Nonclassical ion
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.[2][3]
Examples
[edit | edit source]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]
The 2-norbornyl cation is one of the best characterized carbonium ions:
- C7H10 + H+ → C7H+11
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]
Further reading
[edit | edit source]- Brown, H. C. (with commentary by P. v. R. Schleyer) The Nonclassical Ion Problem; Plenum Press: New York /Springer 1977 [7]
- Moss, R.A. The 2-norbornyl cation: a retrospective J. Phys. Org. Chem. 2014, 27, 374-379 [8]
See also
[edit | edit source]References
[edit | edit source]- ^ a b Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
- ^ 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]