Most people who have studied biology or medicine in any depth in college often have to take an organic chemistry course. During that time, students typically learn about various types of reactions and their mechanisms, including those of acid-catalyzed or acid-promoted reactions. Almost all of these reactions involve positively-charged, “onium ion” intermediates. The species involved can include positively charged oxygen atoms (oxonium ions), nitrogen atoms (ammonium or nitrenium ions), phosphorous (phosphonium ions), sulfur (sulfonium ions), iodine (iodonium or periodonium ions), chlorine (chloronium ions), bromine (bromonium ions), mercury (mercurinium ions), and, of course, carbon (carbocations).
Understanding the significance of carbocations in organic chemistry requires a little perspective. In the early part of the 20th century, physical organic chemistry was beginning to become a thriving field. The introduction of physical methods into chemistry (especially spectroscopy) gave chemists new, extremely powerful methods of characterization of compounds. Chemists desired not only to come up with new transformations, but to apply these physical methods in understanding the steps involved as well. The reigning dogma (propounded by Sir. C. K. Ingold), was that in order to know if an intermediate lay on the reaction pathway, it should be isolated and characterized. What Ingold did not know, that is taken for granted today, is that stability and reactivity are opposing properties. That is, if something can be isolated, it is generally not very reactive. Conversely, the more reactive something is, the more difficult it becomes to isolate.
Thus, after Meerwein and Whitmore introduced the theory of the involvement of carbocations as intermediates in acid-catalyzed organic reactions, attempts were made by numerous chemists to isolate or observe them. The triphenylmethyl cation was able to be isolated and studied (this cation is extremely stable due to the large delocalization of charge into the phenyl rings); this is even done in undergraduate labs today! Solutions of the triphenylmethyl cation in sulfuric acid are extremely stable and have a bright yellow color. However, other carbocations could not be isolated despite numerous attempts, and so chemists had to resort to solvolytic methods for their study. In a solvolysis reaction, the carbocation precursor (with a good nucleofugal (leaving) group) is refluxed in a polar solvent. The product(s) are then isolated and the structure of the carbocation intermediate determined based on the product distribution. This may seem like a crude way of doing things (and in retrospect, it is), but it still allowed some brilliant deductions to be made, attesting to the quality of the minds of the people who were involved in these studies.
Key among these initial investigators was Saul Winstein, who was at UCLA. Winstein is especially noteworthy in that he advanced the concept of the “non-classical” carbocation (although the person who actually coined the term is John D. Roberts). Non-classical carbocations can loosely be described as pentavalent, delocalizing charge through σ rather than π interactions. Winstein also came up with numerous other concepts now in the parlance of organic chemistry such as ion pairing, internal return, anchimeric assistance, and homoaromaticity, among others. He was a brilliant chemist and (in my opinion) undoubtedly would have shared the Nobel Prize with Prof. Olah had it not been for his untimely demise in 1969.
At around the same time this was going on, other developments in acid-promoted organic reactions were taking place. Friedel-Crafts chemistry (which is routinely taught to undergraduates these days) was developing into a field of industrial importance, and initial studies on hydrocarbon isomerization were being carried out (most notably by P. D. Bartlett at Harvard). All of these lay the stage for the rather climactic entrance of George Olah into the world of carbocation chemistry.
George Olah had started his career in the Technical University of Budapest and in a few short years had carved out a reputation for himself as a creative organofluorine chemist. In his biography, he recalled having to literally work on a balcony of the chemistry building, because his former boss was unwilling to grant him any lab space! He also mentioned that it was not possible to procure a lot of the reagents that he needed for his research, such as HF, FSO3H, and BF3, and he actually had to make those himself. Things turned around however, and Olah fled Hungary during the 1957 revolution with his family and migrated to Canada, where he started work for the Dow Chemical Company. There, he was able to pursue his research interests with earnest and became known as a giant in the field of Friedel-Crafts chemistry. It was also at this time that Olah became interested in tackling the chemistry of carbocations, since those are well-known intermediates in Friedel-Crafts reactions. Olah had the intuition to realize that carbocations would be extremely strong Lewis acids, and that their isolation would therefore require discovering even stronger Lewis acids than those known at the time. He then systematically screened all the high-valent metal fluorides known and eventually came across antimony pentafluoride, SbF5. Antimony pentafluoride is a colorless, fuming, extremely viscous liquid, and is still one of the strongest known Lewis acids to this day. Olah found that addition of t-butyl fluoride to excess antimony pentafluoride resulted in a solution of the t-butyl cation (paired with SbF6- or Sb2F11- counterions). This discovery marked a watershed in the field of carbocation chemistry. Up to that point, carbocations were thought of as simply being too energetic or unstable to isolate; Olah’s discovery enabled the direct observation (by NMR spectroscopy) of these otherwise transient species! As the Nobel committee was to later comment, “George Olah gave the cations of carbon long life”. This longer life was several orders of magnitude; carbocations could go from just hanging around for milli- or microseconds to several hours.
Olah later moved to the Western Reserve University in Cleveland, and was one of the people who oversaw the confederation with the neighboring Case Institute of Technology to become Case Western Reserve University, as it is known today. There, he extended his work to the chemistry of superacids. Bronsted superacids are defined as acids stronger than 100% sulfuric acid. Examples are chlorosulfonic acid (ClSO3H), fluorosulfonic acid (FSO3H), trifluoromethanesulfonic (triflic) acid (CF3SO3H), and the newly discovered carborane acids. It is known that the strength of liquid Bronsted acids can be increased by addition of a Lewis acid; this improves the charge delocalization in the counterion and thereby increases the effective proton activity (or acidity). Olah thus combined his prior experience with fluorosulfonic acid with antimony pentafluoride and came up with the acid mixture FSO3H-SbF5, which later became known as “Magic Acid” due to it’s ability to dissolve candle wax. This accidental observation led to new discoveries in superacidic hydrocarbon chemistry. Olah proposed that in superacid solutions, alkanes are protonated along C-H or C-C σ bonds, in a 3-center, 2-electron fashion. These then break down to yield either H2 or trivalent carbocations. This is simply a consequence of the extreme acidity of these systems; they are strong enough to protonate the weakest bases possible, σ bonds. This hypothesis was borne out through a series of papers with careful mechanistic studies. The applications of this chemistry are enormous; the industrial cracking of high-molecular weight hydrocarbons into lighter fractions is carried out over solid acid catalysts (usually zeolites) and is a multibillion-dollar industry; understanding the mechanistic details of these processes is therefore of great importance.
Olah was also well-known for his involvement in the norbornyl (or non-classical) ion controversy, where he took the opposite stance against H. C. Brown. Thanks to his discoveries enabling the direct observation of carbocations at variable temperature, Olah was able to prove, through careful NMR experiments, that the norbornyl ion has a bridged (or nonclassical) structure. The controversy was well-documented, and a lot of old-time chemists remember those days vividly.
Recently, Olah has been involved in promoting the “Methanol Economy” as a way of ensuring a sustainable future. I’ll mention more about this later.
For readers, I apologize for not including structures; this is just a consequence of my laziness and also due to the fact that almost everything mentioned here can be looked up in seconds on Wikipedia anyway.