Carbonium ion explained

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 in three-center, two-electron bonds. The more stable members are often bi- or polycyclic.^[1]

2-Norbornyl cation

The 2-Norbornyl cation is one of the best characterized carbonium ion. 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 RCH₂⁺ 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. he 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 (SbF₅·SO₂ClF·SO₂F₂, 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 homoallyl cation and cyclobutyl cation) and a more highly delocalized non-classical structure (the tricyclobutonium ion), are less stable.^[2]

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.^[3]

Methanium and ethanium

The simplest carbonium ions are also the least accessible. In methanium carbon is covalently bonded to five hydrogen atoms.^{[4] [5] [6] [7]}

The ethanium ion has been characterized by infrared spectroscopy.^[8] The isomers of octonium (protonated octane,) have been studied.^[9]

Pyramidal carbocations

Pyramidal Carbocations
		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 square pyramidal ion will carry a charge of +1, the Pentagonal pyramidal ion will carry +2. X-ray crystallography confirms that hexamethylbenzene dication ([C<sub>6</sub>(CH<sub>3</sub>)<sub>6</sub>]2+) is pentagonal-pyramidal.[10]
An example of the monovalent carbocation	An example of the divalent carbocation

Applications

Carbonium ions are intermediates in the isomerization of alkanes catalyzed by very strong solid acids.^[11] Such carbonium ions are invoked in cracking (Haag-Dessau mechanism).^{[12] [13] [14]}

See also

• The complex pentakis(triphenylphosphinegold(I))methanium .^[15]
• Fluxional molecules
• More carbonium ions called non-classical ions are found in certain norbornyl systems
• Onium compounds
• Carbenium ion

Notes and References

1. Book: Mechanism and Theory in Organic Chemistry, Second Edition. Thomas H. Lowery. Kathleen Schueller Richardson. Harper and Rowe. 1981. 0-06-044083-X. 396.
2. Olah. George A.. Surya Prakash. G. K.. Rasul. Golam. July 2008. Ab Initio/GIAO-CCSD(T) Study of Structures, Energies, and ¹³C NMR Chemical Shifts of and Ions: Relative Stability and Dynamic Aspects of the Cyclopropylcarbinyl vs Bicyclobutonium Ions. Journal of the American Chemical Society. en. 130. 28. 9168–9172. 10.1021/ja802445s. 18570420 . 0002-7863.
3. Book: F.A. . Carey . R.J. . Sundberg . Advanced Organic Chemistry Part A . 2nd.
4. 10.1063/1.470138. Infrared spectroscopy of the molecular hydrogen solvated carbonium ions, (n = 1–6). The Journal of Chemical Physics. 103. 2. 520. 1995. Boo. Doo Wan. Lee. Yuan T. 1995JChPh.103..520B.
5. . 309 . 5738 . 1219–1222 . Understanding the Infrared Spectrum of Bare CH₅⁺ . O. . Asvany . P. . Kumar P . B. . Redlich . 2005 . I. . Hegemann . S. . Schlemmer . D. . Marx . 10.1126/science.1113729 . 15994376. 2005Sci...309.1219A . 28745636 . free .
6. Xiao-Gang Wang . Tucker Carrington Jr . Calculated rotation-bending energy levels of CH₅⁺ and a comparison with experiment . Journal of Chemical Physics . 2016 . 144 . 20 . 204304 . 10.1063/1.4948549 . 27250303 . 2016JChPh.144t4304W .
7. H. Schmiedt . Per Jensen . S. Schlemmer . Rotation-vibration motion of extremely flexible molecules - The molecular superrotor . Chemical Physics Letters . 2017 . 672 . 34–46 . 10.1016/j.cplett.2017.01.045. 2017CPL...672...34S . free .
8. 10.1021/ja00197a015. Infrared spectroscopy of the pentacoordinated carbonium ion . Journal of the American Chemical Society. 111. 15. 5597. 1989. Yeh. L. I. Price. J. M. Lee. Yuan T.
9. 10.1021/jp021724v. Isomers of Protonated Octane, . The Journal of Physical Chemistry A. 106. 47. 11653. 2002. Seitz. Christa. East. Allan L. L. 2002JPCA..10611653S.
10. Malischewski. Moritz. Seppelt. K.. 2016-11-25. Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication . Angewandte Chemie International Edition. en. 56. 1. 368–370. 10.1002/anie.201608795. 27885766. 1433-7851.
11. 10.1351/pac200072122309. Carbenium and carbonium ions in liquid- and solid-superacid-catalyzed activation of small alkanes. Pure and Applied Chemistry. 72. 12. 2309. 2000. Sommer. J. Jost. R. free.
12. Office of Energy Efficiency and Renewable Energy, U.S. DOE (2006). "Energy Bandwidth for Petroleum Refining Processes"
13. Kotrel. S.. Knözinger. H.. Gates. B.C.. April 2000. The Haag–Dessau mechanism of protolytic cracking of alkanes. Microporous and Mesoporous Materials. en. 35-36. 11–20. 10.1016/S1387-1811(99)00204-8. 2000MicMM..35...11K .
14. 10.1007/s11144-016-0978-9 . Pentane Transformations over Sulfated Alumina catalyst . 2016 . Marczewski . M. . Popielarska . D. . Marczewska . H. . Reaction Kinetics, Mechanisms and Catalysis . 118 . 267–280 . free .
15. Book: George A. Olah . 1998 . Onium Ions . John Wiley & Sons . 9780471148777.