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Structure of the 2-norbornyl cation, an iconic nonclassical ion. The structure was deduced from X-ray crystallography. C-C bond lengths not labeled are normal (ca. 1.5 Å).[1]
Historically, nonclassical ions were invoked to explain unusually fast solvolyses of steroidal, norbornyl, and cyclopropyl halides.[4][5] Explanations for these rates was once controversial.[6]
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,[1] it has a symmetric structure with an RCH2+ 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.[7][8]
^Thomas H. Lowery; Kathleen Schueller Richardson (1981). Mechanism and Theory in Organic Chemistry, Second Edition. Harper and Rowe. p. 396. ISBN0-06-044083-X.
^Anslyn, E.V., Dougherty, D.A Modern Physical Organic Chemistry University Science Books 2005[1]
^Sykes P. A guide book to mechanism in organic chemistry 6th Ed., New Delhi: Orient Longman, 1986, p. 111 ff [2]
^Capon, B., McManus, S. P. Neighboring Group Participation Vol. 1, Plenum, New York, 1976 [3]
^Schneider, H.-J. The Controversy about Nonclassical Ions – Abandoned too Early? J. Phys. Org. Chem. 2018,[4]
^Saunders, M., Laidig, K.E., Wiberg, K.B., Schleyer Structures, energies, and modes of interconversion of C4H+7 ions J. Am. Chem. Soc., 1988, 110, 7652–7659 [5]
^Siehl, H. U. The Conundrum of the (C4H7) Cation: Bicyclobutonium and Related Carbocations Adv. Phys. Org. Chem., 2018, 52, 1-47 [6]
