Methylene (compound)

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Ball-and-stick model of triplet methylene Triplet-methylene-3D-balls.png
Ball-and-stick model of triplet methylene
Space filling model of triplet methylene Triplet-methylene-3D-SF.png
Space filling model of triplet methylene
IUPAC name
Dihydridocarbon(2•) [1]
Preferred IUPAC name
Methylidene [2]
Other names
Methene [1]
3D model (JSmol)
MeSH carbene
PubChem CID
  • InChI=1S/CH2/h1H2 Yes check.svgY
  • [CH2]
Molar mass 14.0266 g mol−1
AppearanceColourless gas
Conjugate acid Methenium
Std molar
193.93 J K−1 mol−1
386.39 kJ mol−1
Related compounds
Related compounds
Methyl (CH3)
Methylidyne (CH)
Carbide (C)
Silylene (SiH2)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Methylene (IUPAC name: Methylidene, also called carbene or methene) is an organic compound with the chemical formula CH
(also written [CH
). It is a colourless gas that fluoresces in the mid-infrared range, and only persists in dilution, or as an adduct.


Methylene is the simplest carbene. [3] :p.7 [4] It is usually detected only at very low temperatures, or as a short-lived intermediate in chemical reactions. [5]


The trivial name carbene is the preferred IUPAC name.[ citation needed ] The systematic names methylidene and dihydridocarbon, valid IUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively.

Methylidene is viewed as methane with two hydrogen atoms removed. By default, this name pays no regard to the radicality of the methylene. Although in a context where the radicality is considered, it can also name the non-radical excited state, whereas the radical ground state with two unpaired electrons is named methanediyl.

Methylene is also used as the trivial name for the substituent groups methanediyl (>CH
), and methylidene (=CH

Methylene has an electron affinity of 0.65 eV. [6]

Discovery and preparation

Using the technique of flash photolysis with the compound diazomethane, Gerhard Herzberg and Jack Shoosmith [7] were the first to produce and spectroscopically characterize the methylene molecule. In their work they obtained the ultraviolet spectrum of gas phase methylene at around 141.5 nm. Their analysis of the spectrum lead them to the conclusion that the ground electronic state was an electronic triplet state and that the equilibrium structure was either linear, or else it had a large bond angle of about 140°. It turns out that the latter is correct. [8] The reactions of methylene were also studied around 1960, by infrared spectroscopy in frozen gas matrix isolation experiments. [9] [10]

Methylene can be prepared, under suitable conditions, by decomposition of compounds with a methylidene or methanediyl group, such as ketene (ethenone) (CH
=CO), diazomethane (linear CH
), diazirine (cyclic [-CH
-N=N-]) and diiodomethane (I-CH
-I). The decomposition can be effected by photolysis, photosensitized reagents (such as benzophenone), or thermal decomposition. [5] [11]

The methylene molecule (CH2) was mentioned for the first time by Donald Duck in a comic in 1944. [12] [13]

Chemical properties


Many of methylene's electronic states lie relatively close to each other, giving rise to varying degrees of radical chemistry. The ground state is a triplet radical with two unpaired electrons (X̃3B1), [11] and the first excited state is a singlet non-radical (ã1A1). With the singlet non-radical only 38 kJ above the ground state, [11] a sample of methylene exists as a mixture of electronic states even at room temperature, giving rise to complex reactions. For example, reactions of the triplet radical with non-radical species generally involves abstraction, whereas reactions of the singlet non-radical not only involves abstraction, but also insertion or addition.

2•(X̃3B1) + H
+ [HO]
(ã1A1) + H
+ H
or H

The singlet state is also more stereospecific than the triplet. [11]

Unsolvated methylene will spontaneously autopolymerise to form various excited oligomers, the simplest of which, is the excited form of the alkene ethylene. The excited oligomers, decompose rather than decay to a ground state. For example, the excited form of ethylene decomposes to acetylene and atomic hydrogen. [11]

2 CH
→ HCCH + 2 H

Unsolvated, excited methylene will form stable ground state oligomers.

2 CH*


The ground state of methylene has an ionisation energy of 10.396  eV. It has a bent configuration, with H-C-H angle of 133.84°, [11] and is thus paramagnetic. (The correct prediction of this angle was an early success of ab initio quantum chemistry. [11] ) However conversion to a linear configuration requires only 5.5  kcal/mol. [11]

The singlet state has a slightly higher energy (by about 9 kcal/mol) than the triplet state, [11] and its H-C-H angle is smaller, about 102°. In dilute mixtures with an inert gas, the two states will convert to each other until reaching an equilibrium. [11]

Chemical reactions

Organic chemistry

Neutral methylene complexes undergo different chemical reactions depending on the pi character of the coordinate bond to the carbon centre. A weak contribution, such as in diazomethane, yields mainly substitution reactions, whereas a strong contribution, such as in ethenone, yields mainly addition reactions. Upon treatment with a standard base, complexes with a weak contribution convert to a metal methoxide. With strong acids (e.g., fluorosulfuric acid), they can be protonated to give CH
. Oxidation of these complexes yields formaldehyde, and reduction yields methane.

