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{{short description|Chemical group (–CH₃) derived from methane}}(File:Methyl Group General Formulae V.1.png|thumb|right|110px|Different ways of representing a methyl group (highlighted in blue))In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula {{chem2|CH3}} (whereas normal methane has the formula {{chem2|CH4}}). 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, bonded to the rest of the molecule by a single covalent bond ({{chem2|sCH3}}), it can be found on its own in any of three forms: methanide anion ({{chem2|CH3−}}), methylium cation ({{chem2|CH3+}}) or methyl radical ({{chem|CH|3|•}}). The anion has eight valence electrons, the radical seven and the cation six. All three forms are highly reactive and rarely observed.BOOK, March, Jerry, Advanced organic chemistry: reactions, mechanisms, and structure, 1992, John Wiley & Sons, 0-471-60180-2,

Methyl cation, anion, and radical

Methyl cation

The methylium cation ({{chem2|CH3+}}) exists in the gas phase, but is otherwise not encountered. Some compounds are considered to be sources of the {{chem2|CH3+}} 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 methyl cation has been detected in interstellar space.NEWS, Sauers, Elisha, Webb telescope just found something unprecedented in the Orion Nebula - Astronomers are excited about the detection of a special molecule in space.,mashable.com/article/james-webb-space-telescope-orion-nebula, 27 June 2023, Mashable, live,archive.today/20230627153343/https://mashable.com/article/james-webb-space-telescope-orion-nebula, 27 June 2023, 27 June 2023, JOURNAL, Berne, Olivier, et al., Formation of the Methyl Cation by Photochemistry in a Protoplanetary Disk,www.nature.com/articles/s41586-023-06307-x, 26 June 2023, Nature (journal), Nature, 10.1038/s41586-023-06307, live,archive.today/20230627160651/https://doi.org/10.1038/s41586-023-06307, 27 June 2023, 27 June 2023,

Methyl anion

The methanide anion ({{chem2|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.G. Barney Ellison , P. C. Engelking , W. C. Lineberger (1978), “An experimental determination of the geometry and electron affinity of methyl radical CH3” Journal of the American Chemical Society, volume 100, issue 8, pages 2556–2558. {{doi|10.1021/ja00476a054}} In discussing mechanisms of organic reactions, methyl lithium and related Grignard reagents are often considered to be salts of {{chem2|CH3−}}; 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 {{chem|CH|3|•}}. It exists in dilute gases, but in more concentrated form it readily dimerizes to ethane. It is routinely produced by various enzymes of the radical SAM and methylcobalamin varieties.JOURNAL, 10.1021/cr020428b, Radical Catalysis in Coenzyme B12-Dependent Isomerization (Eliminating) Reactions, 2003, Toraya, Tetsuo, Chemical Reviews, 103, 6, 2095–2128, 12797825, JOURNAL, 10.1021/acs.chemrev.8b00715, Organocobalt Complexes as Sources of Carbon-Centered Radicals for Organic and Polymer Chemistries, 2019, Demarteau, Jérémy, Debuigne, Antoine, Detrembleur, Christophe, Chemical Reviews, 119, 12, 6906–6955, 30964644, 106409337,

Reactivity

The reactivity of a methyl group depends on the adjacent substituents. Methyl groups can be quite unreactive. For example, in organic compounds, the methyl group resists attack by even the strongest acids.{{citation needed|date=April 2024}}

Oxidation

The oxidation of a methyl group occurs widely in nature and industry. The oxidation products derived from methyl are hydroxymethyl group {{chem2|sCH2OH}}, formyl group {{chem2|sCHO}}, and carboxyl group {{chem2|sCOOH}}. For example, permanganate often converts a methyl group to a carboxyl ({{chem2|sCOOH}}) group, e.g. the conversion of toluene to benzoic acid. Ultimately oxidation of methyl groups gives protons and carbon dioxide, as seen in combustion.

