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*Moss, R.A. ''The 2-norbornyl cation: a retrospective'' J. Phys. Org. Chem. 2014 , 27, 374-37
In chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, a nonclassical ion usually refers to carbonium ion
In chemistry, a carbonium ion is a cation that has a pentacoordinated carbon atom. They are a type of carbocation. In older literature, the name "carbonium ion" was used for what is today called carbenium. Carbonium ions charge is delocalized ...
s, 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. The2-norbornyl cation
In organic chemistry, the term 2-norbornyl cation (or 2-bicyclo .2.1eptyl cation) describes a carbonium ionic derivative of norbornane. A salt of the 2-norbornyl cation was crystallized and characterized by X-ray crystallography confirmed the n ...
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
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique based on re-orientation of atomic nuclei with non-zero nuclear spins in an external magnetic f ...
and confirmed by X-ray crystallography, 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. Siehl , H. U. ''The Conundrum of the (C4H7) Cation: Bicyclobutonium and Related Carbocations'' Adv. Phys. Org. Chem. , 2018, 52, 1-4/ref>
Further reading
*Brown, H. C. (with commentary by P. v. R. Schleyer) ''The Nonclassical Ion Problem''; Plenum Press: New York /Springer 197*Moss, R.A. ''The 2-norbornyl cation: a retrospective'' J. Phys. Org. Chem. 2014 , 27, 374-37
See also
*2-Norbornyl cation
In organic chemistry, the term 2-norbornyl cation (or 2-bicyclo .2.1eptyl cation) describes a carbonium ionic derivative of norbornane. A salt of the 2-norbornyl cation was crystallized and characterized by X-ray crystallography confirmed the n ...
* Neighbouring group participation
In organic chemistry, neighbouring group participation (NGP, also known as anchimeric assistance) has been defined by the International Union of Pure and Applied Chemistry (IUPAC) as the interaction of a reaction centre with a lone pair of electro ...
* Carbocation
Carbocation is a general term for ions with a positively charged carbon atom. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further ...
* Steric effects
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is generally a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape (conformational isomerism, co ...
* Solvation
Solvations describes the interaction of a solvent with dissolved molecules. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of the solute, includi ...
References
{{reflist Carbocations Ions Organic chemistry