Inorganic chemistry,amethyl groupis analkylderived frommethane,containing onecarbonatombondedto threehydrogenatoms, having chemical formulaCH3(whereas normal methane has the formulaCH4). Informulas,the group is oftenabbreviatedasMe.Thishydrocarbongroup 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]
Methyl cation, anion, and radical
editMethyl cation
editThe methylium cation (CH+3) exists in thegas phase,but is otherwise not encountered. Some compounds are considered to be sources of theCH+3cation, and this simplification is used pervasively in organic chemistry. For example,protonationof methanol gives an electrophilic methylating reagent that reacts by the SN2 pathway:
- CH3OH + H+→ [CH3OH2]+
Similarly,methyl iodideand methyltriflateare 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]
Methyl anion
editThe methanide anion (CH−3) exists only in rarefied gas phase or under exotic conditions. It can be produced by electrical discharge inketeneat low pressure (less than onetorr) and itsenthalpy of reactionis determined to be about 252.2 ± 3.3kJ/mol.[4]It is a powerfulsuperbase;only thelithium monoxide anion(LiO−) and thediethynylbenzene dianionsare known to be stronger.[5]
In discussing mechanisms of organic reactions,methyl lithiumand relatedGrignard reagentsare 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:
- 2 M + CH3X → MCH3+ MX
where M is analkali metal.
Methyl radical
editThe methylradicalhas the formulaCH•
3.It exists in dilute gases, but in more concentrated form it readilydimerizestoethane.It is routinely produced by various enzymes of theradical SAMandmethylcobalaminvarieties.[6][7]
Reactivity
editThe 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.[citation needed]
Oxidation
editTheoxidationof 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,permanganateoften converts a methyl group to a carboxyl (−COOH) group, e.g. the conversion oftoluenetobenzoic acid.Ultimately oxidation of methyl groups givesprotonsandcarbon dioxide,as seen in combustion.
Methylation
editDemethylation (the transfer of the methyl group to another compound) is a common process, andreagentsthat 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.[8]Together with ubiquitin and phosphorylation, methylation is a major biochemical process for modifying protein function.[9]The field ofepigeneticsfocuses on the influence of methylation on gene expression.[10]
Deprotonation
editCertain methyl groups can be deprotonated. For example, the acidity of the methyl groups inacetone((CH3)2CO) is about 1020times more acidic than methane. The resultingcarbanionsare key intermediates in many reactions inorganic synthesisandbiosynthesis.Fatty acidsare produced in this way.
Free radical reactions
editWhen placed inbenzylicorallylicpositions, the strength of theC−Hbond is decreased, and the reactivity of the methyl group increases. One manifestation of this enhanced reactivity is thephotochemicalchlorinationof the methyl group intolueneto givebenzyl chloride.[11]
Chiral methyl
editIn the special case where one hydrogen is replaced bydeuterium(D) and another hydrogen bytritium(T), the methyl substituent becomeschiral.[12]Methods exist to produce optically pure methyl compounds, e.g., chiralacetic acid(deuterotritoacetic acidCHDTCO2H). Through the use of chiral methyl groups, thestereochemicalcourse of severalbiochemicaltransformations have been analyzed.[13]
Rotation
editA methyl group may rotate around theR−Caxis. This is a free rotation only in the simplest cases like gaseousmethyl chlorideCH3Cl.In most molecules, the remainder R breaks theC∞symmetry of theR−Caxis 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.[14]
Etymology
editFrench chemistsJean-Baptiste DumasandEugene 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)".[15][16]The term "methyl" was derived in about 1840 byback-formationfrom "methylene", and was then applied to describe "methyl alcohol" (which since 1892 is called "methanol").
Methylis theIUPAC nomenclature of organic chemistryterm for analkane(or alkyl) molecule, using the prefix "meth-" to indicate the presence of a single carbon.
See also
editReferences
edit- ^March, Jerry (1992).Advanced organic chemistry: reactions, mechanisms, and structure.John Wiley & Sons.ISBN0-471-60180-2.
- ^Sauers, Elisha (27 June 2023)."Webb telescope just found something unprecedented in the Orion Nebula - Astronomers are excited about the detection of a special molecule in space".Mashable.Archivedfrom the original on 27 June 2023.Retrieved27 June2023.
- ^Berne, Olivier; et al. (26 June 2023)."Formation of the Methyl Cation by Photochemistry in a Protoplanetary Disk".Nature.doi:10.1038/s41586-023-06307.Archivedfrom the original on 27 June 2023.Retrieved27 June2023.
- ^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
- ^Poad, Berwyck L. J.; Reed, Nicholas D.; Hansen, Christopher S.; Trevitt, Adam J.; Blanksby, Stephen J.; Mackay, Emily G.; Sherburn, Michael S.; Chan, Bun; Radom, Leo (2016)."Preparation of an ion with the highest calculated proton affinity: ortho-diethynylbenzene dianion".Chemical Science.7(9): 6245–6250.doi:10.1039/C6SC01726F.PMC6024202.PMID30034765.
- ^Toraya, Tetsuo (2003). "Radical Catalysis in Coenzyme B12-Dependent Isomerization (Eliminating) Reactions ".Chemical Reviews.103(6): 2095–2128.doi:10.1021/cr020428b.PMID12797825.
- ^Demarteau, Jérémy; Debuigne, Antoine; Detrembleur, Christophe (2019). "Organocobalt Complexes as Sources of Carbon-Centered Radicals for Organic and Polymer Chemistries".Chemical Reviews.119(12): 6906–6955.doi:10.1021/acs.chemrev.8b00715.PMID30964644.S2CID106409337.
- ^Thauer, R. K., "Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson", Microbiology, 1998, volume 144, pages 2377–2406.
- ^Clarke, Steven G. (2018)."The ribosome: A hot spot for the identification of new types of protein methyltransferases".Journal of Biological Chemistry.293(27): 10438–10446.doi:10.1074/jbc.AW118.003235.PMC6036201.PMID29743234.
- ^Bird, Adrian (2002-01-01)."DNA methylation patterns and epigenetic memory".Genes & Development.16(1): 6–21.doi:10.1101/gad.947102.ISSN0890-9369.
- ^M. Rossberg et al. “Chlorinated Hydrocarbons” in Ullmann’s Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim.doi:10.1002/14356007.a06_233.pub2
- ^"Archived copy"(PDF).Archived fromthe original(PDF)on 2010-07-14.Retrieved2013-11-26.
{{cite web}}:CS1 maint: archived copy as title (link) - ^Heinz G. Floss, Sungsook Lee "Chiral methyl groups: small is beautiful" Acc. Chem. Res., 1993, volume 26, pp 116–122. doi:10.1021/ar00027a007
- ^Press,W: Single-particle rotation in molecular crystals (Springer tracts in modern physics 92), Springer: Berlin (1981).
- ^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; frompage 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 hulē, wood; that is, wine or spirit of wood.)
- ^Note that the correct Greek word for the substance "wood" isxylo-.