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]

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]

Examples

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

• 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