Chemistry:Carbonium ion

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Not to be confused with Carbenium ion.
Short description: Any cation that has a pentavalent carbon atom
Structure of the 2-norbornyl cation based on X-ray crystallography. All other C-C bond lengths are normal (ca. 1.5 Å).[1]

In chemistry, a carbonium ion is a cation that has a pentacoordinated carbon atom.[2] 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.[3]

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,[1] 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 C4H+7 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 C4H+7 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 SbF–6 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.[4]

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.[5]

Methanium and ethanium

The simplest carbonium ions are also the least accessible. In methanium (CH+
5), carbon is covalently bonded to five hydrogen atoms.[6][7][8][9]

The ethanium ion C
2H+
7 has been characterized by infrared spectroscopy.[10] The isomers of octonium (protonated octane, C
8H+
19) have been studied.[11]

Pyramidal carbocations

  • An example of the monovalent carbocation
    An example of the monovalent carbocation
  • An example of the divalent carbocation
    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 square pyramidal ion will carry a charge of +1, the pentagonal pyramidal ion will carry +2.

X-ray crystallography confirms that hexamethylbenzene dication ([C6(CH3)6]2+) is pentagonal-pyramidal.[12]

Applications

Carbonium ions are intermediates in the isomerization of alkanes catalyzed by very strong solid acids.[13] Such carbonium ions are invoked in cracking (Haag-Dessau mechanism).[14][15][16]

See also

  • The complex pentakis(triphenylphosphinegold(I))methanium (Ph
    3    PAu)
    5    C+
    .[17]
  • Fluxional molecules
  • More carbonium ions called non-classical ions are found in certain norbornyl systems.
  • Onium compounds
  • Carbenium ion

References

  1. ↑ 1.0 1.1 Scholz, F.; Himmel, D.; Heinemann, F. W.; Schleyer, P. v R.; Meyer, K.; Krossing, I. (2013-07-05). "Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation" (in en). Science 341 (6141): 62–64. doi:10.1126/science.1238849. ISSN 0036-8075. PMID 23828938. Bibcode: 2013Sci...341...62S. 
  2. ↑ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Carbonium ion". doi:10.1351/goldbook.C00839
  3. ↑ Thomas H. Lowery; Kathleen Schueller Richardson (1981). Mechanism and Theory in Organic Chemistry, Second Edition. Harper and Rowe. p. 396. ISBN 0-06-044083-X. 
  4. ↑ 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 C4H+7 and C5H+9 Ions: Relative Stability and Dynamic Aspects of the Cyclopropylcarbinyl vs Bicyclobutonium Ions" (in en). Journal of the American Chemical Society 130 (28): 9168–9172. doi:10.1021/ja802445s. ISSN 0002-7863. PMID 18570420. https://pubs.acs.org/doi/10.1021/ja802445s. 
  5. ↑ Carey, F.A.; Sundberg, R.J.. Advanced Organic Chemistry Part A (2nd ed.). 
  6. ↑ Boo, Doo Wan; Lee, Yuan T (1995). "Infrared spectroscopy of the molecular hydrogen solvated carbonium ions, CH+5(H2)n (n = 1–6)". The Journal of Chemical Physics 103 (2): 520. doi:10.1063/1.470138. Bibcode: 1995JChPh.103..520B. https://zenodo.org/record/1232938. 
  7. ↑ Asvany, O.; Kumar P, P.; Redlich, B.; Hegemann, I.; Schlemmer, S.; Marx, D. (2005). "Understanding the Infrared Spectrum of Bare CH5+". Science 309 (5738): 1219–1222. doi:10.1126/science.1113729. PMID 15994376. Bibcode: 2005Sci...309.1219A. 
  8. ↑ Xiao-Gang Wang; Tucker Carrington Jr (2016). "Calculated rotation-bending energy levels of CH5+ and a comparison with experiment". Journal of Chemical Physics 144 (20): 204304. doi:10.1063/1.4948549. PMID 27250303. Bibcode: 2016JChPh.144t4304W. 
  9. ↑ H. Schmiedt; Per Jensen; S. Schlemmer (2017). "Rotation-vibration motion of extremely flexible molecules - The molecular superrotor". Chemical Physics Letters 672: 34–46. doi:10.1016/j.cplett.2017.01.045. Bibcode: 2017CPL...672...34S. 
  10. ↑ Yeh, L. I; Price, J. M; Lee, Yuan T (1989). "Infrared spectroscopy of the pentacoordinated carbonium ion C2H+7". Journal of the American Chemical Society 111 (15): 5597. doi:10.1021/ja00197a015. 
  11. ↑ Seitz, Christa; East, Allan L. L (2002). "Isomers of Protonated Octane, C8H+19". The Journal of Physical Chemistry A 106 (47): 11653. doi:10.1021/jp021724v. Bibcode: 2002JPCA..10611653S. 
  12. ↑ Malischewski, Moritz; Seppelt, K. (2016-11-25). "Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication C6(CH3)2+6" (in en). Angewandte Chemie International Edition 56 (1): 368–370. doi:10.1002/anie.201608795. ISSN 1433-7851. PMID 27885766. 
  13. ↑ Sommer, J; Jost, R (2000). "Carbenium and carbonium ions in liquid- and solid-superacid-catalyzed activation of small alkanes". Pure and Applied Chemistry 72 (12): 2309. doi:10.1351/pac200072122309. 
  14. ↑ Office of Energy Efficiency and Renewable Energy, U.S. DOE (2006). "Energy Bandwidth for Petroleum Refining Processes"
  15. ↑ Kotrel, S.; Knözinger, H.; Gates, B.C. (April 2000). "The Haag–Dessau mechanism of protolytic cracking of alkanes" (in en). Microporous and Mesoporous Materials 35-36: 11–20. doi:10.1016/S1387-1811(99)00204-8. Bibcode: 2000MicMM..35...11K. https://linkinghub.elsevier.com/retrieve/pii/S1387181199002048. 
  16. ↑ Marczewski, M.; Popielarska, D.; Marczewska, H. (2016). "Pentane Transformations over Sulfated Alumina catalyst". Reaction Kinetics, Mechanisms and Catalysis 118: 267–280. doi:10.1007/s11144-016-0978-9. 
  17. ↑ George A. Olah (1998). Onium Ions. John Wiley & Sons. ISBN 9780471148777. 



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