 | |||
| |||
| Names | |||
|---|---|---|---|
| IUPAC name Dihydridocarbon(2•)[1] | |||
| Preferred IUPAC name Methylidene[2] | |||
| Other names | |||
| Identifiers | |||
3D model (JSmol) | |||
| 1696832 | |||
| ChEBI | |||
| ChemSpider |
| ||
| 56 | |||
| MeSH | carbene | ||
| |||
| |||
| Properties | |||
| CH 22• | |||
| Molar mass | 14.0266 g/mol | ||
| Appearance | Colourless gas | ||
| Reacts | |||
| Conjugate acid | Methenium | ||
| Thermochemistry | |||
Std molar entropy(S⦵298) | 193.93 J/(K⋅mol) | ||
Std enthalpy of formation(ΔfH⦵298) | 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 theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Methylene (IUPAC name:methylidene, also calledcarbene ormethene) is anorganic compound with thechemical formulaCH
2 (also written[CH
2] and not to be confused withcompressed hydrogen, which is also denotedCH
2). It is a colourless gas that fluoresces in the mid-infrared range, and only persists in dilution, or as anadduct.
Methylene is the simplestcarbene.[3]: p.7 [4] It is usually detected only atvery low temperatures or as a short-lived intermediate inchemical reactions.[5]
Thetrivial namecarbene is thepreferred IUPAC name.[2] The systematic namesmethylidene anddihydridocarbon, validIUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively.
Methylidene is viewed asmethane 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-radicalexcited state, whereas the radicalground state with two unpaired electrons is namedmethanediyl.
Methylene is also used as the trivial name for thesubstituent groupsmethanediyl (>CH
2), andmethylidene (=CH
2). It was introduced as early as 1835 by French chemistsJean-Baptiste Dumas andEugene Peligot after determiningmethanol's chemical structure. They coined it theGreekμέθυ (methy) "wine" andὕλη (hȳlē) "wood, patch of trees" (even though the correct Greek word for the substance "wood" isxylo-) with the intention of highlighting its origins, 'alcohol made from wood (substance)'.[6]
Amethylidene group is any part of a molecule that consists of a CH2= group.[7] 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 twosingle bonds.[8] 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 moleculeCH2, also known ascarbene.[9] 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 ofsubstituting a methylidene group for twoadjacent hydrogen atoms of some parent molecule (even if they are not actually obtained that way). Thus, for example,methylenecyclopropene is named aftercyclopropene.
Methylene can be prepared by decomposition of compounds with a methylidene or methanediyl group, such asketene (ethenone) (CH2=CO),diazomethane (linearCH2=N2),diazirine (cyclic[−CH2−N=N−]) anddiiodomethane (I−CH2−I). The decomposition can be effected byphotolysis,photosensitized reagents (such asbenzophenone), or thermal decomposition.[5][10] Methylene can be produced byphotolysis ofdiazomethane.[11] In itsultraviolet spectrum, gaseous methylene absorbs at around 141.5 nm. It was shown to have a bond angle of about 140°.[12]
The reactions of methylene were also studied around 1960 byinfrared spectroscopy usingmatrix isolation experiments.[13][14]
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),[10] and the first excited state is a singlet non-radical (ã1A1). With the singlet non-radical only 38 kJ above the ground state,[10] 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.
The singlet state is also morestereospecific than the triplet.[10]
Methylene spontaneously autopolymerises to form various excited oligomers, the simplest of which, is the excited form of thealkeneethylene. The excited oligomers, decompose rather than decay to a ground state. For example, the excited form of ethylene decomposes to acetylene and atomic hydrogen.[10]
Unsolvated, excited methylene will form stable ground state oligomers.
The ground state of methylene has anionisation energy of 10.396 eV. It has a bent configuration, with H–C–H angle of 133.84°,[10] and is thusparamagnetic. (The correct prediction of this angle was an early success ofab initio quantum chemistry.[10]) However conversion to a linear configuration requires only 5.5 kcal/mol.[10]
The singlet state has a slightly higher energy (by about 9 kcal/mol) than the triplet state,[10] 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.[10]
Neutral methylene complexes undergo differentchemical 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 inethenone, 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 giveCH
3L+
. Oxidation of these complexes yields formaldehyde, and reduction yields methane.
Free methylene undergoes the typicalchemical reactions of acarbene.Addition reactions are very fast and exothermic.[15]
When the methylene molecule is in itsstate of lowest energy, the unpaired valence electrons are in separateatomic orbitals with independentspins, a configuration known astriplet state.
Methylene may gain an electron yielding a monovalentanionmethanidyl (CH•−
2), which can be obtained as thetetramethylammonium ((CH
3)4N+
)salt by the reaction ofphenyl sodium (C
6H
5Na) withtetramethylammonium bromide ((CH
3)4N+
Br−
).[5] The ion has bent geometry, with a H-C-H angle of about 103°.[10]
Methylene is also a commonligand incoordination compounds, such ascopper methyleneCuCH
2.[16]
Methylene can bond as a terminal ligand, which is calledmethylidene, or as a bridging ligand, which is calledmethanediyl.
The formula of the methylene molecule (CH2) was mentioned as part of aDisney comic by theDonald 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 andGeorge S. Hammond.[17][18] The comic has been cited in other sources since, including a widely adopted textbook in organic chemistry by Robert Morrison and Robert Boyd.[19]
{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)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).
