The Nonclassical Ion Problem - Twenty Years Later

Twenty years have passed since the author first questioned the importance of nonclassical structures for carbonium ions at the 1962 Symposium on "The Transition State" at Sheffield. At that time nonclassical structures were being advanced for essentially all carbocations known to man. There appeared to be general agreement that the 2-norbornyl cation was the most favorable example of a nonclassical ion. Accordingly, this cation was made the central feature of a careful reexamination of the evidence. After 20 years study, it is possible to say that many of the arguments and experimental evidence advanced to support the symmetrical abridged structure for the 2-norbornyl cation are no longer valid. Indeed, at this time, there is no definitive evidence which requires the nonclassical structure for the 2-norbornyl cation. Moreover, there are many experimental results which are clearly incompatible with this structure. INTRODUCTI ON In 1937 there was a famous Discussion of the Faraday Society on the topic, "Reaction Kinetics." This discussion was very influential in stimulating the development of our current ideas about the mechanisms of chemical reactions. To celebrate the twenty-fifth anniversary of this renowned Discussion, in 1962 The Chemical Society (London) sponsored a Symposium on the topic, "The Transition State," at the University in Sheffield, England. There I presented an invited lecture on the topic, "Strained Transition States" (Ref. 1). The main topic of my lecture was the role of steric strains in influencing the rate and course of chemical reactions (Ref. 2). However, I also discussed some of the unease I felt toward the rapidly proliferating nonclassical structures for carbocations which had been appearing in the literature for the past ten or SO years. This lecture brought about a vigorous reaction from supporters of the nonclassical position. Unfortunately, many of the papers published by these supporters appeared to have as their objective proving me wrong, rather than providing an irrefutable experimental basis for the concept. It was not possible for one individual with a few coworkers in this area to undertake to examine all of the nonclassical structures which had been proposed. There appeared to be general agreement that the 2-norbornyl cation represented the best possible example for a nonclassical ion (Ref. 3). Accordingly, we decided to concentrate our efforts on this system. HOW DID I GET INVOLVED? In 1946 the author proposed that the solvolysis of highly branched tertiary alkyl derivatives should be facilitated by the relief of steric strain (Eqn. 1) (Ref. 4). R R R—X —> + X (1) / R R R tetrahedral, planar, strained less strained * Based on a lecture presented at the Sixth IUPAC Conference on Physical Organic Chemistry at the Universit Catholique de Louvain, Louvain-la—Neuve, Belgium. 1783