Free methylene undergoes the typical chemical reactions of a carbene. Addition reactions are very fast and exothermic. [14]

When the methylene molecule is in its state of lowest energy, the unpaired valence electrons are in separate atomic orbitals with independent spins, a configuration known as triplet state.

Methylene may gain an electron yielding a monovalent anion methanidyl (CH•−
), which can be obtained as the trimethylammonium ((CH
) salt by the reaction of phenyl sodium (C
) with trimethylammonium bromide ((CH
). [5] The ion has bent geometry, with a H-C-H angle of about 103°. [11]

Reactions with inorganic compounds

Methylene is also a common ligand in coordination compounds, such as copper methylene CuCH
. [15]

Methylene can bond as a terminal ligand, which is called methylidene, or as a bridging ligand, which is called methanediyl.

See also

Related Research Articles

<span class="mw-page-title-main">Functional group</span> Set of atoms in a molecule which augment its chemical and/or physical properties

In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest of the molecule's composition. This enables systematic prediction of chemical reactions and behavior of chemical compounds and the design of chemical synthesis. The reactivity of a functional group can be modified by other functional groups nearby. Functional group interconversion can be used in retrosynthetic analysis to plan organic synthesis.

<span class="mw-page-title-main">Photochemistry</span> Sub-discipline of chemistry

Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet, visible light (400–750 nm) or infrared radiation (750–2500 nm).

In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is R−:C−R' or R=C: where the R represents substituents or hydrogen atoms.

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

Silylene is a chemical compound with the formula SiH2. It is the silicon analog of methylene, the simplest carbene. Silylene is a stable molecule as a gas but rapidly reacts in a bimolecular manner when condensed. Unlike carbenes, which can exist in the singlet or triplet state, silylene (and all of its derivatives) are singlets.

In chemistry, a nitrene or imene is the nitrogen analogue of a carbene. The nitrogen atom is uncharged and univalent, so it has only 6 electrons in its valence level—two covalent bonded and four non-bonded electrons. It is therefore considered an electrophile due to the unsatisfied octet. A nitrene is a reactive intermediate and is involved in many chemical reactions. The simplest nitrene, HN, is called imidogen, and that term is sometimes used as a synonym for the nitrene class.

<span class="mw-page-title-main">Singlet oxygen</span> Oxygen with all of its electrons spin paired

Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with the formula O=O (also written as 1
or 1
), which is in a quantum state where all electrons are spin paired. It is kinetically unstable at ambient temperature, but the rate of decay is slow.

<span class="mw-page-title-main">Photosensitizer</span> Type of molecule reacting to light

Photosensitizers are light absorbers that alters the course of a photochemical reaction. They usually are catalysts. They can function by many mechanisms, sometimes they donate an electron to the substrate, sometimes they abstract a hydrogen atom from the substrate. At the end of this process, the photosensitizer returns to its ground state, where it remains chemically intact, poised to absorb more light. One branch of chemistry which frequently utilizes photosensitizers is polymer chemistry, using photosensitizers in reactions such as photopolymerization, photocrosslinking, and photodegradation. Photosensitizers are also used to generate prolonged excited electronic states in organic molecules with uses in photocatalysis, photon upconversion and photodynamic therapy. Generally, photosensitizers absorb electromagnetic radiation consisting of infrared radiation, visible light radiation, and ultraviolet radiation and transfer absorbed energy into neighboring molecules. This absorption of light is made possible by photosensitizers' large de-localized π-systems, which lowers the energy of HOMO and LUMO orbitals to promote photoexcitation. While many photosensitizers are organic or organometallic compounds, there are also examples of using semiconductor quantum dots as photosensitizers.

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Methylidyne, or (unsubstituted) carbyne, is an organic compound whose molecule consists of a single hydrogen atom bonded to a carbon atom. It is the parent compound of the carbynes, which can be seen as obtained from it by substitution of other functional groups for the hydrogen.

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Trimethylenemethane is a chemical compound with formula C
. It is a neutral free molecule with two unsatisfied valence bonds, and is therefore a highly reactive free radical. Formally, it can be viewed as an isobutylene molecule C
with two hydrogen atoms removed from the terminal methyl groups.

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    Footnote 91 cites the relevant issue of Walt Disney's Comics and Stories
  13. "If I mix CH2 with NH4 and boil the atoms in osmotic fog, I should get speckled nitrogen." Walt Disney's Comics and Stories , issue 44, 1944
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  15. Sou-Chan Chang, Zakya H. Kafafi, Robert H. Hauge, W. Edward Billups, and John L. Margrave (1987), Isolation and characterization of copper methylene (CuCH2) via FTIR matrix isolation spectroscopy . Journal of the American Chemical Society, volume 109 pages 4508-4513. doi : 10.1021/ja00249a013.