Chemical group (?CH₃) derived from methane
In
organic chemistry
, a
methyl group
is an
alkyl
derived from
methane
, containing one
carbon
atom
bonded
to three
hydrogen
atoms, having chemical formula
CH
3
(whereas normal methane has the formula
CH
4
). 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 (
?CH
3
), 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]
Methyl cation, anion, and radical
[
edit
]
Methyl cation
[
edit
]
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 S
N
2 pathway:
- CH
3
OH + H
+
→ [CH
3
OH
2
]
+
Similarly,
methyl iodide
and methyl
triflate
are viewed as the equivalent of the methyl cation because they readily undergo S
N
2 reactions by weak
nucleophiles
.
The methyl cation has been detected in
interstellar space
.
[2]
[3]
Methyl anion
[
edit
]
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 + CH
3
X → MCH
3
+ MX
where M is an
alkali metal
.
Methyl radical
[
edit
]
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
[
edit
]
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
[
edit
]
The
oxidation
of a methyl group occurs widely in nature and industry. The oxidation products derived from methyl are
hydroxymethyl group
?CH
2
OH
,
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
[
edit
]
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
[
edit
]
Certain methyl groups can be deprotonated. For example, the acidity of the methyl groups in
acetone
(
(CH
3
)
2
CO
) is about 10
20
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
[
edit
]
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
[
edit
]
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
CHDTCO
2
H
). Through the use of chiral methyl groups, the
stereochemical
course of several
biochemical
transformations have been analyzed.
[13]
Rotation
[
edit
]
A methyl group may rotate around the
R?C
axis. This is a free rotation only in the simplest cases like gaseous
methyl chloride
CH
3
Cl
. 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
CH
3
CH
3
, 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
[
edit
]
French chemists
Jean-Baptiste Dumas
and
Eugene Peligot
, after determining methanol's chemical structure, introduced "
methylene
" from the
Greek
μ?θυ
(
methy
) "wine" and
?λη
(
h?l?
) "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
[
edit
]
References
[
edit
]
- ^
March, Jerry (1992).
Advanced organic chemistry: reactions, mechanisms, and structure
. John Wiley & Sons.
ISBN
0-471-60180-2
.
- ^
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
.
- ^
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
.
- ^
G. Barney Ellison , P. C. Engelking , W. C. Lineberger (1978), "An experimental determination of the geometry and electron affinity of methyl radical CH
3
" Journal of the American Chemical Society, volume 100, issue 8, pages 2556?2558.
doi
:
10.1021/ja00476a054
- ^
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
.
- ^
Toraya, Tetsuo (2003). "Radical Catalysis in Coenzyme B
12
-Dependent Isomerization (Eliminating) Reactions".
Chemical Reviews
.
103
(6): 2095?2128.
doi
:
10.1021/cr020428b
.
PMID
12797825
.
- ^
Demarteau, Jeremy; 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
.
- ^
Thauer, R. K., "Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson", Microbiology, 1998, volume 144, pages 2377?2406.
- ^
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
.
- ^
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
.
- ^
M. Rossberg et al. “Chlorinated Hydrocarbons” in Ullmann’s Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim.
doi
:
10.1002/14356007.a06_233.pub2
- ^
"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
)
- ^
Heinz G. Floss, Sungsook Lee "Chiral methyl groups: small is beautiful" Acc. Chem. Res., 1993, volume 26, pp 116?122.
doi
:
10.1021/ar00027a007
- ^
Press,W: Single-particle rotation in molecular crystals (Springer tracts in modern physics 92), Springer: Berlin (1981).
- ^
J. Dumas and E. Peligot (1835) "Memoire sur l'espirit de bois et sur les divers composes etheres 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 methylene (1) a un radical … (1) μεθυ, vin, et υλη, bois; c'est-a-dire vin ou liqueur spiritueuse du bois.
(We will give the name "methylene" (1) to a radical … (1) methy, wine, and hul?, wood; that is, wine or spirit of wood.)
- ^
Note that the correct Greek word for the substance "wood" is
xylo-
.
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