In chemistry, a carbonium ion is a cation that has a pentacoordinated carbon atom.Thomas H. Lowery (1981). 006044083X, Harper and Rowe. 006044083X
2-Norbornyl cation
The 2-norbornyl cation is one of the best-characterized carbonium ions. It is 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 RCH
2+ group bonded to an
alkene group, stabilized by a bicyclic structure.
Cyclopropylmethyl cation
A non-classical structure for is supported by substantial experimental evidence from
solvolysis experiments and NMR studies. One or both of two structures, the cyclopropylcarbinyl cation and the bicyclobutonium cation, were invoked to account for the observed reactivity. The NMR spectrum consists of two
13C NMR signals, even at temperatures as low as −132 °C. Computations suggest that the energetic landscape of the system is very flat. The bicyclobutonium structure is computed to be 0.4 kcal/mol more stable than the cyclopropylcarbinyl structure. In the solution phase (SbF
5·SO
2ClF·SO
2F
2, with as the counterion), the bicyclobutonium structure predominates over the cyclopropylcarbinyl structure in a 84:16 ratio at −61 °C. Three other possible structures, two classical structures (the
Homoallylic cation and
cyclobutyl cation) and a more highly delocalized non-classical structure (the tricyclobutonium ion), are less stable.
The low-temperature NMR spectrum of a dimethyl derivative shows two methyl signals, indicating that the molecular conformation of this cation is not perpendicular (as in A), which possesses a mirror plane, but is bisected (as in B) with the empty p-orbital parallel to the cyclopropyl ring system:
In terms of
bent bond theory, this preference is explained by assuming favorable
orbital overlap between the filled cyclopropane bent bonds and the empty p-orbital.
Methanium and ethanium
The simplest carbonium ions are also the least accessible. In
methanium (), carbon is
covalently bonded to five
hydrogen atoms.
The ethanium ion has been characterized by infrared spectroscopy.
Pyramidal carbocations
File:Pyramidal ion 4 sided with numbers.jpg|An example of the monovalent carbocation
File:Pyramidal dikation, hexamethyl.jpg|An example of the divalent carbocation
One class of carbonium ions are called pyramidal carbocations. In these ions, a single carbon atom hovers over a four- or five-sided
polygon, in effect forming a
pyramid. The
ion will carry a charge of +1, the pentagonal pyramidal ion will carry +2.
X-ray crystallography confirms that hexamethylbenzene dication (C6(CH3)62+) is pentagonal-pyramidal.
Applications
Carbonium ions are intermediates in the isomerization of
catalyzed by very strong
.
Such carbonium ions are invoked in cracking (Haag-Dessau mechanism).
[Office of Energy Efficiency and Renewable Energy, U.S. DOE (2006). "Energy Bandwidth for Petroleum Refining Processes"]
See also
-
The complex pentakis(triphenylphosphinegold(I))methanium .
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Fluxional molecules
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More carbonium ions called non-classical ions are found in certain Norbornane systems.
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Onium compounds
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Carbenium ion