Methylation

Demethylation (the transfer of the methyl group to another compound) is a common process, and reagents that undergo this reaction are called methylating agents. Common methylating agents are dimethyl sulfate, methyl iodide, and methyl triflate. Methanogenesis, the source of natural gas, arises via a demethylation reaction.Thauer, R. K., “Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson”, Microbiology, 1998, volume 144, pages 2377–2406. Together with ubiquitin and phosphorylation, methylation is a major biochemical process for modifying protein function.JOURNAL, 10.1074/jbc.AW118.003235, 29743234, 6036201, The ribosome: A hot spot for the identification of new types of protein methyltransferases, Journal of Biological Chemistry, 293, 27, 10438–10446, 2018, Clarke, Steven G., free, The field of epigenetics focuses on the influence of methylation on gene expression.JOURNAL, Bird, Adrian, 2002-01-01, DNA methylation patterns and epigenetic memory,genesdev.cshlp.org/lookup/doi/10.1101/gad.947102, Genes & Development, en, 16, 1, 6–21, 10.1101/gad.947102, 0890-9369, free,

Deprotonation

Certain methyl groups can be deprotonated. For example, the acidity of the methyl groups in acetone ({{chem2|(CH3)2CO}}) is about 1020 times more acidic than methane. The resulting carbanions are key intermediates in many reactions in organic synthesis and biosynthesis. Fatty acids are produced in this way.

Free radical reactions

When placed in benzylic or allylic positions, the strength of the {{chem2|CsH}} bond is decreased, and the reactivity of the methyl group increases. One manifestation of this enhanced reactivity is the photochemical chlorination of the methyl group in toluene to give benzyl chloride.M. Rossberg et al. “Chlorinated Hydrocarbons” in Ullmann’s Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim.{{doi|10.1002/14356007.a06_233.pub2}}

Chiral methyl

In the special case where one hydrogen is replaced by deuterium (D) and another hydrogen by tritium (T), the methyl substituent becomes chiral.WEB,www2.lsdiv.harvard.edu/labs/evans/pdf/smnr_2000-2001_Burch_Jason.pdf, Archived copy, 2013-11-26, dead,www2.lsdiv.harvard.edu/labs/evans/pdf/smnr_2000-2001_Burch_Jason.pdf," title="web.archive.org/web/20100714113055www2.lsdiv.harvard.edu/labs/evans/pdf/smnr_2000-2001_Burch_Jason.pdf,">web.archive.org/web/20100714113055www2.lsdiv.harvard.edu/labs/evans/pdf/smnr_2000-2001_Burch_Jason.pdf, 2010-07-14, Methods exist to produce optically pure methyl compounds, e.g., chiral acetic acid (deuterotritoacetic acid {{chem2|CHDTCO2H}}). Through the use of chiral methyl groups, the stereochemical course of several biochemical transformations have been analyzed.Heinz G. Floss, Sungsook Lee “Chiral methyl groups: small is beautiful” Acc. Chem. Res., 1993, volume 26, pp 116–122.{{doi|10.1021/ar00027a007}}

Rotation

{{See also|Rotamer}}A methyl group may rotate around the {{chem2|RsC}} axis. This is a free rotation only in the simplest cases like gaseous methyl chloride {{chem2|CH3Cl}}. In most molecules, the remainder R breaks the C∞ symmetry of the {{chem2|RsC}} axis and creates a potential V(φ) that restricts the free motion of the three protons. For the model case of ethane {{chem2|CH3CH3}}, this is discussed under the name ethane barrier.In condensed phases, neighbour molecules also contribute to the potential. Methyl group rotation can be experimentally studied using quasielastic neutron scattering.Press,W: Single-particle rotation in molecular crystals (Springer tracts in modern physics 92), Springer: Berlin (1981).

Etymology

French chemists Jean-Baptiste Dumas and Eugene Peligot, after determining methanol’s chemical structure, introduced “methylene” from the Greek {{wikt-lang|grc|μέθυ}} (methy) “wine” and {{wikt-lang|grc|ὕλη}} (hȳlÄ“) “wood, patch of trees” with the intention of highlighting its origins, “alcohol made from wood (substance)”.J. Dumas and E. Péligot (1835) “Mémoire sur l’espirit de bois et sur les divers composés ethérés qui en proviennent” (Memoir on spirit of wood and on the various ethereal compounds that derive therefrom), Annales de chimie et de physique, 58 : 5-74; from page 9: Nous donnerons le nom de méthylène (1) à un radical … (1) μεθυ, vin, et υλη, bois; c’est-à-dire vin ou liqueur spiritueuse du bois. (We will give the name “methylene” (1) to a radical … (1) methy, wine, and hulÄ“, wood; that is, wine or spirit of wood.)Note that the correct Greek word for the substance “wood” is xylo-. The term “methyl” was derived in about 1840 by back-formation from “methylene”, and was then applied to describe “methyl alcohol” (which since 1892 is called “methanol“).Methyl is the IUPAC nomenclature of organic chemistry term for an alkane (or alkyl) molecule, using the prefix “meth-” to indicate the presence of a single carbon.

See also

• AdoMet
• Methylation

References

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