Methyl radical
A methyl radical, with the formula CH3· (previously called a methyl free radical) is a radical centered on the carbon atom, consisting of one carbon atom and three hydrogen atoms, with one unpaired electron. It derives from methane, (CH4), by elimination of a hydrogen atom, although it can be obtained from other substances. It exists as an independent molecular unit although its half-life is very short.
It must be differentiated from the methyl group CH3-, present as a substituent in many organic compounds as a part of their molecule, and never in isolation.
The carbon atom has sp2 hybridization (planar triangular geometry) in the methyl radical, instead of the typical sp3 (tetrahedral) hybridization of compounds of C with single bonds. The odd electron stays in an unhybridized p orbital.
Synthesis of the methyl radical
Methyl radicals were first obtained by thermal decomposition of organometallic compounds such as tetramethyl lead, in 1929:
- PbMe4 → Pb + 4Me·
It can also be obtained by photolysis of acetone under ultraviolet light of 320 nm wavelength.
- CH3-CO-CH3 → CH3-CO· + ·CH3
and later, the acyl radical formed in the previous reaction degrades spontaneously
- CH3-CO· → CO + ·CH3
It is also obtained as an intermediate product of the chlorination of methane, under ultraviolet light: the light dissociates the chlorine molecule into two atoms or chlorine radicals:
- Cl2 → 2 Cl·
, said chlorine radical attacks a hydrogen atom of methane:
- Cl· + CH4 → HCl+ H3C·
and subsequently the methyl radicals formed can be attached to some of the radicals present.
Reactivity of the methyl radical
Like other radicals, they are highly reactive. When isolated, their half-life is of the order of a few milliseconds as they undergo rapid dimerization with the formation of ethane. It can be observed in gaseous form at low pressure.
- 2CH3· → CH3-CH3
It binds to oxygen to give the more stable methylperoxyl radical.
CH3 + O2 → CH3-O-O·
Other alkyl radicals
The methyl radical is part of a "family" of chemical entities called alkyl or alkyl radicals, since they follow the same pattern: they only contain carbon and hydrogen atoms, linked by a single covalent bond, and are not stable in isolation, because they have an unpaired electron. They are named the same as alkyl groups with the same number of carbon atoms.