Methylene (compound)

Last updated
Methylene
Carbene.svg
Ball-and-stick model of triplet methylene Triplet-methylene-3D-balls.png
Ball-and-stick model of triplet methylene
Space filling model of triplet methylene Triplet-methylene-3D-SF.png
Space filling model of triplet methylene
Names
IUPAC name
Dihydridocarbon(2•) [1]
Preferred IUPAC name
Methylidene [2]
Other names
Dihydridocarbon
Carbene
Methylene
Methene [1]
Identifiers
3D model (JSmol)
1696832
ChEBI
ChemSpider
56
MeSH carbene
PubChem CID
  • InChI=1S/CH2/h1H2 Yes check.svgY
    Key: HZVOZRGWRWCICA-UHFFFAOYSA-N Yes check.svgY
  • [CH2]
Properties
CH
22•
Molar mass 14.0266 g mol−1
AppearanceColourless gas
Reacts
Conjugate acid Methenium
Thermochemistry
Std molar
entropy (S⦵298)
193.93 J K−1 mol−1
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).

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.

Contents

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 trivial name carbene is the preferred IUPAC name. [2] 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)'. [6]

Methylidene group

The blue part of this diagram of a propene molecule is a methylidene group. Propene Methylidene Structural Formulae V.1.svg
The blue part of this diagram of a propene molecule is a methylidene group.

A methylidene 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 two single bonds. [8] The distinction is often important, because the double bond is chemically different from two single bonds.

3-Methylidenecycloprop-1-ene is named as a cyclopropene with a methylidene substituent. Methylenecyclopropene.svg
3-Methylidenecycloprop-1-ene is named as a cyclopropene with a methylidene substituent.

The same name (methylidene) was used for the distinct molecule CH2, also known as carbene. [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 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 [-CH
2-N=N-]) and diiodomethane (I-CH
2-I). The decomposition can be effected by photolysis, photosensitized reagents (such as benzophenone), or thermal decomposition. [5] [10] Methylene can be produced by photolysis of diazomethane. [11] In its ultraviolet 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 by infrared spectroscopy using matrix isolation experiments. [13] [14]

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 (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.

[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 stereospecific than the triplet. [10]

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. [10]

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°, [10] and is thus paramagnetic. (The correct prediction of this angle was an early success of ab 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]

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. [15]

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−
). [5] The ion has bent geometry, with a H-C-H angle of about 103°. [10]

Reactions with inorganic compounds

Methylene is also a common ligand in coordination compounds, such as copper methylene CuCH
2. [16]

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

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. [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]

See also

Related Research Articles

<span class="mw-page-title-main">Alkene</span> Hydrocarbon compound containing one or more C=C bonds

In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or in the terminal position. Terminal alkenes are also known as α-olefins.

<span class="mw-page-title-main">Functional group</span> Group of atoms giving a molecule characteristic properties

In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest of the molecule's composition. This enables systematic prediction of chemical reactions and behavior of chemical compounds and the design of chemical synthesis. The reactivity of a functional group can be modified by other functional groups nearby. Functional group interconversion can be used in retrosynthetic analysis to plan organic synthesis.

<span class="mw-page-title-main">Photochemistry</span> Sub-discipline of chemistry

Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet, visible (400–750 nm), or infrared radiation (750–2500 nm).

In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is R−:C−R' or R=C: where the R represents substituents or hydrogen atoms.

<span class="mw-page-title-main">Silylene</span> Chemical compound

Silylene is a chemical compound with the formula SiH2. It is the silicon analog of methylene, the simplest carbene. Silylene is a stable molecule as a gas but rapidly reacts in a bimolecular manner when condensed. Unlike carbenes, which can exist in the singlet or triplet state, silylene (and all of its derivatives) are singlets.

<span class="mw-page-title-main">Singlet oxygen</span> Oxygen with all of its electrons spin paired

Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemical with the formula O=O (also written as 1
[O
2] or 1
O
2), which is in a quantum state where all electrons are spin paired. It is kinetically unstable at ambient temperature, but the rate of decay is slow.

A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene. Carbene complexes have been synthesized from most transition metals and f-block metals, using many different synthetic routes such as nucleophilic addition and alpha-hydrogen abstraction. The term carbene ligand is a formalism since many are not directly derived from carbenes and most are much less reactive than lone carbenes. Described often as =CR2, carbene ligands are intermediate between alkyls (−CR3) and carbynes (≡CR). Many different carbene-based reagents such as Tebbe's reagent are used in synthesis. They also feature in catalytic reactions, especially alkene metathesis, and are of value in both industrial heterogeneous and in homogeneous catalysis for laboratory- and industrial-scale preparation of fine chemicals.

