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Concept
Méthyle
Résumé
In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in many organic compounds. It is a very stable group in most molecules. While the methyl group is usually part of a larger molecule, bounded to the rest of the molecule by a single covalent bond (), it can be found on its own in any of three forms: methanide anion (), methylium cation () or methyl radical (CH3•). The anion has eight valence electrons, the radical seven and the cation six. All three forms are highly reactive and rarely observed.
Methenium
The methylium cation () exists in the gas phase, but is otherwise not encountered. Some compounds are considered to be sources of the cation, and this simplification is used pervasively in organic chemistry. For example, protonation of methanol gives an electrophilic methylating reagent that reacts by the SN2 pathway:
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 () 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 252.2kJ/mol. It is a powerful superbase; only the lithium monoxide anion () and the diethynylbenzene dianions are known to be stronger.
In discussing mechanisms of organic reactions, methyl lithium and related Grignard reagents are often considered to be salts of ; and though the model may be useful for description and analysis, it is only a useful fiction. Such reagents are generally prepared from the methyl halides:
where M is an alkali metal.
Methyl radical
The methyl radical has the formula CH3•. It exists in dilute gases, but in more concentrated form it readily dimerizes to ethane.
Source officielle
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