Methylene (compound)

Last updated
Methylene
Carbene.svg
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
Names
IUPAC name
Dihydridocarbon(2•) [1]
Preferred IUPAC name
Methylidene [2]
Other names
Dihydridocarbon
Carbene
Methylene
Methene [1]
Identifiers
3D model (JSmol)
1696832
ChEBI
ChemSpider
56
MeSH carbene
PubChem CID
  • InChI=1S/CH2/h1H2 Yes check.svgY
    Key: HZVOZRGWRWCICA-UHFFFAOYSA-N Yes check.svgY
  • [CH2]
Properties
CH
22•
Molar mass 14.0266 g mol−1
AppearanceColourless gas
Reacts
Conjugate acid Methenium
Thermochemistry
Std molar
entropy (S⦵298)
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
2 (also written [CH
2]). It is a colourless gas that fluoresces in the mid-infrared range, and only persists in dilution, or as an adduct.

Contents

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]

Nomenclature

The trivial name carbene is the preferred IUPAC name. [2] 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
2), and methylidene (=CH
2).

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
2=CO), diazomethane (linear CH
2=N
2), diazirine (cyclic [-CH
2-N=N-]) and diiodomethane (I-CH
2-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

Radicality

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.

[CH
2]2•(X̃3B1) + H
2O → [CH
3]• + [HO]•
[CH
2](ã1A1) + H
2O → H
2CO + H
2 or H
3COH

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
2 → H
2CCH*
2 → HCCH + 2 H

Unsolvated, excited methylene will form stable ground state oligomers.

2 CH*
2 → H
2CCH
2

Structure

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
3L+
. 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•−
2), which can be obtained as the trimethylammonium ((CH
3)4N+
) salt by the reaction of phenyl sodium (C
6H
5Na) with trimethylammonium bromide ((CH
3)4N+
Br−
). [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
2. [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">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.

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
[O
2] or 1
O
2), 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 alter 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.

<span class="mw-page-title-main">Triplet oxygen</span> Triplet state of the dioxygen molecule

Triplet oxygen, 3O2, refers to the S = 1 electronic ground state of molecular oxygen (dioxygen). Molecules of triplet oxygen contain two unpaired electrons, making triplet oxygen an unusual example of a stable and commonly encountered diradical: it is more stable as a triplet than a singlet. According to molecular orbital theory, the electron configuration of triplet oxygen has two electrons occupying two π molecular orbitals (MOs) of equal energy (that is, degenerate MOs). In accordance with Hund's rules, they remain unpaired and spin-parallel, which accounts for the paramagnetism of molecular oxygen. These half-filled orbitals are antibonding in character, reducing the overall bond order of the molecule to 2 from the maximum value of 3 that would occur when these antibonding orbitals remain fully unoccupied, as in dinitrogen. The molecular term symbol for triplet oxygen is 3Σ−
g.

<span class="mw-page-title-main">Persistent carbene</span> Type of carbene demonstrating particular stability

A persistent carbene (also known as stable carbene) is a type of carbene demonstrating particular stability. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC) (sometimes called Arduengo carbenes), for example diaminocarbenes with the general formula (R2N)2C:, where the four R moieties are typically alkyl and aryl groups. The groups can be linked to give heterocyclic carbenes, such as those derived from imidazole, imidazoline, thiazole or triazole.

In chemistry, a diradical is a molecular species with two electrons occupying molecular orbitals (MOs) which are degenerate. The term "diradical" is mainly used to describe organic compounds, where most diradicals are extremely reactive and in fact rarely isolated. Diradicals are even-electron molecules but have one fewer bond than the number permitted by the octet rule.

<span class="mw-page-title-main">Cyclopropanation</span> Chemical process which generates cyclopropane rings

In organic chemistry, cyclopropanation refers to any chemical process which generates cyclopropane rings. It is an important process in modern chemistry as many useful compounds bear this motif; for example pyrethroid insecticides and a number of quinolone antibiotics. However, the high ring strain present in cyclopropanes makes them challenging to produce and generally requires the use of highly reactive species, such as carbenes, ylids and carbanions. Many of the reactions proceed in a cheletropic manner.

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

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.

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

Diatomic carbon (systematically named dicarbon and 1λ2,2λ2-ethene), is a green, gaseous inorganic chemical with the chemical formula C=C (also written [C2] or C2). It is kinetically unstable at ambient temperature and pressure, being removed through autopolymerisation. It occurs in carbon vapor, for example in electric arcs; in comets, stellar atmospheres, and the interstellar medium; and in blue hydrocarbon flames. Diatomic carbon is the second simplest of the allotropes of carbon (after atomic carbon), and is an intermediate participator in the genesis of fullerenes.

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

Atomic carbon, systematically named carbon and λ0-methane, is a colourless gaseous inorganic chemical with the chemical formula C. It is kinetically unstable at ambient temperature and pressure, being removed through autopolymerisation.

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

Trimethylenemethane is a chemical compound with formula C
4H
6. 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
4H
8 with two hydrogen atoms removed from the terminal methyl groups.

