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.

Examples

Historically, nonclassical ions were invoked to explain unusually fast solvolyses of steroidal, norbornyl, and cyclopropyl halides. Explanations for these rates was once controversial.

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.

Further reading

• Brown, H. C. (with commentary by P. v. R. Schleyer) The Nonclassical Ion Problem; Plenum Press:  New York /Springer 1977 https://www.springer.com/de/book/9781461341208

• Moss, R.A. The 2-norbornyl cation: a retrospective J. Phys. Org. Chem. 2014, 27, 374-379 https://onlinelibrary.wiley.com/doi/full/10.1002/poc.3290

See also

• 2-Norbornyl cation

• Neighbouring group participation

• Carbocation

• Steric effects

• Solvation

References