Methylene (compound)
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| Names | |||
|---|---|---|---|
| IUPAC name
Dihydridocarbon(2•)[1]
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| Preferred IUPAC name
Methylidene[2] | |||
| Other names | |||
| Identifiers | |||
3D model (
JSmol ) |
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| 1696832 | |||
| ChEBI | |||
| ChemSpider | |||
| 56 | |||
| MeSH | carbene | ||
PubChem CID
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CompTox Dashboard (EPA)
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| Properties | |||
| CH 22• | |||
| Molar mass | 14.0266 g mol−1 | ||
| Appearance | Colourless gas | ||
| Reacts | |||
Conjugate acid
|
Methenium | ||
| Thermochemistry | |||
Std molar
entropy (S⦵298) |
193.93 J K−1 mol−1 | ||
Std enthalpy of (ΔfH⦵298)formation |
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).
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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.
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
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
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
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
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
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
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
Methylene can bond as a terminal ligand, which is called methylidene, or as a bridging ligand, which is called methanediyl.
See also
References
- ^ a b "methanediyl (CHEBI:29357)". Chemical Entities of Biological Interest. UK: European Bioinformatics Institute. 14 January 2009. IUPAC Names. Retrieved 2 January 2012.
- ^ ISBN 978-0-85404-182-4.
- ISBN 0-19-853093-5
- ^ a b c
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
- ^ "Methylene". webbook.nist.gov. Retrieved 12 April 2018.
- S2CID 4272040.
- ^ 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
- .
- doi:10.1021/ja00886a006.)
{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link - ^ a b c d e f g h i j k 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
- 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 - ^ "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
- ^
Milan Lazár (1989), Free radicals in chemistry and biology. CRC Press. ISBN 0-8493-5387-4
- ^ 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. .