Methyl group

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Different ways of representing a methyl group (highlighted in blue) Methyl Group General Formulae V.1.png
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 CH3 (whereas normal methane has the formula 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 (−CH3), it can be found on its own in any of three forms: methanide anion (CH−3), methylium 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]

Contents

Methyl cation, anion, and radical

Methyl cation

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 ketene at low pressure (less than one torr) and its enthalpy of reaction is determined to be about 252.2 ± 3.3 kJ/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, methyl lithium and related 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

The methyl radical has the formula 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. [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.[ citation needed ]

Oxidation

The oxidation of a methyl group occurs widely in nature and industry. The oxidation products derived from methyl are hydroxymethyl group −CH2OH, formyl group −CHO, and 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

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. [8] Together with ubiquitin and phosphorylation, methylation is a major biochemical process for modifying protein function. [9] The field of 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 benzylic or 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 deuterium (D) and another hydrogen by tritium (T), the methyl substituent becomes 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 methyl chloride CH3Cl. In most molecules, the remainder R breaks the C∞ symmetry of the R−C axis and creates a potential V(φ) that restricts the free motion of the three protons. For the model case of ethane 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. [14]

Etymology

French chemists Jean-Baptiste Dumas and Eugene Peligot, after determining methanol's chemical structure, introduced "methylene" from the Greek μέθυ (methy) "wine" and ὕλη (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

Related Research Articles

<span class="mw-page-title-main">Carboxylic acid</span> Organic compound containing a –C(=O)OH group

In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group attached to an R-group. The general formula of a carboxylic acid is often written as R−COOH or R−CO2H, sometimes as R−C(O)OH with R referring to an organyl group, or hydrogen, or other groups. Carboxylic acids occur widely. Important examples include the amino acids and fatty acids. Deprotonation of a carboxylic acid gives a carboxylate anion.

<span class="mw-page-title-main">Ether</span> Organic compounds made of alkyl/aryl groups bound to oxygen (R–O–R)

In organic chemistry, ethers are a class of compounds that contain an ether group—an oxygen atom bonded to two organyl groups. They have the general formula R−O−R′, where R and R′ represent the organyl groups. Ethers can again be classified into two varieties: if the organyl groups are the same on both sides of the oxygen atom, then it is a simple or symmetrical ether, whereas if they are different, the ethers are called mixed or unsymmetrical ethers. A typical example of the first group is the solvent and anaesthetic diethyl ether, commonly referred to simply as "ether". Ethers are common in organic chemistry and even more prevalent in biochemistry, as they are common linkages in carbohydrates and lignin.

In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are Lewis bases.

<span class="mw-page-title-main">Methyl radical</span> Chemical compound

Methyl radical is an organic compound with the chemical formula CH•
3. It is a metastable colourless gas, which is mainly produced in situ as a precursor to other hydrocarbons in the petroleum cracking industry. It can act as either a strong oxidant or a strong reductant, and is quite corrosive to metals.

<span class="mw-page-title-main">Ethyl group</span> Chemical group (–CH₂–CH₃)

In organic chemistry, an ethyl group is an alkyl substituent with the formula −CH2CH3, derived from ethane. Ethyl is used in the International Union of Pure and Applied Chemistry's nomenclature of organic chemistry for a saturated two-carbon moiety in a molecule, while the prefix "eth-" is used to indicate the presence of two carbon atoms in the molecule.

In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term alkyl is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of −CnH2n+1. A cycloalkyl group is derived from a cycloalkane by removal of a hydrogen atom from a ring and has the general formula −CnH2n−1. Typically an alkyl is a part of a larger molecule. In structural formulae, the symbol R is used to designate a generic (unspecified) alkyl group. The smallest alkyl group is methyl, with the formula −CH3.

In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons.

In organic chemistry, a carbanion is an anion in which carbon is negatively charged.

<span class="mw-page-title-main">Organolithium reagent</span> Chemical compounds containing C–Li bonds

In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C−Li bond is highly ionic. Owing to the polar nature of the C−Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric.

<span class="mw-page-title-main">Allyl group</span> Chemical group (–CH₂–CH=CH₂)

In organic chemistry, an allyl group is a substituent with the structural formula −CH2−HC=CH2. It consists of a methylene bridge attached to a vinyl group. The name is derived from the scientific name for garlic, Allium sativum. In 1844, Theodor Wertheim isolated an allyl derivative from garlic oil and named it "Schwefelallyl". The term allyl applies to many compounds related to H2C=CH−CH2, some of which are of practical or of everyday importance, for example, allyl chloride.

<span class="mw-page-title-main">Radical anion</span> Free radical species

In organic chemistry, a radical anion is a free radical species that carries a negative charge. Radical anions are encountered in organic chemistry as reduced derivatives of polycyclic aromatic compounds, e.g. sodium naphthenide. An example of a non-carbon radical anion is the superoxide anion, formed by transfer of one electron to an oxygen molecule. Radical anions are typically indicated by .

