Methyl
A methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms — CH3. In formulas, the group is often abbreviated Me. Such hydrocarbon groups occur in many organic compounds. It is a very stable group in most molecules. While the methyl group is usually part of a larger molecule, it can be found on its own in any of three forms: anion, cation or radical. The anion has eight valence electrons, the radical seven and the cation six. All three forms are highly reactive and rarely observed.
The methylium cation (CH3+) exists in the gas phase, but is otherwise not encountered. Some compounds are considered to be sources of the CH3+ cation, and this simplification is used pervasively in organic chemistry. For example, protonation of methanol gives a strongly electrophilic methylating reagent:
Similarly, methyl iodide and methyl triflate are viewed as the equivalent of the methyl cation because they readily undergo SN2 reactions by weak nucleophiles.
The methanide anion (CH3−) exists only in rarefied gas phase or under exotic conditions. It can be produced by electrical discharge in ketene at low pressure (less than one torr) and its enthalpy of reaction is determined to be about 252.2±3.3 kJ/mol.
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