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Methylene (IUPAC name: methylidene, also called carbene or methene) is an organic compound with the chemical formula (also written and not to be confused with compressed hydrogen, which is also denoted ). 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.Roald Hoffman (2005), Molecular Orbitals of Transition Metal Complexes. Oxford. . It is usually detected only at cryogenics or as a short-lived intermediate in chemical reactions.W. B. DeMore and S. W. Benson (1964), Preparation, properties, and reactivity of methylene. In Advances in Photochemistry, John Wiley & Sons, 453 pages.
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 (), and methylidene (). 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 Ancient Greek ( methy) "wine" and ( 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)'.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.)
In contrast, methylene is connected to the rest of the molecule by two . 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 , 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 substituent a methylidene group for two geminal hydrogen atoms of some parent molecule (even if they are not actually obtained that way). Thus, for example, methylenecyclopropene is named after cyclopropene.
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The reactions of methylene were also studied around 1960 by infrared spectroscopy using matrix isolation experiments.
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.
Unsolvated, excited methylene will form stable ground state oligomers.
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.
Free methylene undergoes the typical chemical reactions of a carbene. Addition reactions are very fast and exothermic.
When the methylene molecule is in its ground state, the unpaired valence electrons are in separate with independent spins, a configuration known as triplet state.
Methylene may gain an electron yielding a monovalent anion methanidyl (), which can be obtained as the tetramethylammonium (()4) salt by the reaction of phenyl sodium () with tetramethylammonium bromide (()4). The ion has bent geometry, with a H-C-H angle of about 103°.
Methylene can bond as a terminal ligand, which is called methylidene, or as a bridging ligand, which is called methanediyl.
Structure
The ground state of methylene has an ionisation energy of 10.396 electronvolt. 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 kilocalorie/mol.
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 . Oxidation of these complexes yields formaldehyde, and reduction yields methane.
Milan Lazár (1989), ''Free radicals in chemistry and biology''. CRC Press.
Reactions with inorganic compounds
Methylene is also a common ligand in coordination compounds, such as copper methylene .
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.
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. 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
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