<span class="mw-page-title-main">Cyclopropanation</span> Chemical process which generates cyclopropane rings

In organic chemistry, cyclopropanation refers to any chemical process which generates cyclopropane rings. It is an important process in modern chemistry as many useful compounds bear this motif; for example pyrethroid insecticides and a number of quinolone antibiotics. However, the high ring strain present in cyclopropanes makes them challenging to produce and generally requires the use of highly reactive species, such as carbenes, ylids and carbanions. Many of the reactions proceed in a cheletropic manner.

<span class="mw-page-title-main">Methylidyne radical</span> Chemical compound

Methylidyne, or (unsubstituted) carbyne, is an organic compound whose molecule consists of a single hydrogen atom bonded to a carbon atom. It is the parent compound of the carbynes, which can be seen as obtained from it by substitution of other functional groups for the hydrogen.

<span class="mw-page-title-main">Atomic carbon</span> Chemical compound

Atomic carbon, systematically named carbon and λ0-methane, is a colourless gaseous inorganic chemical with the chemical formula C. It is kinetically unstable at ambient temperature and pressure, being removed through autopolymerisation.

<span class="mw-page-title-main">Trimethylenemethane</span> Chemical compound

Trimethylenemethane is a chemical compound with formula C
4H
6. It is a neutral free molecule with two unsatisfied valence bonds, and is therefore a highly reactive free radical. Formally, it can be viewed as an isobutylene molecule C
4H
8 with two hydrogen atoms removed from the terminal methyl groups.

In spectroscopy and quantum chemistry, the multiplicity of an energy level is defined as 2S+1, where S is the total spin angular momentum. States with multiplicity 1, 2, 3, 4, 5 are respectively called singlets, doublets, triplets, quartets and quintets.

<span class="mw-page-title-main">Radical (chemistry)</span> Atom, molecule, or ion that has an unpaired valence electron; typically highly reactive

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<span class="mw-page-title-main">Methylene group</span> Chemical group (–CH₂–)

A methylene group is any part of a molecule that consists of two hydrogen atoms bound to a carbon atom, which is connected to the remainder of the molecule by two single bonds. The group may be represented as −CH2− or >CH2, where the '>' denotes the two bonds.

In organic chemistry, a methylene bridge, methylene spacer, or methanediyl group is any part of a molecule with formula −CH2−; namely, a carbon atom bound to two hydrogen atoms and connected by single bonds to two other distinct atoms in the rest of the molecule. It is the repeating unit in the skeleton of the unbranched alkanes.

In organic chemistry, a methylidyne group or just methylidyne is a neutral part of a molecule with formula ≡CH, consisting of a carbon atom bonded to a hydrogen atom by one single bond and to the rest of the molecule by one triple bond. For example, a methylidyne group is present in n-methylidyne-1-hexanaminium, H3C−(CH2)5−N+≡CH.

<span class="mw-page-title-main">Fluorenylidene</span> Chemical compound

9-Fluorenylidene is an aryl carbene derived from the bridging methylene group of fluorene. Fluorenylidene has the unusual property that the triplet ground state is only 1.1 kcal/mol lower in energy than the singlet state. For this reason, fluorenylidene has been studied extensively in organic chemistry.

Methylidenecarbene (systematically named λ2-ethene and dihydrido-1κ2H-dicarbon(C—C)) is an organic compound with the chemical formula C=CH
2 (also written [CCH
2] or C
2H
2). It is a metastable proton tautomer of acetylene, which only persists as an adduct. It is a colourless gas that phosphoresces in the far-infrared range. It is the simplest unsaturated carbene.

In organic chemistry, methylenation is a chemical reaction that inserts a methylene group into a chemical compound:

In organic chemistry, alkylidene is a general term for divalent functional groups of the form R2C=, where each R is an alkane or hydrogen. They can be considered the functional group corresponding to mono- or disubstituted divalent carbenes, or as the result of removing two hydrogen atoms from the same carbon atom in an alkane.

References

  1. 1 2 "methanediyl (CHEBI:29357)". Chemical Entities of Biological Interest. UK: European Bioinformatics Institute. 14 January 2009. IUPAC Names. Retrieved 2 January 2012.
  2. 1 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. 1 2 3 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. 1 2 3 4 5 6 7 8 9 10 11 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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