|Jmol-3D images||Image 1|
|Molar mass||14.0266 g mol−1|
|Solubility in water||Reacts|
|193.93 J K−1 mol−1|
|Std enthalpy of
|386.39 kJ mol−1|
|Related compounds||Methyl (CH3)
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
Methylene (systematically named λ2-methane, and dihydridocarbon), also called carbene 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.
The trivial name carbene is the preferred IUPAC name. The systematic names, λ2-methane and dihydridocarbon, valid IUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively.
In appropriate contexts, methylene can be viewed as methane with two hydrogen atoms removed, and as such, methylidene may be used as a context-specific systematic name, according to substitutive nomenclature. By default, this name pays no regard to the radicality of the methylene molecule. Although, in even more specific context, it can also name the non-radical singlet state, whereas the diradical ground state is named methanediyl.
Methylene is also used, non-systematically, to refer to the substituent groups methanediyl (>CH
2), and methylidene (=CH
2). Care should be taken to avoid confusing the names of the groups for the context-specific names for methylene given above.
Discovery and preparation
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), diazirine (cyclic [-CH
2-N=N-]) and diiodomethane (I-CH
2-I). The decomposition can be effected by photolysis, photosensistized reagents (such as benzophenone), or thermal decomposition.
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.
- 2 CH
2 → H
2 → HCCH + 2 H
Unsolvated, excited methylene will form stable ground state oligomers.
- 2 CH*
2 → H
Calculations determine that the ground state of methylene is a diradical triplet state. In this state, 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 molecule also occurs in a singlet state, where the two electrons share the same orbital (and therefore have opposite spins). This 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.
The singlet state is considerably more reactive than the triplet state, and behaves very differently in chemical reactions. With alkanes, for example, singlet methylene will insert after a methyl end group, while CH2•
2 removes a hydrogen leaving a free radical:
2 + H
3C-R → H-CH
2 + H-R → CH•
3 + R•
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.
Methylene may gain an electron yielding a monovalent anion methanidyl (CH•−
2), which can be obtained as the trimethylammonium (CH
salt by the reaction of phenyl sodium C
5Na with trimethylammonium bromide. The ion is bent too, with a H-C-H angle of about 103°.
Reactions with inorganic compounds
Methylene can bond as a terminal ligand, which is called methylidene, or as a bridging ligand, which is called methanediyl.
- "methanediyl (CHEBI:29357)". Chemical Entities of Biological Interest. UK: European Bioinformatics Institute. 14 January 2009. IUPAC Names. Retrieved 2 January 2012.
- Roald Hoffman (2005), Molecular Orbitals of Transition Metal Complexes. Oxford. ISBN 0-19-853093-5
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "carbenes".
- 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
- William B. DeMore, H. O. Pritchard, and Norman Davidson (1959), Photochemical experiments in rigid media at low temperatures. II. The reactions of methylene, cyclopentadienylene and diphenylmethylene. Journal of the American Chemical Society, volume 81, issue 22, pages 5874–5879. doi:10.1021/ja01531a008
- Marilyn E. Jacox and Dolphus E. Milligan (1963), 'Infrared study of the reactions of CH2 and NH with C2H2 and C2H4 in solid argon.Journal of the American Chemical Society, volume 85, issue 3, pages 278–282. doi:10.1021/ja00886a006
- "The Spin States of Carbenes. (No. 3036)", P.P. Gaspar and G.S. Hammond, Chapter 12 in "Carbene Chemistry", Vol. 1. W. Kirmse, Editor, Academic Press, New York, pp 235-274 (1964)
- "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
- 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
- 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. doi:10.1021/ja00249a013.