The Nobel Prize in Chemistry 1994
George A. Olah
Presentation Speech by Professor Salo
Gronowitz of the Royal
Swedish Academy of Sciences
Translation from the Swedish text
Your Majesties, Your Royal Highnesses,
Ladies and Gentlemen,
The preparation of complicated organic compounds from simple and inexpensive starting materials is one of the prerequisites of our civilization, the chemical era in which we live. Organic synthesis has given us efficient methods to obtain drugs, vitamins, textile fibers, plastics, insecticides and herbicides, lacquers, paints and fuels. For chemists it is very important to understand in detail what is going on when the molecules in the starting materials react with each other and create the molecules characteristic of the product. This is the process of determining the mechanism of the reaction. Knowledge about mechanisms makes it possible to develop better and less expensive methods to prepare products of technical importance. My teacher, the late Professor Arne Fredga, said from this podium in 1975: "Not knowing the mechanism is like seeing the first and the last scene of Hamlet." The performance would not be very rewarding, and one would wonder what actually happened!
In many cases the reactions proceed via so-called intermediates, which are products with very short lifetimes. One type of reactive intermediates is called "carbocations." Charged atoms and groups of atoms are common in inorganic chemistry. All of us know about table salt, which consists of positively charged sodium ions, or cations, and negatively charged chloride ions, or anions. The opposite is true of the large number of organic compounds, especially hydrocarbons, which are composed of only two elements, carbon and hydrogen. There were many indications that carbocations were intermediates in two common and frequently used reactions in synthetic organic chemistry. In that case these intermediates had to have an extremely short lifetime, a billionth of a second or less, and due to their high reactivity their concentrations had to be very low. Their existence has been indicated by measurements of reaction rates and observations of the spatial arrangement of the atoms in space. For these purposes a variety of ingenious experiments have been carried out. However, nobody was able to see these carbocations, not even with the most powerful microscopes or by spectroscopic methods. These techniques can be regarded as extensions of human vision. Consequently there was no evidence for the existence of carbocations, in other words whether they were a reality independent of human consciousness, or were only created by human imagination in order to describe the experimental results.
Because it was not possible to detect carbocations with spectroscopic methods, different scientists interpreted their experiments differently, and a scientific feud took place in organic chemistry during the 1960s and 1970s.
Through a series of brilliant experiments Professor George Olah solved the problem. He created methods to prepare long-lived carbocations in high concentrations, which made it possible to study their structure, stability and reactions with spectroscopic methods. He achieved this by using special solvents, which did not react with the cations. He observed that in these solvents at low temperatures, carbocations could be prepared with the aid of superacids, acids eighteen powers of ten stronger than concentrated sulfuric acid. Through Olah's pioneering work he and the scientists who followed in his footsteps could obtain detailed knowledge about the structure and reactivity of carbocations. Olah's discovery resulted in a complete revolution for scientific studies of carbocations, and his contributions occupy a prominent place in all modern textbooks of organic chemistry.
Olah found that there are two groups of carbocations, the trivalent ones called carbenium ions, in which the positive carbon atom is surrounded by three atoms, and those in which the positive carbon is surrounded by five atoms, called carbonium ions. The disputed existence of these penta-coordinated carbocations was the reason for the scientific feud. By providing convincing proof that penta-coordinated carbocations exist, Olah demolished the dogma that carbon in organic compounds could at most be tetra-coordinated, or bind a maximum of four atoms. This had been one of the cornerstones of structural organic chemistry since the days of Kekulé in the 1860s.
Olah found that the superacids were so strong that they could donate a proton to simple saturated hydrocarbons, and that these penta-coordinated carbonium ions could undergo further reactions. This fact has contributed to a better understanding of the most important reactions in petrochemistry. His discoveries have led to the development of methods for the isomerization of straight chain alkanes, which have low octane numbers when used in combustion engines, to produce branched alkanes with high octane numbers. Furthermore, these branched alkanes are important as starting materials in industrial syntheses. Olah has also shown that with the aid of superacids it is possible to prepare larger hydrocarbons with methane as the building block. With superacid catalysis it is also possible to crack heavy oils and liquefy coal under surprisingly mild conditions.
I have in these few minutes tried to explain your immense impact on physical organic chemistry through your fundamental investigations of the structure, stability and reactions of carbocations. In recognition of your important contribution, the Royal Swedish Academy of Sciences has decided to confer upon you this year's Nobel Prize for Chemistry. It is an honour and pleasure for me to extend to you the congratulations of the Royal Swedish Academy of Sciences and to ask you to receive your Prize from the hands of his Majesty the King.
From Nobel Lectures, Chemistry 1991-1995, Editor Bo G. Malmström, World Scientific Publishing Co., Singapore, 1997
Copyright © The Nobel Foundation 1994