ‘‘superelectrophile.’’ Protolytic solvation, however, can suffice in itself to achieve activation without necessarily forming the free NO2H2 dication.

Whereas the proton (H ) can be considered the ultimate Brønsted acid (having no electron), the helium dication (He2 ) is an even stronger, doubly electron-deficient electron acceptor. In a theoretical, calculational study we found that the helionitronium trication (NO2He3 ) has a minimum structure isoelectronic and isostructural with NO2H2 .

Charge-charge repulsion effects in protolytically activating charged electrophiles certainly play a significant role, which must be overcome. Despite these effects multidentate protolytic interactions with superacids can take place, increasing the electrophilic nature of varied reagents.

We found a way to overcome charge-charge repulsion when activating the nitronium ion when Lewis acids were used instead of strong Brønsted acids. The Friedel-Crafts nitration of deactivated aromatics and some aliphatic hydrocarbons was efficiently carried out with the NO2Cl/3AlCl3 system. In this case, the nitronium ion is coordinated to AlCl3.

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The protolytic activation of hydrocarbon ions (carbocations) can also be achieved. We have found that the electrophilic nature of trivalent carbenium ion centers can be greatly enhanced by decreasing neighboring group participation not only by - or n-donor ligands but also by hyperconjugatively interacting -donor alkyl groups. For example, the hyperconjugative effect of the methyl groups in the tert-butyl cation can be protosolvolytically decreased, resulting in the limiting case in a dipositive carbenium, carbonium dication.

This may be a factor while acid-catalyzed transformations (isomerization, alkylation) of saturated hydrocarbons proceed preferentially in excess strong acid media.

My group’s studies on superelectrophilic activation found that not only superacidic solutions but also solid acid systems can bring about such activation. Solid strong acids, possessing both Brønsted and Lewis acid sites, are of increasing significance. They range from supported or intercalated systems to highly acidic perfluorinated resinsulfonic acids (such as Nafion-H and its analogues) and to certain zeolites (such as H-ZSM-5) and acidic oxides. One of the major difficulties in characterizing solid acids is the accurate determination of their acidity. Frequently used methods are based on kinetic rather than thermodynamic measurements, which can give data on catalytic activity of the solid acids but not necessarily on their acidities.

To explain how solid acids such as Nafion-H or HZSM-5 can show remarkable catalytic activity in hydrocarbon transformations, the nature of activation at the acidic sites of such solid acids must be considered. Nafion-H contains acidic -SO3H groups in clustered pockets.

In the acidic zeolite H-ZSM-5 the active Brønsted and Lewis acid sites

˚ are in close proximity ( 2.5 A).

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Figure 12.1. Perfluorinated resinsulfonic acid similar to Nafion-H, showing clustering of SO3H groups; PTFE = poly(tetrafluoroethylene).

In these (and other) solid superacid catalyst systems, bior multidentate interactions are thus possible, forming highly reactive intermediates. This amounts to the solid-state equivalent of protosolvation resulting in superelectrophilic activation.

Nature is able to perform its own transformations in ways that chemists have only begun to understand. At enzymatic sites many significant transformations take place that, in a generalized sense, are also acid catalyzed (for example, electron-deficient metal ion-catalyzed processes). Because the unique geometry at enzyme sites can make multidentate interactions possible, these superelectrophilic chemical activations may have their equivalent in some enzymatic process. In recent years Thauer, for example, found an active metal free dehydrogenase enzyme. Because the involved mechanism cannot be that of metal coordination, it was suggested to involve carbocationic transformations (in a way again similar to superelectrophilic activation). Although I was never involved with enzymatic studies, sometimes in the literature reference is made to this ‘‘Olah-type’’ enzyme.

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My recent work on superelectrophiles emerged from my previous studies on superacidic carbocationic and onium systems. It led to the realization that a variety of electrophiles capable of further interaction (coordination) with strong Brønsted or Lewis acids can be greatly activated by them. Examples mentioned were onium and carboxonium ions, acyl cations, halonium, azonium, carbazonium ions, and even certain substituted carbocations and the like. This activation produces what I suggested should be called superelectrophiles, that is, electrophiles of doubly electron-deficient (dipositive) nature whose reactivity significantly exceeds that of their parents. Superelectrophiles are the de facto reactive intermediates of many electrophilic reactions in superacidic systems (including those involving solid superacids) and should be differentiated from energetically lower-lying, much more stable intermediates, which frequently are observable and even isolable but are not necessarily reactive enough without further activation.

Examples of some superelectrophiles so far studied and their parents are

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It should be recognized that superelectrophilic reactions can also proceed with only ‘‘electrophilic assistance’’ (solvation, association) by the superacids without forming distinct depositive intermediates. Protosolvolytic activation of electrophiles should always be considered in this context.

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and fuel our cars and airplanes, they form carbon dioxide and water. Thus they are used up and are nonrenewable (at least on the human time scale). We must find ways to replace our diminishing natural resources with hydrocarbons made by ourselves in a renewable, economical, and environmentally adaptable, clean way. In my view, this represents a most significant challenge for chemistry in the twenty-first century.

In an increasingly technological society, the world’s per capita resources have difficulty keeping up. Society’s demands, however, must be satisfied while at the same time safeguarding the environment to allow future generations to continue to enjoy planet Earth as a hospitable home. Establishing an equilibrium between mankind’s needs and the environment is a challenge we must meet.

