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Methylidyne radical

Methylidyne radical
Names
Systematic IUPAC name
Hydridocarbon
hydridocarbon(•),[1]
Hydridocarbon(3•)[2]
Identifiers
3D model (JSmol)
7801830
ChEBI
24689
Properties
CH, CH, 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).
Infobox references

Methylidyne, or (unsubstituted) carbyne, is an organic compound whose molecule consists of a single hydrogen atom bonded to a carbon atom. It is the parent compound of the carbynes, which can be seen as obtained from it by substitution of other functional groups for the hydrogen.

The carbon atom is left with either one or three unpaired electrons (unsatisfied valence bonds), depending on the molecule's excitation state; making it a radical. Accordingly, the chemical formula can be CH or CH3• (also written as CH); each dot representing an unpaired electron. The corresponding systematic names are methylylidene or hydridocarbon(•), and methanetriyl or hydridocarbon(3•). However, the formula is often written simply as CH.

Methylidyne 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).[3]

Nomenclature

The trivial name carbyne is the preferred IUPAC name.

Following the substitutive nomenclature, the molecule is viewed as methane with three hydrogen atoms removed, yielding the systematic name "methylidyne".

Following the additive nomenclature, the molecule is viewed as a hydrogen atom bonded to a carbon atom, yielding the name "hydridocarbon".

By default, these names pay no regard to the excitation state of the molecule. When that attribute is considered, the states with one unpaired electron are named "methylylidene" or "hydridocarbon(•)", whereas the excited states with three unpaired electrons are named "methanetriyl" or "hydridocarbon(3•)".

Bonding

HCCo
3
(CO)
9
, a metal cluster complex with a methylidyne ligand.

As an odd-electron species, CH is a radical. The ground state is a doublet (X2Π). The first two excited states are a quartet (with three unpaired electrons) (a4Σ) and a doublet (A2Δ). The quartet lies at 71 kJ/mol above the ground state.[4]

Reactions of the doublet radical with non-radical species involves insertion or addition:

[CH](X2Π) + H
2
O
→ [CHO] + H
2
or [CH
2
(OH)]

whereas reactions of the quartet radical generally involves only abstraction:

[CH]3•(a4Σ) + H
2
O
[CH
2
]
+ [HO]

Methylidyne can bind to metal atoms as tridentate ligand in coordination complexes. An example is methylidynetricobaltnonacarbonyl HCCo
3
(CO)
9
.[5]

Occurrence and reactivity

Fischer–Tropsch intermediate

Methylidyne-like species are implied intermediates in the Fischer–Tropsch process, the hydrogenation of CO to produce hydrocarbons. Methylidyne entities are assumed to bond to the catalyst's surface. A hypothetical sequence is:[5]

MnCO + 1/2 H2 → MnCOH
MnCOH + H2 → MnCH + H2O
MnCH + 1/2 H2 → MnCH2

The MnCH intermediate has a tridentate methylidine ligand. The methylene ligand (H2C) is then poised couple to CO or to another methylene, thereby growing the C–C chain.[6]

Amphotericity

The methylylidyne group can exhibit both Lewis-acidic and Lewis basic character.[7] Such behavior is only of theoretical interest since it is not possible to produce methylylidyne.

In interstellar space

In October 2016, astronomers reported that the very basic chemical ingredients of life – the methylidyne radical CH, the carbon-hydrogen positive ion :CH+, and the carbon ion C+ – are the result of ultraviolet light from stars, rather than in other ways, such as the result of turbulent events related to supernovae and young stars, as thought earlier.[8] These results have given new light to the formation of organic compounds in the early development of life on earth.

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. ^ "hydridocarbon(3•)". Substance page at the Chemical Entities of Biological Interest (ChEBI) website, European Bioinformatics Institute, UK. Accessed on 2019-04-20
  3. ^ 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
  4. ^ 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. 67 (11): 5146–5151. doi:10.1063/1.434743.
  5. ^ a b c 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. (ed.). Inorganic Syntheses, Vol. 20. John Wiley & Sons, Inc. pp. 226–229. doi:10.1002/9780470132517.ch53. ISBN 9780470132517.
  6. ^ 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
  7. ^ 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. 93 (3): 1042–1048. doi:10.1021/j100340a007.
  8. ^ Landau, Elizabeth (12 October 2016). "Building Blocks of Life's Building Blocks Come From Starlight". NASA. Retrieved 13 October 2016.