Contemporary Problems in Carbonium Ion Chemistry I/II

Description

In 1965 a book by P. Bartlett appeared under the title "The Nonclassical Ions" 1). The book is a collection of papers reprinted from various journals. The many reviews that have appeared since 2-22) are either antiquated (the book published in 1972 12) covers the literature mainly before 1968) or relatively biased (e.g., 3.4,10» on brief 2, 7,11). This review attempts to discuss the various points of view on the "nonclassical" carbocations. The main point is to establish the relative role of "nonclassical" and "classical" ions in various chemical processes. The author has followed P. Bartlett's advice 1) that when setting forth the achievements of the human mind one should see how we came to the modern understanding of a given problem (" ... how we know what we know"). The theory of "nonclassical" ions offers an explanation of many unique chemical, stereochemical and kinetic peculiarities of bicyclic compounds. It has expanded our knowledge on chemical bonds in carbocations by introducing electron-deficient bonds (as in boron hydrides). It has accounted for many rearrangements of stable cations. As a "side" result our knowledge has been extended about ionization processes in a solution, as well as about stereochemical methods. 2 Main Terms of Nonclassical Carbocations In 1939 Hevell, Salas and Wilson 23) assumed an intermediate, "bridge" ion 2 to be formed when camphene hydrochloride 1 is rearranged into isobornyl chloride 3. This happened 17 years after Meerwein first postulated the intermediate formation of "carbonium" ions in chemical reactions.