Contents

Methyl radical

Methyl radical

Names
IUPAC name Methyl[1]
Identifiers
CAS Number	2229-07-4 Y
3D model (JSmol)	Interactive image
Beilstein Reference	1696831
ChEBI	CHEBI:29309
ChemSpider	2299212 Y
Gmelin Reference	57
MeSH	Methyl+radical
PubChem CID	3034819
UNII	S19006ZD7R Y
CompTox Dashboard (EPA)	DTXSID60176836
InChI InChI=1S/CH3/h1H3 Y Key: WCYWZMWISLQXQU-UHFFFAOYSA-N Y
SMILES [CH3]
Properties
Chemical formula	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). Infobox references

Chemical compound

Methyl radical is an organic compound with the chemical formula CH^•
₃ (also written as [CH
₃]^•). 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[edit]

Its first ionization potential (yielding the methenium ion, CH⁺
₃) is 9.837±0.005 eV.^[2]

Redox behaviour[edit]

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

CH^•
₃ + R^• → RCH
₃

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:

2 CH^•
₃ + 2 H
₂O → 2 CH
₃OH + H
₂

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).^[3]

Chemical reactions[edit]

Methyl undergoes the typical chemical reactions of a radical. Below approximately 1,100 °C (1,400 K), 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, 1,400 °C (1,700 K), methyl decomposes to produce methylidyne and elemental hydrogen, or to produce methylene and atomic hydrogen:

CH^•
₃ → CH^• + H
₂

CH^•
₃ → CH^•
₂ + H^•

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

M + n CH^•
₃ → M(CH₃)_n

Production[edit]

Biosynthesis[edit]

Some radical SAM enzymes generate methyl radicals by reduction of S-adenosylmethionine.^[4]

Acetone photolysis[edit]

It can be produced by the ultraviolet photodissociation of acetone vapour at 193 nm:^[5]

C
₃H
₆O → CO + 2 CH^•
₃

Halomethane photolysis[edit]

It is also produced by the ultraviolet dissociation of halomethanes:

CH
₃X → X^• + CH^•
₃

Methane oxidation[edit]

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

OH^• + CH₄ → CH^•
₃ + H₂O

This process begins the major removal mechanism of methane from the atmosphere. The reaction occurs in the troposphereorstratosphere. 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.^[6]

Azomethane pyrolysis[edit]

Methyl radicals can also be obtained by pyrolysisofazomethane, CH₃N=NCH₃, in a low-pressure system.

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.^[7]

References[edit]