<span class="mw-page-title-main">Dimethyl sulfate</span> Chemical compound

Dimethyl sulfate (DMS) is a chemical compound with formula (CH3O)2SO2. As the diester of methanol and sulfuric acid, its formula is often written as (CH3)2SO4 or Me2SO4, where CH3 or Me is methyl. Me2SO4 is mainly used as a methylating agent in organic synthesis. Me2SO4 is a colourless oily liquid with a slight onion-like odour. Like all strong alkylating agents, Me2SO4 is toxic. Its use as a laboratory reagent has been superseded to some extent by methyl triflate, CF3SO3CH3, the methyl ester of trifluoromethanesulfonic acid.

<span class="mw-page-title-main">Sulfonium</span> Cation of the form [SR3]+

In organic chemistry, a sulfonium ion, also known as sulphonium ion or sulfanium ion, is a positively-charged ion featuring three organic substituents attached to sulfur. These organosulfur compounds have the formula [SR3]+. Together with a negatively-charged counterion, they give sulfonium salts. They are typically colorless solids that are soluble in organic solvent.

<span class="mw-page-title-main">Diazonium compound</span> Group of organonitrogen compounds

Diazonium compounds or diazonium salts are a group of organic compounds sharing a common functional group [R−N+≡N]X− where R can be any organic group, such as an alkyl or an aryl, and X is an inorganic or organic anion, such as a halide. The parent compound where R is hydrogen, is diazenylium.

<span class="mw-page-title-main">Trimethylsilyldiazomethane</span> Chemical compound

Trimethylsilyldiazomethane is the organosilicon compound with the formula (CH3)3SiCHN2. It is classified as a diazo compound. Trimethylsilyldiazomethane, which is a commercially available, reagent used in organic chemistry as a methylating agent of carboxylic acids. Its behavior is akin to the reagent diazomethane, but the trimethylsilyl (TMS) analog is nonexplosive.

In chemistry, a reaction intermediate, or intermediate, is a molecular entity arising within the sequence of a stepwise chemical reaction. It is formed as the reaction product of an elementary step, from the reactants and/or preceding intermediates, but is consumed in a later step. It does not appear in the chemical equation for the overall reaction.

<span class="mw-page-title-main">Trimethyloxonium tetrafluoroborate</span> Chemical compound

Trimethyloxonium tetrafluoroborate is the organic compound with the formula [(CH3)3O]+[BF4]−. This salt is a strong methylating agent, being a synthetic equivalent of CH+3. It is a white solid that rapidly decomposes upon exposure to atmospheric moisture, although it is robust enough to be weighed quickly without inert atmosphere protection. Triethyloxonium tetrafluoroborate is a closely related compound.

Methylene is an organic compound with the chemical formula CH
2. It is a colourless gas that fluoresces in the mid-infrared range, and only persists in dilution, or as an adduct.

<span class="mw-page-title-main">Hexamethylbenzene</span> Chemical compound

Hexamethylbenzene, also known as mellitene, is a hydrocarbon with the molecular formula C12H18 and the condensed structural formula C6(CH3)6. It is an aromatic compound and a derivative of benzene, where benzene's six hydrogen atoms have each been replaced by a methyl group. In 1929, Kathleen Lonsdale reported the crystal structure of hexamethylbenzene, demonstrating that the central ring is hexagonal and flat and thereby ending an ongoing debate about the physical parameters of the benzene system. This was a historically significant result, both for the field of X-ray crystallography and for understanding aromaticity.

<span class="mw-page-title-main">Methenium</span> Ion of carbon with three hydrogens

In organic chemistry, methenium is a cation with the formula CH+
3. It can be viewed as a methylene radical with an added proton, or as a methyl radical with one electron removed. It is a carbocation and an enium ion, making it the simplest of the carbenium ions.

References

  1. ↑ March, Jerry (1992). Advanced organic chemistry: reactions, mechanisms, and structure. John Wiley & Sons. 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. ↑ 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. Archived from the original on 27 June 2023. Retrieved 27 June 2023.
  4. ↑ 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
  5. ↑ 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 . PMC   6024202 . PMID   30034765.
  6. ↑ Toraya, Tetsuo (2003). "Radical Catalysis in Coenzyme B12-Dependent Isomerization (Eliminating) Reactions". Chemical Reviews. 103 (6): 2095–2128. doi:10.1021/cr020428b. PMID   12797825.
  7. ↑ 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. PMID   30964644. S2CID   106409337.
  8. ↑ Thauer, R. K., "Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson", Microbiology, 1998, volume 144, pages 2377–2406.
  9. ↑ 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 . PMC   6036201 . PMID   29743234.
  10. ↑ Bird, Adrian (2002-01-01). "DNA methylation patterns and epigenetic memory". Genes & Development. 16 (1): 6–21. doi: 10.1101/gad.947102 . ISSN   0890-9369.
  11. ↑ M. Rossberg et al. “Chlorinated Hydrocarbons” in Ullmann’s Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim. 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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