Inorganic chemistry, amethyl group is analkyl derived frommethane, containing onecarbon atombonded to threehydrogen atoms, having chemical formulaCH3 (whereas normal methane has the formulaCH4). Informulas, the group is oftenabbreviated asMe. Thishydrocarbon group occurs in manyorganic compounds. It is a very stable group in most molecules. While the methyl group is usually part of a largermolecule, bonded to the rest of the molecule by a single covalent bond (−CH3), it can be found on its own in any of three forms: methanideanion (CH−3), methyliumcation (CH+3) or methylradical (CH•
3). The anion has eightvalence electrons, the radical seven and the cation six. All three forms are highly reactive and rarely observed.[1]
The methylium cation (CH+3) exists in thegas phase, but is otherwise not encountered. Some compounds are considered to be sources of theCH+3 cation, and this simplification is used pervasively in organic chemistry. For example,protonation of methanol gives an electrophilic methylating reagent that reacts by theSN2 pathway:
Similarly,methyl iodide and methyltriflate are viewed as the equivalent of the methyl cation because they readily undergo SN2 reactions by weaknucleophiles.
The methyl cation has been detected ininterstellar space.[2][3]
The methanide anion (CH−3) exists only in rarefied gas phase or under exotic conditions. It can be produced by electrical discharge inketene at low pressure (less than onetorr) and itsenthalpy of reaction is determined to be about 252.2 ± 3.3kJ/mol.[4] It is a powerfulsuperbase; only thelithium monoxide anion (LiO−) and thediethynylbenzene dianions are known to be stronger.[5]
In discussing mechanisms of organic reactions,methyl lithium and relatedGrignard reagents are often considered to be salts ofCH−3; and though the model may be useful for description and analysis, it is only a useful fiction. Such reagents are generally prepared from themethyl halides:
where M is analkali metal.
The methylradical has the formulaCH•
3. It exists in dilute gases, but in more concentrated form it readilydimerizes toethane. It is routinely produced by various enzymes of theradical SAM andmethylcobalamin varieties.[6][7]
The reactivity of a methyl group depends on the adjacentsubstituents. Methyl groups can be quite unreactive. For example, in organic compounds, the methyl group resists attack by even the strongestacids.[8]
Theoxidation of a methyl group occurs widely in nature and industry. The oxidation products derived from methyl arehydroxymethyl group−CH2OH,formyl group−CHO, andcarboxyl group−COOH. For example,permanganate often converts a methyl group to a carboxyl (−COOH) group, e.g. the conversion oftoluene tobenzoic acid. Ultimately oxidation of methyl groups givesprotons andcarbon dioxide, as seen in combustion.
Demethylation (the transfer of the methyl group to another compound) is a common process, andreagents that undergo this reaction are called methylating agents. Common methylating agents aredimethyl sulfate,methyl iodide, andmethyl triflate.Methanogenesis, the source of natural gas, arises via a demethylation reaction.[9] Together with ubiquitin and phosphorylation, methylation is a major biochemical process for modifying protein function.[10] The field ofepigenetics focuses on the influence of methylation on gene expression.[11]
Certain methyl groups can be deprotonated. For example, the acidity of the methyl groups inacetone ((CH3)2CO) is about 1020 times more acidic than methane. The resultingcarbanions are key intermediates in many reactions inorganic synthesis andbiosynthesis.Fatty acids are produced in this way.
When placed inbenzylic orallylic positions, the strength of theC−H bond is decreased, and the reactivity of the methyl group increases. One manifestation of this enhanced reactivity is thephotochemicalchlorination of the methyl group intoluene to givebenzyl chloride.[12]
In the special case where one hydrogen is replaced bydeuterium (D) and another hydrogen bytritium (T), the methyl substituent becomeschiral.[13] Methods exist to produce optically pure methyl compounds, e.g., chiralacetic acid (deuterotritoacetic acidCHDTCO2H). Through the use of chiral methyl groups, thestereochemical course of severalbiochemical transformations have been analyzed.[14]
A methyl group may rotate around theR−C axis. This is a free rotation only in the simplest cases like gaseousmethyl chlorideCH3Cl. In most molecules, the remainder R breaks theC∞ symmetry of theR−C axis and creates a potentialV(φ) that restricts the free motion of the three protons. For the model case ofethaneCH3CH3, this is discussed under the nameethane barrier.In condensed phases, neighbour molecules also contribute to the potential. Methyl group rotation can be experimentally studied usingquasielastic neutron scattering.[15]
French chemistsJean-Baptiste Dumas andEugene Peligot, after determining methanol's chemical structure, introduced "methylene" from theGreekμέθυ (methy) "wine" andὕλη (hȳlē) "wood, patch of trees" with the intention of highlighting its origins, "alcohol made from wood (substance)".[16][17] The term "methyl" was derived in about 1840 byback-formation from "methylene", and was then applied to describe "methyl alcohol" (which since 1892 is called "methanol").
Methyl is theIUPAC nomenclature of organic chemistry term for analkane (or alkyl) molecule, using the prefix "meth-" to indicate the presence of a single carbon.
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