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Methylidyne radical

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Methylidyne radical

Methylidyne radical
Names
Systematic IUPAC name
Hydridocarbon[1]
Identifiers
7801830
ChEBI  Y
24689
Jmol-3D images Image
Properties
CH3•
Molar mass 13.0186 g mol−1
Appearance Colourless gas
Reacts
Thermochemistry
183.04 J K−1 mol−1
594.13 kJ mol−1
Related compounds
Related compounds
Methyl (CH3)
Methylene (CH2)
Carbide (C)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Methylidyne (also systematically named hydridocarbon(•)), also called carbyne, is an chemical formula CH (also written as [CH]).

Methylidyne is the simplest carbyne. It is a highly reactive gas, that is quickly destroyed in ordinary conditions but is abundant in the interstellar medium (and was one of the first molecules to be detected there).[2]

Contents

  • Nomenclature 1
  • Chemical properties 2
    • Radicality 2.1
    • Amphotericity 2.2
    • Fischer-Tropsch intermediate 2.3
  • Preparation 3
  • See also 4
  • References 5

Nomenclature

The trivial name carbyne is the preferred IUPAC name. The systematic names methylidyne, and hydridocarbon(•), valid IUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively.

Methylidyne is viewed as methane with three hydrogen atoms removed. By default, this name pays no regard to the radicality of methylidyne. When the radicality is considered, the radical states with one unpaired electron are named methylylidene, whereas the radical excited states with three unpaired electrons are named methanetriyl.

Chemical properties

Radicality

All electronic states of methylidyne yield one unpaired electron, giving rise to varying degrees of radical chemistry. The ground state is a doublet radical with one unpaired electron (X2Π), and the first two excited states are a quartet radical with three unpaired electrons (a4Σ) and a doublet radical with one unpaired electron (A2Δ). With the quartet radical only 71 kJ above the ground state, a sample of methylidyne exists as a mixture of electronic states even at room temperature, giving rise to complex reactions.[3] For example, reactions of the doublet radical with non-radical species involves insertion or addition, whereas reactions of the quartet radical generally involves only abstraction.

[CH](X2Π) + H
2
O
→ [CHO] + H
2
or [CH
2
(OH)]
[CH]3•(a4Σ) + H
2
O
[CH
2
]
+ [HO]

Amphotericity

The methylylidyne group (-C) in carbynes such as methylidyne can accept an electron-pair donating ligand into the molecule by adduction:

[CH] + L → [CHL]

Because of this acceptance of the electron-pair donating ligand, methylidyne has Lewis-acidic character.[4]

The methylylidyne group can also donate an electron-pair to an accepting centre by adduction:

M + [CH][M(CH)]

Because of this donation of the electron-pair, methylidyne has Lewis-basic character. An example is HCCo
3
(CO)
9
.[5] Since a Lewis base is also Brønsted base,[6] methylidyne can in theory be protonated to form a conjugate acid, which is methyliumyl or dihydridocarbon(•+) (CH•+
2
).

Fischer-Tropsch intermediate

Methylidyne molecules bound to the catalyst's surface are assumed to occur as intermediates in the Fischer-Tropsch process for conversion of CO to hydrocarbons. A hypothetical sequence is

CO → COH → HC3• → H2C2•

The methylene radical H2C2• would then insert itself into an hydrocarbon's backbone, extending it.[7]

Preparation

Methylidine can be prepared from bromoform.[5]

See also

References

  1. ^ "hydridocarbon (CHEBI:51382)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute. IUPAC Name. 
  2. ^ Encyclopedia of Astrobiology, Volume 1 edited by Ricardo Amils, José Cernicharo Quintanilla, Henderson James Cleaves, William M. Irvine, Daniele Pinti, Michel Viso. 2011, Springer: Heidelberg
  3. ^ Brooks, Bernard R.; Schaefer III, Henry F. (1 December 1977). "Reactions of Carbynes. Potential Energy Surfaces for the Doublet and Quartet Methylidyne (CH) Reactions with Molecular Hydrogen". The Journal of Chemical Physics (AIP Publishing) 67 (11): 5146–5151.  
  4. ^ Anderson, Stuart, M.; McCurdy, K. E.; Kolb, C. E. (February 1989). "The Methylidyne Radical + Carbon Monoxide Reaction: Rate Coefficient for Carbon Atom Exchange at 294 K". The Journal of Physical Chemistry (ACS Publications) 93 (3): 1042–1048.  
  5. ^ a b Nestle, Mara O.; Hallgren, John E.; Seyferth, Dietmar; Dawson, Peter; Robinson, Brian H. (1 January 1980). "μ3-Methylidyne and μ3-Benzylidyne-Tris(Tricarbonylcobalt)". In Busch, Daryle H. Inorganic Syntheses, Vol. 20 (PDF). John Wiley & Sons, Inc. pp. 226–229.  
  6. ^ Housecroft, Catherine E.; Sharpe, Alan G. (2012). "Acids, bases and ions in aqueous solution". Inorganic Chemistry (4th ed.). Pearson Education, Ltd. p. 227.  
  7. ^ W. A. Herrmann "Organometallic Aspects of the Fischer-Tropsch Synthesis" Angewandte Chemie International Edition in English, 1982, Volume 21, Issue 2, pages 117–130. doi:10.1002/anie.198201171
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