View source for Methyl group

{{distinguish|Ethyl group}}
{{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 <span style="color:blue;">'''blue'''</span>)]]

In [[organic chemistry]], a '''methyl group''' is an [[alkyl]] derived from [[methane]], containing one [[carbon]] atom [[chemical bond|bonded]] to three [[hydrogen]] atoms, having chemical formula {{chem2|CH3}} (whereas normal methane has the formula {{chem2|CH4}}). In [[chemical formula|formulas]], the group is often [[skeletal formula#Pseudoelement symbols|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 (chemistry)|radical]] ({{chem|CH|3|•}}). The anion has eight [[valence electron]]s, the radical seven and the cation six. All three forms are highly reactive and rarely observed.<ref>{{cite book| last = March| first = Jerry| title = Advanced organic chemistry: reactions, mechanisms, and structure| year = 1992| publisher = John Wiley & Sons| isbn = 0-471-60180-2}}</ref>

==Methyl cation, anion, and radical==

===Methyl cation===
{{main|Methenium}}

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 reaction|S<sub>N</sub>2]] pathway:
:{{chem2|CH3OH + H+ → [CH3OH2]+}}

Similarly, [[methyl iodide]] and methyl [[triflate]] are viewed as the equivalent of the methyl cation because they readily undergo S<sub>N</sub>2 reactions by weak [[nucleophile]]s.

The methyl cation has been detected in [[interstellar space]].<ref name="MSH-20230627">{{cite news |last=Sauers |first=Elisha |title=Webb telescope just found something unprecedented in the Orion Nebula - Astronomers are excited about the detection of a special molecule in space.|url=https://mashable.com/article/james-webb-space-telescope-orion-nebula |date=27 June 2023 |work=[[Mashable]] |url-status=live |archiveurl=https://archive.today/20230627153343/https://mashable.com/article/james-webb-space-telescope-orion-nebula |archivedate=27 June 2023 |accessdate=27 June 2023}}</ref><ref name="NAT-20230626">{{cite journal |author=Berne, Olivier |display-authors=et al. |title=Formation of the Methyl Cation by Photochemistry in a Protoplanetary Disk |url=https://www.nature.com/articles/s41586-023-06307-x |date=26 June 2023 |journal=[[Nature (journal)|Nature]] |doi=10.1038/s41586-023-06307 |url-status=live |archiveurl=https://archive.today/20230627160651/https://doi.org/10.1038/s41586-023-06307 |archivedate=27 June 2023 |accessdate=27 June 2023}}</ref>

===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 [[Kilojoule|kJ]]/[[mol (unit)|mol]].<ref name="Ellison78">G. Barney Ellison , P. C. Engelking , W. C. Lineberger (1978), "An experimental determination of the geometry and electron affinity of methyl radical CH<sub>3</sub>" Journal of the American Chemical Society, volume 100, issue 8, pages 2556–2558. {{doi|10.1021/ja00476a054}}
</ref> It is a powerful [[superbase]]; only the [[lithium monoxide anion]] ({{chem2|LiO-}}) and the [[diethynylbenzene dianion]]s are known to be stronger.<ref>{{cite journal |last1=Poad |first1=Berwyck L. J. |last2=Reed |first2=Nicholas D. |last3=Hansen |first3=Christopher S. |last4=Trevitt |first4=Adam J. |last5=Blanksby |first5=Stephen J. |last6=Mackay |first6=Emily G. |last7=Sherburn |first7=Michael S. |last8=Chan |first8=Bun |last9=Radom |first9=Leo |title=Preparation of an ion with the highest calculated proton affinity: ortho-diethynylbenzene dianion |journal=Chemical Science |date=2016 |volume=7 |issue=9 |pages=6245–6250 |doi=10.1039/C6SC01726F |pmid=30034765 |pmc=6024202 |doi-access=free}}</ref>

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 halide]]s:
:{{chem2|2 M + CH3X → MCH3 + MX}}
where M is an [[alkali metal]].

===Methyl radical===
{{main|Methyl radical}}
The methyl [[Radical (chemistry)|radical]] has the formula {{chem|CH|3|•}}. It exists in dilute gases, but in more concentrated form it readily [[Dimer (chemistry)|dimer]]izes to [[ethane]]. It is routinely produced by various enzymes of the [[radical SAM]] and [[methylcobalamin]] varieties.<ref>{{cite journal |doi=10.1021/cr020428b |title=Radical Catalysis in Coenzyme B<sub>12</sub>-Dependent Isomerization (Eliminating) Reactions |date=2003 |last1=Toraya |first1=Tetsuo |journal=Chemical Reviews |volume=103 |issue=6 |pages=2095–2128 |pmid=12797825 }}</ref><ref>{{cite journal |doi=10.1021/acs.chemrev.8b00715 |title=Organocobalt Complexes as Sources of Carbon-Centered Radicals for Organic and Polymer Chemistries |date=2019 |last1=Demarteau |first1=Jérémy |last2=Debuigne |first2=Antoine |last3=Detrembleur |first3=Christophe |journal=Chemical Reviews |volume=119 |issue=12 |pages=6906–6955 |pmid=30964644 |s2cid=106409337 }}</ref>

==Reactivity==
The reactivity of a methyl group depends on the adjacent [[substituent]]s. Methyl groups can be quite unreactive. For example, in organic compounds, the methyl group resists attack by even the strongest [[acid]]s.{{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 [[proton]]s and [[carbon dioxide]], as seen in combustion.

