My Search for Carbocations and Their Role in Chemistry (Nobel Lecture)

  • Olah G
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" Every generation of scientific men (i.e. scientists) starts where the previous generation left off; and the most advanced discov-eries of one age constitute elementary axioms of the next. ---Aldous Huxley INTRODUCTION Hydrocarbons are compounds of the elements carbon and hydrogen. They make up natural gas and oil and thus are essential for our modern life. Burning of hydrocarbons is used to generate energy in our power plants and heat our homes. Derived gasoline and diesel oil propel our cars, trucks, air-planes. Hydrocarbons are also the feed-stock for practically every man-made material from plastics to pharmaceuticals. What nature is giving us needs, however, to be processed and modified. We will eventually also need to make hydrocarbons ourselves, as our natural resources are depleted. Many of the used processes are acid catalyzed involving chemical reactions proceeding through positive ion intermediates. Consequently, the knowledge of these intermediates and their chemistry is of substantial significance both as fun-damental, as well as practical science. Carbocations are the positive ions of carbon compounds. It was in 1901 that Norris la and Kehrman lb independently discovered that colorless triphe-nylmethyl alcohol gave deep yellow solutions in concentrated sulfuric acid. Triphenylmethyl chloride similarly formed orange complexes with alumi-num and tin chlorides. von Baeyer (Nobel Prize, 1905) should be credited for having recognized in 1902 the salt like character of the compounds for-med (equation 1). 1c He then proceeded to suggest a correlation between the appearance of color and formation of salt -the so called " halochromy " . Gomberg ld (who had just 150 Chemistry 1994 shortly before discovered the related stable triphenylmethyl radical) as well as Walden le contributed to the evolving understanding of the structure of related dyes, such as malachite green 1. Stable carbocationic dyes were soon found to be even present in nature. The color of red wine as well as of many flowers, fruits, leaves, etc. is due in part to flavylium and anthocyanin compounds formed upon cleavage of their respective glycosides. The constitution of flavylium and anthocyanin compounds envolved based on Robinson's and Willstatter's pioneering studies. Werner formulated lf the parent benzopyrilium or xanthylium salts 2 and 3 as oxonium salts, while Baeyer lg emphasized their great similarity to triarylmethylium salts and con-sidered them as carbenium salts. Time has indeed justified both point of view with the realization of the significance of the contribution of both oxonium and carbenium ion resonance forms. Whereas the existence of ionic triarylmethyl and related dyes was established at around the turn of the 20th century, the more general significance of car-bocations in chemistry was for long unrecognized. Triarylmethyl cations were considered as an isolated curiosity of chemistry, not unlike Gomberg's triarylmethyl radicals. Simple hydrocarbon cations in general were believed not only to be of unstable nature but their fleeting existence was even doubted. One of the most original and significant ideas in organic chemistry was the suggestion that carbocations (as we now call all the positive ions of carbon compounds) might be intermediates in the course of reactions that start from nonionic reactants and lead to nonionic covalent products. It was Hans Meerwein 2 who in 1922, while studying the Wagner rearran-gement of camphene hydrochloride 4 to isobornyl chloride 5, found that the rate of the reaction increased with the dielectric constant of the solvent. Further, he found that certain Lewis acid chlorides -such as SbCl 5 , SnCl 4 ,

Author-supplied keywords

  • Carbocations
  • Hydrocarbons
  • Nobel lecture
  • Reactive intermediates

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  • George A. Olah

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