Methenium

In organic chemistry, methenium (also called methylium, carbenium,[1] methyl cation, or protonated methylene) is a cation with the formula CH+
3. It can be viewed as a methylene radical (:CH
2) with an added proton (H+
), or as a methyl radical (•CH
3) with one electron removed. It is a carbocation and an enium ion, making it the simplest of the carbenium ions.[2]

Methenium
Names
IUPAC name
Carbanylium
Other names
Methyl cation; Methylium
Identifiers
3D model (JSmol)
ChEBI
UNII
Properties
CH3
Molar mass 15.035 g·mol−1
Related compounds
borane
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Structure

Experiments and calculations generally agree that the methenium ion is planar, with threefold symmetry.[2] The carbon atom is a prototypical (and exact) example of sp2 hybridization.

Preparation and reactions

For mass spectrometry studies at low pressure, methenium can be obtained by ultraviolet photoionization of methyl radical,[2] or by collisions of monatomic cations such as C+
 and Kr+
 with neutral methane.[3] In such conditions, it will react with acetonitrile CH
3CN to form the ion (CH
3)
2CN+
.[4]

Upon capture of a low-energy electron (less than 1 eV), it will spontaneously dissociate.[5]

It is seldom encountered as an intermediate in the condensed phase. It is proposed as a reactive intermediate that forms upon protonation or hydride abstraction of methane with FSO3H-SbF5. The methenium ion is very reactive, even towards alkanes.[6]

References
  1. "Ions, Free Radicals, and Radical-Ion", Nomenclature of Organic Compounds, Advances in Chemistry, 126, AMERICAN CHEMICAL SOCIETY, June 1974, pp. 216–224, doi:10.1021/ba-1974-0126.ch028, ISBN 978-0841201910
  2. L. Golob, N. Jonathan, A. Morris, M. Okuda, K.J. Ross (1972), "The first ionization potential of the methyl radical as determined by photoelectron spectroscopy". Journal of Electron Spectroscopy and Related Phenomena, volume 1, issue 5, pp. 506–508 doi:10.1016/0368-2048(72)80022-7
  3. R.B. Sharma, N.M. Semo, W.S. Koski (1987), "Dynamics of the reactions of methylium, methylene radical cation, and methyliumylidene with acetylene". Journal of Physical Chemistry, volume 91, issue 15, pp. 4127–4131 doi:10.1021/j100299a037
  4. Murray J. McEwan, Arthur B. Denison, Wesley T. Huntress Jr., Vincent G. Anicich, J. Snodgrass, M.T. Bowers (1989), "Association reactions at low pressure. 2. The methylium/methyl cyanide system". Journal of Physical Chemistry, volume 93, issue 10, pp. 4064–4068. doi:10.1021/j100347a039
  5. E.M. Bahati, M. Fogle, C.R. Vane, M.E. Bannister, R.D. Thomas and V. Zhaunerchyk (2009), "Electron-impact dissociation of CD+
    3     and CH+
    3     ions producing CD+
    2    , CH+
     and C+
     fragment ions". Physical Review, volume 79, article 052703 doi:10.1103/PhysRevA.79.052703
  6. Hogeveen, H.; Lukas, J.; Roobeek, C. F. (1969). "Trapping of the methyl cation by carbon monoxide; formation of acetic acid from methane". Journal of the Chemical Society D: Chemical Communications. 0 (16): 920. doi:10.1039/c29690000920. ISSN 0577-6171.
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