===Methylation===
{{main|Methylation}}
Demethylation (the transfer of the methyl group to another compound) is a common process, and [[reagent]]s 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.<ref>Thauer, R. K., "Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson", Microbiology, 1998, volume 144, pages 2377–2406.</ref> Together with ubiquitin and phosphorylation, methylation is a major biochemical process for modifying protein function.<ref>{{cite journal|doi=10.1074/jbc.AW118.003235|pmid=29743234|pmc=6036201|title=The ribosome: A hot spot for the identification of new types of protein methyltransferases|journal=Journal of Biological Chemistry|volume=293|issue=27|pages=10438–10446|year=2018|last1=Clarke|first1=Steven G.|doi-access=free}}</ref>  The field of [[epigenetic]]s focuses on the influence of methylation on gene expression.<ref>{{Cite journal |last=Bird |first=Adrian |date=2002-01-01 |title=DNA methylation patterns and epigenetic memory |url=http://genesdev.cshlp.org/lookup/doi/10.1101/gad.947102 |journal=Genes & Development |language=en |volume=16 |issue=1 |pages=6–21 |doi=10.1101/gad.947102 |issn=0890-9369 |doi-access=free}}</ref>

===Deprotonation===
Certain methyl groups can be deprotonated. For example, the acidity of the methyl groups in [[acetone]] ({{chem2|(CH3)2CO}}) is about 10<sup>20</sup> times more acidic than methane. The resulting [[carbanion]]s are key intermediates in many reactions in [[organic synthesis]] and [[biosynthesis]]. [[Fatty acid]]s are produced in this way.

===Free radical reactions===
When placed in [[benzylic]] or [[allylic]] positions, the strength of the {{chem2|C\sH}} bond is decreased, and the reactivity of the methyl group increases. One manifestation of this enhanced reactivity is the [[Photochemistry|photochemical]] [[Halogenation|chlorination]] of the methyl group in [[toluene]] to give [[benzyl chloride]].<ref name="Ullmann">M. Rossberg et al. “Chlorinated Hydrocarbons” in Ullmann’s Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim.{{doi|10.1002/14356007.a06_233.pub2}}</ref>

==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]].<ref>{{cite web |url=http://www2.lsdiv.harvard.edu/labs/evans/pdf/smnr_2000-2001_Burch_Jason.pdf |title=Archived copy |access-date=2013-11-26 |url-status=dead |archive-url=https://web.archive.org/web/20100714113055/http://www2.lsdiv.harvard.edu/labs/evans/pdf/smnr_2000-2001_Burch_Jason.pdf |archive-date=2010-07-14}}</ref> 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 [[Stereochemistry|stereochemical]] course of several [[Biochemistry|biochemical]] transformations have been analyzed.<ref>Heinz G. Floss, Sungsook Lee "Chiral methyl groups: small is beautiful" Acc. Chem. Res., 1993, volume 26, pp 116–122.
{{doi|10.1021/ar00027a007}}</ref>

==Rotation==
{{See also|Rotamer}}
A methyl group may rotate around the {{chem2|R\sC}} 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''<sub>∞</sub> symmetry of the {{chem2|R\sC}} 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]].<ref>Press,W: Single-particle rotation in molecular crystals (Springer tracts in modern physics 92), Springer: Berlin (1981).</ref>

==Etymology==
French chemists [[Jean-Baptiste Dumas]] and [[Eugene Peligot]], after determining methanol's chemical structure, introduced "[[methylene (disambiguation)|methylene]]" from the [[Ancient Greek|Greek]] {{wikt-lang|grc|μέθυ}} (''methy'') "wine" and {{wikt-lang|grc|ὕλη}} (''hȳlē'') "wood, patch of trees" with the intention of highlighting its origins, "alcohol made from wood (substance)".<ref>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 [https://books.google.com/books?id=94c5AAAAcAAJ&pg=PA9 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 hulē, wood; that is, wine or spirit of wood.)</ref><ref>Note that the correct Greek word for the substance "wood" is ''xylo-''.</ref> 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==
{{Reflist}}

{{Functional Groups}}
{{Molecules detected in outer space}}
{{Authority control}}

{{DEFAULTSORT:Methyl Group}}
[[Category:Alkyl groups]]
[[Category:Functional groups]]