In spectroscopy and quantum chemistry, the multiplicity of an energy level is defined as 2S+1, where S is the total spin angular momentum. States with multiplicity 1, 2, 3, 4, 5 are respectively called singlets, doublets, triplets, quartets and quintets.

<span class="mw-page-title-main">Radical (chemistry)</span> Atom, molecule, or ion that has an unpaired valence electron; typically highly reactive

In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes.

<span class="mw-page-title-main">Imidogen</span> Inorganic radical with the chemical formula NH

Imidogen is an inorganic compound with the chemical formula NH. Like other simple radicals, it is highly reactive and consequently short-lived except as a dilute gas. Its behavior depends on its spin multiplicity.

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

A methylene group is any part of a molecule that consists of two hydrogen atoms bound to a carbon atom, which is connected to the remainder of the molecule by two single bonds. The group may be represented as −CH2− or >CH2, where the '>' denotes the two bonds.

In organic chemistry, a methylene bridge, methylene spacer, or methanediyl group is any part of a molecule with formula −CH2−; namely, a carbon atom bound to two hydrogen atoms and connected by single bonds to two other distinct atoms in the rest of the molecule. It is the repeating unit in the skeleton of the unbranched alkanes.

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

9-Fluorenylidene is an aryl carbene derived from the bridging methylene group of fluorene. Fluorenylidene has the unusual property that the triplet ground state is only 1.1 kcal/mol lower in energy than the singlet state. For this reason, fluorenylidene has been studied extensively in organic chemistry.

Methylidenecarbene (systematically named λ2-ethene and dihydrido-1κ2H-dicarbon(C—C)) is an organic compound with the chemical formula C=CH
2 (also written [CCH
2] or C
2H
2). It is a metastable proton tautomer of acetylene, which only persists as an adduct. It is a colourless gas that phosphoresces in the far-infrared range. It is the simplest unsaturated carbene.

References

  1. 1 2 "methanediyl (CHEBI:29357)". Chemical Entities of Biological Interest. UK: European Bioinformatics Institute. 14 January 2009. IUPAC Names. Retrieved 2 January 2012.
  2. 1 2 Henri A. Favre; Warren H. Powell (2014). Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013. Cambridge, England: Royal Society of Chemistry. p. 1054. ISBN   978-0-85404-182-4.
  3. ↑ Roald Hoffman (2005), Molecular Orbitals of Transition Metal Complexes. Oxford. ISBN   0-19-853093-5
  4. ↑ IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) " carbenes ". doi : 10.1351/goldbook.C00806
  5. 1 2 3 W. B. DeMore and S. W. Benson (1964), Preparation, properties, and reactivity of methylene . In Advances in Photochemistry, John Wiley & Sons, 453 pages. ISBN   0470133597
  6. ↑ "Methylene". webbook.nist.gov. Retrieved 12 April 2018.
  7. ↑ Herzberg, G.; Shoosmith, J. (1959). "Spectrum and Structure of the Free Methylene Radical". Nature. 183 (4678): 1801–1802. Bibcode:1959Natur.183.1801H. doi:10.1038/1831801a0. S2CID   4272040.
  8. ↑ P.R. Bunker, 'The Spectrum, Structure, and Singlet-Triplet Splitting in Methylene CH2.' Chapter in ‘Comparison of Ab Initio Quantum Chemistry with Experiment for small molecules’, ed. Rodney J. Bartlett, Reidel Dordrecht The Netherlands (1985). ISBN   978-9027721297
  9. ↑ Demore, William B; Pritchard, H. O; Davidson, Norman (1959). "Photochemical Experiments in Rigid Media at Low Temperatures. II. The Reactions of Methylene, Cyclopentadienylene and Diphenylmethylene". Journal of the American Chemical Society. 81 (22): 5874. doi:10.1021/ja01531a008.
  10. ↑ Jacox, [ILL] E; Milligan, Dolphus E (1963). "Infrared Study of the Reactions of CH2and NH with C2H2and C2H4in Solid Argon". Journal of the American Chemical Society. 85 (3): 278. doi:10.1021/ja00886a006.{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  11. 1 2 3 4 5 6 7 8 9 10 11 Isaiah Shavitt (1985), Geometry and singlet-triplet energy gap in methylene: A critical review of experimental and theoretical determinations. Tetrahedron, volume 41, issue 8, page 1531 doi : 10.1016/S0040-4020(01)96393-8
  12. ↑ Gaspar, Peter P.; Hammond, George S. (1964). "Chapter 12: The Spin States of Carbenes". In Kirmse, Wolfgang (ed.). Carbene Chemistry. Vol. 1. New York: Academic Press. pp. 235–274. OCLC   543711. Among experiments which have not, to our knowledge, been carried out as yet is one of a most intriguing nature suggested in the literature of no less than 19 years ago (91).
    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
  14. ↑ Milan Lazár (1989), Free radicals in chemistry and biology. CRC Press. ISBN   0-8493-5387-4
  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.
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