HOME

TheInfoList



OR:

In
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clay ...
, a methyl group is an
alkyl 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 . A cycloalkyl is derived from a cycloal ...
derived from
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ear ...
, containing one
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...
atom bonded to three
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
group occurs in many
organic compounds In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...
. It is a very stable group in most molecules. While the methyl group is usually part of a larger
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
, bounded to the rest of the molecule by a single covalent bond (), it can be found on its own in any of three forms: methanide
anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
(), methylium
cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
() or methyl radical (). The anion has eight
valence electron In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a shared pair f ...
s, the radical seven and the cation six. All three forms are highly reactive and rarely observed.


Methyl cation, anion, and radical


Methyl cation

The methylium cation () exists in the gas phase, but is otherwise not encountered. Some compounds are considered to be sources of the 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 Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one ...
and methyl triflate are viewed as the equivalent of the methyl cation because they readily undergo SN2 reactions by weak nucleophiles.


Methyl anion

The methanide anion () 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 .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. It is a powerful superbase; only the lithium monoxide anion () and the diethynylbenzene dianions are known to be stronger. In discussing mechanisms of organic reactions,
methyl lithium Methyllithium is the simplest organolithium reagent with the empirical formula CH3Li. This s-block organometallic compound adopts an oligomeric structure both in solution and in the solid state. This highly reactive compound, invariably used i ...
and related Grignard reagents are often considered to be salts of ; 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 . It exists in dilute gases, but in more concentrated form it readily dimerizes to ethane. It can be produced by thermal decomposition of only certain compounds, especially those with an –N=N– linkage.


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
oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
of a methyl group occurs widely in nature and industry. The oxidation products derived from methyl are –, –CHO, and –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 Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
, 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 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)2 SO4 or Me2SO4, where CH3 or Me is methyl. Me2SO4 is mainly used as a methylating agen ...
,
methyl iodide Iodomethane, also called methyl iodide, and commonly abbreviated "MeI", is the chemical compound with the formula CH3I. It is a dense, colorless, volatile liquid. In terms of chemical structure, it is related to methane by replacement of one ...
, and methyl triflate. Methanogenesis, the source of natural gas, arises via a demethylation reaction. Together with ubiquitin and phosphorylation, methylation is a major biochemical process for modifying protein function.


Deprotonation

Certain methyl groups can be deprotonated. For example, the acidity of the methyl groups in
acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscible wi ...
() is about 1020 times more acidic than methane. The resulting carbanions are key intermediates in many reactions in organic synthesis and biosynthesis.
Fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...
s 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.M. Rossberg et al. “Chlorinated Hydrocarbons” in Ullmann’s Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim.


Chiral methyl

In the special case where one hydrogen is replaced by
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
(D) and another hydrogen by
tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus ...
(T), the methyl substituent becomes chiral. Methods exist to produce optically pure methyl compounds, e.g., chiral
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...
(). Through the use of chiral methyl groups, the stereochemical course of several biochemical transformations have been analyzed.


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 . 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 , 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.


Etymology

French chemists Jean-Baptiste Dumas and Eugene Peligot, after determining methanol's chemical structure, introduced "methylene" from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
''methy'' "wine" and ''hȳlē'' "wood, patch of trees" with the intention of highlighting its origins, "alcohol made from wood (substance)".Note that the correct Greek word for the substance "wood" is ''xylo-''. The term "methyl" was derived in about 1840 by
back-formation In etymology, back-formation is the process or result of creating a new word via inflection, typically by removing or substituting actual or supposed affixes from a lexical item, in a way that expands the number of lexemes associated with the ...
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 In organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms tha ...
(or alkyl) molecule, using the prefix "meth-" to indicate the presence of a single carbon.


See also

* AdoMet


References

{{DEFAULTSORT:Methyl Group Alkyl groups Functional groups