Nature has given us a remarkable gift in the form of oil and natural gas. However, what was created over the ages, man is using up rather rapidly. The large-scale use of petroleum and natural gas to generate energy, and also as raw materials for diverse man-made materials and products (fuels, plastics, pharmaceuticals, dyes, etc.), developed mainly during the twentieth century. The U.S. energy consumption (and that of the majority of the world) is very heavily based on fossil fuels. Atomic energy and other sources (hydro, geothermal, solar) represent only 11–12%.

U.S. Energy Sources (%)

Some Western industrial countries, in contrast, get a significant part of their energy from nonfossil sources, such as hydro and atomic energy (vide infra). Regrettably, alternate energy sources such as hydroenergy or even solar energy (via various conversion process) still represent only a small part of our overall energy picture. Whereas atomic

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energy is man’s best hope for clean, practically unlimited energy for the foreseeable future, we must make it safer and solve problems of disposal and storage of radioactive waste byproduct.

Power Generated in Industrial Countries by Nonfossil Fuels (1990)

Oil use has grown to the point where the world consumption is 70– 75 million barrels a day (a barrel equals 42 gallons, i.e., some 160 liters) or some 10 million metric tons. Oil and gas are mixtures of hydrocarbons. As already mentioned, once we burn hydrocarbons, they are irreversibly used up and are not renewable on the human time scale. Fortunately, we still have significant worldwide fossil fuel reserves, including heavy oils, shale and tar-sands, and even larger deposits of coals (a complex mixture of carbon compounds more deficient in hydrogen) that can be eventually utilized, albeit at a higher cost. I am not suggesting that our resources will run out in the foreseeable future, but it is clear that they will become scarcer and much more expensive and will not last for very long. With the world population at 6 billion and rapidly growing, the demand for oil and gas will inevitably increase. It is true, however, that in the past dire predictions of fast-disappearing oil and gas reserves were always incorrect, and today our proven reserves are significantly higher than they were decades ago.

Proven oil reserves, instead of being depleted, as a matter of fact, tripled over the last 30 years and now are a trillion barrels. Natural gas reserves have grown even more. This seems so impressive that most people assume that there can be no oil or gas shortage in sight. However, inevitably increasing consumption by a growing world population

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Recognized Oil and Natural Gas Reserves (in billion tons) from 1960 to 1990

and the drive for improving standards of living makes it more realistic to consider per capita reserves. Doing this, it becomes evident that our known reserves, if we go on using them as in the past, will last for half a century. Even if we consider all other factors (new findings, savings, alternate sources, etc.) in the twenty-first century, we will face a major problem. Oil and gas will not be exhausted overnight, but market forces of supply and demand will start to drive prices up to levels that no one wants even to contemplate now ($100/barrel for oil may be just a beginning). By the second half of the century, if we do not find new solutions, mankind will face a real crisis with grave economic and societal consequences.

All of mankind wants the advantages an industrialized society can give its citizens. We all rely on energy, but the level of consumption is vastly different in the industrialized versus the developing world. For example, oil consumption per capita in China currently is 5 barrels/ year, but it is well over tenfold that amount in the United States. China’s oil use may, even under conservative estimates, double in a decade, whereas the bulk of its energy needs will continue to come from coal. This increase alone equals the amount of U.S. consumption, reminding us of the size of the problem we are facing. This estimate does not consider that vast numbers of Chinese (or Indians, etc.) would, instead of riding bicycles, drive their own cars and use other energy-consuming conveniences to the level common in the industrial world. However, they will (and may already) ride mopeds and enjoy

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other technical advances of our time, necessitating increasing use of energy. Do we in the industrialized world have a monopoly on a better life? I certainly don’t think so.

Generating energy by burning nonrenewable fossil fuels including oil, gas and coal at the present level, as I pointed out, will be possible only for a relatively short term, and even this causes serious environmental problems (vide infra). The advent of the atomic age opened up for mankind wonderful new possibilities but also created dangers and concerns for safety. I believe that it is tragic that these considerations, however justified, practically brought further development of atomic energy to a standstill in most of the Western world. Whether we like it or not, in the long run we have no known alternative to relying increasingly on atomic energy, but we must solve safety and environmental problems. Finding solutions is essential and entirely within the capability of mankind if we have the will and determination for it. After all, during World War II we created the atom bomb by a great national effort. Could the same country that harnessed the energy of the atom in a wartime effort not also solve problems for its safe, peaceful use? I believe we can and will.

We continue to burn our hydrocarbon resources mainly to generate energy and to use them as fuels. Diminishing supplies (and sharply increasing prices) will lead inevitably to the need to supplement or make them ourselves by synthetic manufacturing. Synthetic oil or gasoline products will be, however, much costlier. Nature’s petroleum oil and natural gas are still the greatest bargains we will ever have. A barrel of oil sells for around $30 (with market fluctuation). No synthetic manufacturing process will be able to come even close to this price, and we will need to get used to this, not as a matter of any government policy, but as a fact of market forces over which we have little control.

Synthetic oil is feasible and can be produced from coal or natural gas via synthesis gas (a mixture of carbon monoxide and hydrogen obtained from incomplete combustion of coal or natural gas). However, these are themselves nonrenewable resources. Coal conversion was used in Germany during World War II by hydrogenation or,