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Methylene

Names
IUPAC name Dihydridocarbon(2•)
Preferred IUPAC name Methylidene
Other names Dihydridocarbon Carbene Methylene Methene
Identifiers
CAS Number	2465-56-7
3D model (JSmol)	Interactive image
Beilstein Reference	1696832
ChEBI	CHEBI:29357
ChemSpider	109779
Gmelin Reference	56
MeSH	carbene
PubChem CID	123164
CompTox Dashboard (EPA)	DTXSID50179391
InChI InChI=1S/CH2/h1H2Key: HZVOZRGWRWCICA-UHFFFAOYSA-N
SMILES
Properties
Chemical formula	CH 2
Molar mass	14.0266 g mol
Appearance	Colourless gas
Solubility in water	Reacts
Conjugate acid	Methenium
Thermochemistry
Std molar entropy (S298)	193.93 J K mol
Std enthalpy of formation (ΔfH298)	386.39 kJ mol
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 , 100 kPa). Infobox references

Methylene (IUPAC name: Methylidene, also called carbene or methene) is an organic compound with the chemical formula CH
₂ (also written ). 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. It is usually detected only at very low temperatures, or as a short-lived intermediate in chemical reactions.

Nomenclature

The trivial name carbene is the preferred IUPAC name. 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). It was introduced as early as 1835 by French chemists Jean-Baptiste Dumas and Eugene Peligot after determining methanol's chemical structure. They coined it the Greek μέθυ (methy) "wine" and ὕλη (hȳlē) "wood, patch of trees" (even though the correct Greek word for the substance "wood" is xylo-) with the intention of highlighting its origins, 'alcohol made from wood (substance)'.

Methylidene group

A methylidene group is any part of a molecule that consists of a CH₂= group. The group may be represented as =CH₂, where the '=' denotes the double bond.

In contrast, methylene is connected to the rest of the molecule by two single bonds. The distinction is often important, because the double bond is chemically different from two single bonds.

The same name (methylidene) was used for the distinct molecule CH₂, also known as carbene. Formerly the methylene name was used for all three isomers (methylene, methylidene, and carbene).

Many organic compounds are named and classified as if they were the result of substituting a methylidene group for two adjacent hydrogen atoms of some parent molecule (even if they are not actually obtained that way). Thus, for example, methylenecyclopropene is named after cyclopropene.

Preparation

Methylene can be prepared 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 ) and diiodomethane (I-CH
₂-I). The decomposition can be effected by photolysis, photosensitized reagents (such as benzophenone), or thermal decomposition. Methylene can be produced by photolysis of diazomethane. In its ultraviolet spectrum, gaseous methylene absorbs at around 141.5 nm. It was shown to have a bond angle of about 140°.

The reactions of methylene were also studied around 1960 by infrared spectroscopy using matrix isolation experiments.

Chemical properties

Radical character

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 (X̃B₁), and the first excited state is a singlet non-radical (ãA₁). With the singlet non-radical only 38 kJ above the ground state, 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.

(X̃B₁) + H
₂O → +

(ãA₁) + H
₂O → H
₂CO + H
₂ or H
₃COH

The singlet state is also more stereospecific than the triplet.

Methylene spontaneously autopolymerises 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.

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°, and is thus paramagnetic. (The correct prediction of this angle was an early success of ab initio quantum chemistry.) However conversion to a linear configuration requires only 5.5 kcal/mol.

The singlet state has a slightly higher energy (by about 9 kcal/mol) than the triplet state, 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.

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.

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
). The ion has bent geometry, with a H-C-H angle of about 103°.

Reactions with inorganic compounds

Methylene is also a common ligand in coordination compounds, such as copper methylene CuCH
₂.

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

In popular culture

The formula of the methylene molecule (CH₂) was mentioned as part of a Disney comic by the Donald Duck character in a comic in 1944 in a humorous vein. In the same spirit, the comic was eventually cited in the scientific literature by Peter Gaspar and George S. Hammond. The comic has been cited in other sources since, including a widely adopted textbook in organic chemistry by Robert Morrison and Robert Boyd.

