In chemistry, a nonclassical ion usually refers to carbonium ions, a family of organic cations. They are characterized by delocalized three-center, two-electron bonds. The more stable members are often bi- or polycyclic.Anslyn, E.V., Dougherty, D.A Modern Physical Organic Chemistry University Science Books 2005
Examples Historically, nonclassical ions were invoked to explain unusually fast solvolyses of steroidal, norbornyl, and cyclopropyl halides.Sykes P. A guide book to mechanism in organic chemistry 6th Ed., New Delhi: Orient Longman, 1986, p. 111 ff Capon, B., McManus, S. P. Neighboring Group Participation Vol. 1, Plenum, New York, 1976 Explanations for these rates was once controversial.Schneider, H.-J. The Controversy about Nonclassical Ions – Abandoned too Early? J. Phys. Org. Chem. 2018,
The 2-norbornyl cation is one of the best characterized carbonium ions:
In fact, it has emerged as the prototype for non-classical ions. As indicated first by low-temperature NMR spectroscopy and confirmed by X-ray crystallography, it has a symmetric structure with an group bonded to an alkene group, stabilized by a bicyclic structure.
Solvolyses of cyclopropylcarbinyl, cyclobutyl, and homoallyl esters are also characterized by very large rates, and have been shown to occur via a common nonclassical ion structure in the form of a bicyclobutonium ion.Saunders, M., Laidig, K.E., Wiberg, K.B., Schleyer Structures, energies, and modes of interconversion of ions J. Am. Chem. Soc., 1988, 110, 7652–7659 Siehl, H. U. The Conundrum of the (C4H7) Cation: Bicyclobutonium and Related Carbocations Adv. Phys. Org. Chem., 2018, 52, 1-47
Further reading Brown, H. C. (with commentary by P. v. R. Schleyer) The Nonclassical Ion Problem; Plenum Press: New York /Springer 1977 Moss, R.A. The 2-norbornyl cation: a retrospective J. Phys. Org. Chem. 2014, 27, 374-379
See also 2-Norbornyl cation Neighbouring group participation Carbocation Steric effects Solvation
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