Methyl (also systematically named trihydridocarbon) is an
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...
with the
chemical formula
In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
(also written as
•). 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
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...
or a strong
reductant
In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ).
Examples of substances that are commonly reducing agents include the Earth meta ...
, and is quite corrosive to metals.
Chemical properties
Its first ionization potential (yielding the
methenium
In organic chemistry, methenium (also called methylium, carbenium, methyl cation, or protonated methylene) is a cation with the formula . It can be viewed as a methylene radical (:) with an added proton (), or as a methyl radical (•) with on ...
ion, ) is .
[ ]
Redox behaviour
The carbon centre in methyl can bond with electron-donating molecules by reacting:
: + R
• →
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
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
and elemental hydrogen:
:2 + 2 → 2 +
Structure
The
molecular geometry
Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that determ ...
of the methyl radical is
trigonal planar
In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands a ...
(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 barrier to
pyramidal inversion In chemistry, pyramidal inversion (also umbrella inversion) is a fluxional process in compounds with a pyramidal molecule, such as ammonia (NH3) "turns inside out". It is a rapid oscillation of the atom and substituents, the molecule or ion passin ...
. 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
The trifluoromethyl group is a functional group that has the formula -CF3. The naming of is group is derived from the methyl group (which has the formula -CH3), by replacing each hydrogen atom by a fluorine atom. Some common examples are trifluoro ...
radical, , with a much more substantial inversion barrier of around .
Chemical reactions
Methyl undergoes the typical chemical reactions of a radical. Below approximately , it rapidly dimerises to form
ethane
Ethane ( , ) is an organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petr ...
. Upon treatment with an alcohol, it converts to
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
and either an alkoxy or hydroxyalkyl. Reduction of methyl gives methane. When heated above, at most, , methyl decomposes to produce
methylidyne and elemental hydrogen, or to produce
methylene and atomic hydrogen:
: → CH
• +
: → + H
•
Methyl is very corrosive to metals, forming methylated metal compounds:
:M + ''n'' → M(CH
3)
''n''
Production
Biosynthesis
Some
radical SAM
Radical SAM is a designation for a superfamily of enzymes that use a +_cluster.html" ;"title="Fe-4Ssup>+ cluster">Fe-4Ssup>+ cluster to reductively cleave ''S''-adenosyl-L-methionine (SAM) to generate a radical, usually a 5′- deoxyadenosyl rad ...
enzymes generate methyl radicals by reduction of S-adenosylmethionine.
Acetone photolysis
It can be produced by the ultraviolet photodissociation of
acetone
Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour.
Acetone is miscib ...
vapour at 193 nm:
: → CO + 2
Halomethane photolysis
It is also produced by the ultraviolet dissociation of
halomethane Halomethane compounds are derivatives of methane () with one or more of the hydrogen atoms replaced with halogen atoms ( F, Cl, Br, or I). Halomethanes are both naturally occurring, especially in marine environments, and human-made, most notably ...
s:
: → X
• +
Methane oxidation
It can also be produced by the reaction of
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
with the
hydroxyl radical
The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals are produced from the ...
:
:OH
• + CH
4 → + H
2O
This process begins the major removal mechanism of methane from the atmosphere. The reaction occurs in the
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
or
stratosphere
The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air ...
. 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
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
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
•O
1D in the stratosphere (120 year lifetime), giving a net lifetime of 8.4 years.
Azomethane pyrolysis
Methyl radicals can also be obtained by
pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements ''py ...
of
azomethane, CH
3N=NCH
3, in a low-pressure system.
In the interstellar medium
Methyl was discovered in
interstellar medium
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...
in 2000 by a team led by Helmut Feuchtgruber who detected it using the
Infrared Space Observatory
The Infrared Space Observatory (ISO) was a space telescope for infrared light designed and operated by the European Space Agency (ESA), in cooperation with ISAS (now part of JAXA) and NASA. The ISO was designed to study infrared light at wavelengt ...
. It was first detected in molecular clouds toward the centre of the Milky Way.
References
{{Hydrides by group
Astrochemistry
Free radicals
Oil refining
Yl