Methyl radical
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| Names | |
|---|---|
| IUPAC name
Methyl[1] | |
| Identifiers | |
3D model (JSmol) |
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| 1696831 | |
| ChEBI | |
| ChemSpider | |
| 57 | |
| MeSH | Methyl+radical |
PubChem CID |
|
| UNII | |
CompTox Dashboard (EPA) |
|
| |
| |
| Properties | |
| CH3 | |
| Molar mass | 15.035 g·mol−1 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
| |
Methyl radical is an organic compound with the chemical formula CH3 (also written as [CH
3]•). Although extremely rare, it is a colourless gas. Indicative of its high reactivity, its half-life is a few minutes in methanol solution at -196 °C.[2]
Properties
[edit]Its first ionization potential (yielding the methenium ion, CH+
3) is 9.837±0.005 eV.[3]
Structure
[edit]The molecular geometry of the methyl radical is trigonal planar (bond angles are 120°), although the energy cost of distortion to a pyramidal geometry is small. All other electron-neutral, non-conjugated alkyl radicals are pyramidalized to some extent, though with very small inversion barriers. For instance, the t-butyl radical has a bond angle of 118° with a 0.7 kcal/mol (2.9 kJ/mol) barrier to pyramidal inversion. On the other hand, substitution of hydrogen atoms by more electronegative substituents leads to radicals with a strongly pyramidal geometry (112°), such as the trifluoromethyl radical, CF•
3, with a much more substantial inversion barrier of around 25 kcal/mol (100 kJ/mol).[4]
Chemical reactions
[edit]Methyl undergoes the typical chemical reactions of an organic radical. At temperatures above -196 °C, it dimerises to form ethane. Upon treatment with an alcohol, it converts to methane and either an alkoxy or hydroxyalkyl.
Production
[edit]Biosynthesis
[edit]Radical SAM enzymes have been proposed to generate methyl radicals by reduction of S-adenosylmethionine.[5]
Acetone photolysis
[edit]It can be produced by the ultraviolet photodissociation of acetone vapour at 193 nm:[6]
- (CH3)2CO → CO + 2 CH3·
Halomethane photolysis
[edit]It is also produced by the ultraviolet dissociation of iodomethane:[7]
- CH3I → I· + CH3·
These studies allowed the recording of the electron spin resonance spectrum of methyl, revealing strong coupling to three equivalent H atoms.
Methane oxidation
[edit]It can also be produced by the reaction of methane with the hydroxyl radical:
- OH• + CH4 → CH•
3 + H2O
This process is a step in the major pathway for removal of methane from the atmosphere. The reaction occurs in the troposphere or stratosphere. In addition to being the largest known sink for atmospheric methane, this reaction is one of the most important sources of water vapor in the upper atmosphere. This reaction in the troposphere gives a methane lifetime of 9.6 years. Two more minor sinks are soil sinks (160 year lifetime) and stratospheric loss by reaction with •OH, •Cl and •O1D in the stratosphere (120 year lifetime), giving a net lifetime of 8.4 years.[8]
Azomethane pyrolysis
[edit]Methyl radicals can also be obtained by pyrolysis of azomethane, CH3N=NCH3, in a low-pressure system.[citation needed]
In the interstellar medium
[edit]Methyl was discovered in interstellar medium in 2000 by a team led by Helmut Feuchtgruber who detected it using the Infrared Space Observatory. It was first detected in molecular clouds toward the centre of the Milky Way.[9]
References
[edit]- ↑ International Union of Pure and Applied Chemistry (2014). Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013. The Royal Society of Chemistry. p. 1051. doi:10.1039/9781849733069. ISBN 978-0-85404-182-4.
- ↑ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 266, ISBN 978-0-471-72091-1
- ↑ Golob, L.; Jonathan, N.; Morris, A.; Okuda, M.; Ross, K.J. (1972). "The first ionization potential of the methyl radical as determined by photoelectron spectroscopy". Journal of Electron Spectroscopy and Related Phenomena. 1 (5): 506–508. Bibcode:1972JESRP...1..506G. doi:10.1016/0368-2048(72)80022-7.
- ↑ Anslyn E.V. and Dougherty D.A., Modern Physical Organic Chemistry (University Science Books, 2006), p.57
- ↑ Hoffman BM, Broderick WE, Broderick JB (June 2023). "Mechanism of Radical Initiation in the Radical SAM Enzyme Superfamily". Annual Review of Biochemistry. 92 (1): 333–349. doi:10.1146/annurev-biochem-052621-090638. PMC 10759928. PMID 37018846. S2CID 257983715.
- ↑ Hall, G. E.; Vanden Bout, D.; Sears, Trevor J. (1991). "Photodissociation of acetone at 193 nm: Rotational- and vibrational-state distributions of methyl fragments by diode laser absorption/gain spectroscopy". The Journal of Chemical Physics. 94 (6). AIP Publishing: 4182. Bibcode:1991JChPh..94.4182H. doi:10.1063/1.460741.
- ↑ Cole, T.; Pritchard, H.O.; Davidson, N.R.; McConnell, H.M. (1958). "Structure of the methyl radical". Molecular Physics. 1 (4): 406–409. doi:10.1080/00268975800100471.
- ↑ "Trace Gases: Current Observations, Trends, and Budgets". Climate Change 2001, IPCC Third Assessment Report. IPCC/United Nations Environment Programme.
- ↑ "ISO detects a new molecule in interstellar space". Science & Technology. European Space Agency. Retrieved 17 June 2013.
