Methyl group

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Different ways of representing a methyl group (highlighted in blue)

In

cation (CH+3) or methyl radical (CH•
3). The anion has eight valence electrons, the radical seven and the cation six. All three forms are highly reactive and rarely observed.[1]

Methyl cation, anion, and radical

Methyl cation

Main article: Methenium

The methylium cation (CH+3) exists in the

gas phase, but is otherwise not encountered. Some compounds are considered to be sources of the CH+3 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:

CH3OH + H+ → [CH3OH2]+

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.[2][3]

Methyl anion

The methanide anion (CH−3) exists only in rarefied gas phase or under exotic conditions. It can be produced by electrical discharge in

mol.[4] It is a powerful superbase; only the lithium monoxide anion (LiO−) and the diethynylbenzene dianions are known to be stronger.[5]

In discussing mechanisms of organic reactions,

Grignard reagents
are often considered to be salts of CH−3; 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:

2 M + CH3X → MCH3 + MX

where M is an alkali metal.

Methyl radical

Main article: Methyl radical

The methyl

dimerizes to ethane. It is routinely produced by various enzymes of the radical SAM and methylcobalamin varieties.[6][7]

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.

Oxidation

The

carboxyl group −COOH. For example, permanganate often converts a methyl group to a carboxyl (−COOH) 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

Main article: Methylation

Demethylation (the transfer of the methyl group to another compound) is a common process, and

epigenetics focuses on the influence of methylation on gene expression.[10]

Deprotonation

Certain methyl groups can be deprotonated. For example, the acidity of the methyl groups in acetone ((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

allylic positions, the strength of the C−H 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.[11]

Chiral methyl

In the special case where one hydrogen is replaced by

chiral.[12] Methods exist to produce optically pure methyl compounds, e.g., chiral acetic acid (deuterotritoacetic acid CHDTCO2H). Through the use of chiral methyl groups, the stereochemical course of several biochemical transformations have been analyzed.[13]

Rotation

A methyl group may rotate around the R−C axis. This is a free rotation only in the simplest cases like gaseous

ethane barrier
. In condensed phases, neighbour molecules also contribute to the potential. Methyl group rotation can be experimentally studied using quasielastic neutron scattering.[14]

Etymology

French chemists

Eugene Peligot, after determining methanol's chemical structure, introduced "methylene" from the Greek methy "wine" and 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 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

References

  1. ISBN 0-471-60180-2
    .
  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. Archived from the original on 27 June 2023. Retrieved 27 June 2023.
  3. doi:10.1038/s41586-023-06307. Archived
    from the original on 27 June 2023. Retrieved 27 June 2023.
  4. doi:10.1021/ja00476a054
  5. PMID 30034765
    .
  6. PMID 12797825
    .
  7. S2CID 106409337
    .
  8. ^ Thauer, R. K., "Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson", Microbiology, 1998, volume 144, pages 2377–2406.
  9. PMID 29743234
    .
  10. ISSN 0890-9369
    .
  11. doi:10.1002/14356007.a06_233.pub2
  12. ^ "Archived copy" (PDF). Archived from the original (PDF) on 2010-07-14. Retrieved 2013-11-26.{{cite web}}: CS1 maint: archived copy as title (link)
  13. ^ Heinz G. Floss, Sungsook Lee "Chiral methyl groups: small is beautiful" Acc. Chem. Res., 1993, volume 26, pp 116–122.
    doi:10.1021/ar00027a007
  14. ^ Press,W: Single-particle rotation in molecular crystals (Springer tracts in modern physics 92), Springer: Berlin (1981).
  15. ^ 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 hulē, wood; that is, wine or spirit of wood.)
  16. ^ Note that the correct Greek word for the substance "wood" is xylo-.
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