In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom chemical bond to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In chemical formula, 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 (), it can be found on its own in any of three forms: methanide anion (), methylium cation () or methyl radical (). The anion has eight , 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 S
N2 pathway:
Similarly, methyl iodide and methyl triflate are viewed as the equivalent of the methyl cation because they readily undergo SN2 reactions by weak .
The methyl cation has been detected in interstellar space.
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 252.2 ± 3.3
Kilojoule/mol.
[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 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 :
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 is routinely produced by various enzymes of the
radical SAM and
methylcobalamin varieties.
Reactivity
The reactivity of a methyl group depends on the adjacent
. Methyl groups can be quite unreactive. For example, in organic compounds, the methyl group resists attack by even the strongest
.
Oxidation
The
oxidation of a methyl group occurs widely in nature and industry. The oxidation products derived from methyl are hydroxymethyl group ,
formyl group , and
carboxyl group . For example,
permanganate often converts a methyl group to a carboxyl () group, e.g. the conversion of
toluene to
benzoic acid. Ultimately oxidation of methyl groups gives
and
carbon dioxide, as seen in combustion.
Methylation
Demethylation (the transfer of the methyl group to another compound) is a common process, and
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.
[Thauer, R. K., "Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson", Microbiology, 1998, volume 144, pages 2377–2406.] Together with ubiquitin and phosphorylation, methylation is a major biochemical process for modifying protein function.
The field of
focuses on the influence of methylation on gene expression.
Deprotonation
Certain methyl groups can be deprotonated. For example, the acidity of the methyl groups in
acetone () is about 10
20 times more acidic than methane. The resulting
are key intermediates in many reactions in organic synthesis and
biosynthesis.
are produced in this way.
Free radical reactions
When placed in
benzylic or
allylic positions, the strength of the bond is decreased, and the reactivity of the methyl group increases. One manifestation of this enhanced reactivity is the
Photochemistry Halogenation 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 (D) and another hydrogen by
tritium (T), the methyl substituent becomes
chiral.
Methods exist to produce optically pure methyl compounds, e.g., chiral
acetic acid (deuterotritoacetic acid ). Through the use of chiral methyl groups, the
Stereochemistry course of several
Biochemistry transformations have been analyzed.
[Heinz G. Floss, Sungsook Lee "Chiral methyl groups: small is beautiful" Acc. Chem. Res., 1993, volume 26, pp 116–122.
]
Rotation
A methyl group may rotate around the 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 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.
[Press,W: Single-particle rotation in molecular crystals (Springer tracts in modern physics 92), Springer: Berlin (1981).]
Etymology
French chemists Jean-Baptiste Dumas and
Eugene Peligot, after determining methanol's chemical structure, introduced "methylene" from the
Ancient Greek (
methy) "wine" and (
hȳlē) "wood, patch of trees" with the intention of highlighting its origins, "alcohol made from wood (substance)".
[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.)][Note that the correct Greek word for the substance "wood" is xylo-.] 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
