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Methyl radical

Methyl radical
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IUPAC name
Methyl
Systematic IUPAC name
λ3-Methyl
Other names
Hydrogen carbide(-III)
Methyl radical
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1696831
2229-07-4 7pxY
ChEBI CHEBI:29309
ChemSpider 2299212 7pxY
57
Jmol-3D images Image
MeSH Methyl+radical
PubChem Template:Chembox PubChem/format
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CH3
Molar mass Lua error in Module:Math at line 495: attempt to index field 'ParserFunctions' (a nil value). g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Methyl (also systematically named trihydridocarbon) is an organic compound with the chemical formula Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom (also written as Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom). It is a metastable colourless gas, which is mainly produced in situ as a precursor to other hydrocarbons in the petroleum cracking industry. It can act as either a strong oxidant or a strong reductant, and is quite corrosive to metals.

Chemical properties

Its first ionization potential (yielding the methenium ion, Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom) is 9.837±0.005 eV.[1]

Redox behaviour

The carbon centre in methyl can bond with electron-donating molecules by reacting:

Template:Chem/atomTemplate:Chem/atom + R· → Template:Chem/atomTemplate:Chem/atom

Because of the capture of the nucleophile (R·), methyl has oxidising character. Methyl is a strong oxidant with organic chemicals. However, it is equally a strong reductant with chemicals such as water. It does not form aqueous solutions, as it reduces water to produce methanol and elemental hydrogen:

Template:Chem/atomTemplate:Chem/atom + 2 Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom → 2 Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom + Template:Chem/atomTemplate:Chem/atom

Structure

The molecular geometry of the methyl radical is quasi-trigonal planar, although the energy cost of distortion to a pyramidal geometry is small. Substitution of hydrogen atoms by more electronegative substituents leads to radicals with a pyramidal geometry, such as the trifluoromethyl radical, CF3.[2]

Chemical reactions

Methyl undergoes the typical chemical reactions of a radical. Below approximately Script error: No such module "convert"., it rapidly dimerises to form ethane. Upon treatment with an alcohol, it converts to methane and either an alkoxy or hydroxyalkyl. Reduction of methyl gives methane. When heated above, at most, Script error: No such module "convert"., methyl decomposes to produce methylidyne and elemental hydrogen, or to produce methylene and atomic hydrogen:

Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom → CH + Template:Chem/atomTemplate:Chem/atom
Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atomTemplate:Chem/atomTemplate:Chem/atom + H

Methyl is very corrosive to metals, forming methylated metal compounds:

M + n Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom → M(CH3)n

Production

Acetone photolysis

It can be produced by the ultraviolet photodissociation of acetone vapour at 193 nm:[3]

Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atomTemplate:Chem/atomTemplate:Chem/atom → CO + 2 Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom

Halomethane photolysis

It is also produced by the ultraviolet dissociation of halomethanes:

Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom → X + Template:Chem/atomTemplate:Chem/atomTemplate:Chem/atom

Methane oxidation

Template:Details3 It can also be produced by the reaction of methane with the hydroxyl radical:

OH + CH4 → CH3 + H2O

This process begins the major removal mechanism 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.[4]

Azomethane pyrolysis

Methyl radicals can also be obtained by pyrolysis of azomethane, CH3-N=N-CH3, in a low-pressure system.

In the interstellar medium

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.[5]

References

  1. ^ 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, pages 506-508 doi:10.1016/0368-2048(72)80022-7
  2. ^ Anslyn E.V. and Dougherty D.A., Modern Physical Organic Chemistry (University Science Books, 2006), p.57
  3. ^ 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 (AIP Publishing) 94 (6): 4182. doi:10.1063/1.460741. 
  4. ^ "Trace Gases: Current Observations, Trends, and Budgets". Climate Change 2001, IPCC Third Assessment Report. IPCC/United Nations Environment Programme. 
  5. ^ "ISO detects a new molecule in interstellar space". Science & Technology. European Space Agency. Retrieved 17 June 2013.