See also

• Methyl radical
• Methylidyne radical
• Atomic carbon
• Alkene
• Methylene group
• Dichlorocarbene

References

1. ^ "methanediyl (CHEBI:29357)". Chemical Entities of Biological Interest. UK: European Bioinformatics Institute. 14 January 2009. IUPAC Names. Retrieved 2 January 2012.
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. ^ 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. 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.)
7. "methylidene (preferred IUPAC name)" (PDF). p. 314.
8. "methylene (preferred IUPAC name)" (PDF). p. 58.
9. "methylidene (preferred IUPAC name)" (PDF). p. 921.
10. ^ Shavitt, Isaiah (1985). "Geometry and singlet-triplet energy gap in methylene: A critical review of experimental and theoretical determinations". Tetrahedron. 41 (8): 1531–1542. doi:10.1016/S0040-4020(01)96393-8.
11. 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.
12. 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
13. 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.
14. 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)
15. Milan Lazár (1989), Free radicals in chemistry and biology. CRC Press. ISBN 0-8493-5387-4
16. Chang, Sou Chan; Kafafi, Zakya H.; Hauge, Robert H.; Billups, W. Edward; Margrave, John L. (1987). "Isolation and characterization of copper methylene (CuCH2) via FTIR matrix isolation spectroscopy". Journal of the American Chemical Society. 109 (15): 4508–4513. doi:10.1021/ja00249a013.
17. 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.
18. "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.
19. Morrison, Robert Thornton; Boyd, Robert Neilson (1973). Organic Chemistry. Allyn and Bacon. ISBN 978-0-205-04136-7.

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Molecules	Diatomic Aluminium monochloride Aluminium monofluoride Aluminium(II) oxide Argonium Carbon cation Carbon monophosphide Carbon monosulfide Carbon monoxide Cyano radical Diatomic carbon Fluoromethylidynium Helium hydride ion Hydrogen chloride Hydrogen fluoride Hydrogen (molecular) Hydroxyl radical Iron(II) oxide Magnesium monohydride Methylidyne radical Nitric oxide Nitrogen (molecular) Imidogen Sulfur mononitride Oxygen (molecular) Phosphorus monoxide Phosphorus mononitride Potassium chloride Silicon carbide Silicon monoxide Silicon monosulfide Sodium chloride Sodium iodide Sulfanyl Sulfur monoxide Titanium(II) oxide Triatomic Aluminium(I) hydroxide Aluminium isocyanide Amino radical Carbon dioxide Carbonyl sulfide CCP radical Chloronium Diazenylium Dicarbon monoxide Disilicon carbide Ethynyl radical Formyl radical Hydrogen cyanide (HCN) Hydrogen isocyanide (HNC) Hydrogen sulfide Hydroperoxyl Iron cyanide Isoformyl Magnesium cyanide Magnesium isocyanide Methylene N2H Nitrous oxide Nitroxyl Ozone Methylidynephosphane Potassium cyanide Trihydrogen cation Sodium cyanide Sodium hydroxide Silicon carbonitride c-Silicon dicarbide SiNC Sulfur dioxide Thioformyl Thioxoethenylidene Titanium dioxide Tricarbon Water Four atoms Acetylene Ammonia Isocyanic acid Cyanoethynyl Formaldehyde Fulminic acid HCCN Hydrogen peroxide Hydromagnesium isocyanide Isocyanic acid Isothiocyanic acid Ketenyl Methylene amidogen Methyl cation Methyl radical Propynylidyne Protonated carbon dioxide Protonated hydrogen cyanide Silicon tricarbide Thioformaldehyde Tricarbon monoxide Tricarbon monosulfide Thiocyanic acid Five atoms Ammonium ion Butadiynyl Carbodiimide Cyanamide Cyanoacetylene Cyanoformaldehyde Cyanomethyl Cyclopropenylidene Formic acid Isocyanoacetylene Ketene Methane Methoxy radical Methylenimine Propadienylidene Protonated formaldehyde Silane Silicon-carbide cluster Six atoms Acetonitrile Cyanobutadiynyl radical E-Cyanomethanimine Cyclopropenone Diacetylene Ethylene Formamide HC4N Ketenimine Methanethiol Methanol Methyl isocyanide Pentynylidyne Propynal Protonated cyanoacetylene Seven atoms Acetaldehyde Acrylonitrile Vinyl cyanide Cyanodiacetylene Ethylene oxide Glycolonitrile Hexatriynyl radical Propyne Methylamine Methyl isocyanate Vinyl alcohol Eight atoms Acetic acid Aminoacetonitrile Cyanoallene Ethanimine Glycolaldehyde Hexapentaenylidene Methylcyanoacetylene Methyl formate Acrolein Nine atoms Acetamide Cyanohexatriyne Dimethyl ether Ethanol Methyldiacetylene Octatetraynyl radical Propene Ethanethiol Propionitrile N-Methylformamide Ten atoms or more Acetone Benzene Benzonitrile Buckminsterfullerene (C60, C60, fullerene, buckyball) C70 fullerene Cyanodecapentayne Ethylene glycol Ethyl formate Methyl acetate Methyl-cyano-diacetylene Methyltriacetylene Propionaldehyde Butyronitrile Pyrimidine Heptatrienyl radical
Deuterated molecules	Ammonia Ammonium ion Formaldehyde Formyl radical Heavy water Hydrogen cyanide Hydrogen deuteride Hydrogen isocyanide Propyne N2D Trihydrogen cation
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