{{About|the compound|the functional group|Methylene group}} {{other uses|Methylene (disambiguation)}} {{Chembox | ImageFile1 = Carbene.svg | ImageFile1_Ref = {{chemboximage|correct|??}} | ImageSize1 = 60 | ImageName1 = Skeletal formula of methylene | ImageFileL2 = Triplet-methylene-3D-balls.png | ImageSizeL2 = 100 | ImageNameL2 = Ball-and-stick model of triplet methylene | ImageFileR2 = Triplet-methylene-3D-SF.png | ImageSizeR2 = 100 | ImageNameR2 = Space filling model of triplet methylene | Name = Methylene | PIN = Methylidene<ref name="Powell 2013 p1054">{{cite book |author1=Henri A. Favre |author2=Warren H. Powell |title=Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 |date=2014 |publisher=Royal Society of Chemistry |location=Cambridge, England |isbn=978-0-85404-182-4 |page=1054}}</ref> | IUPACName = Dihydridocarbon(2•)<ref name="Chemical Entities of Biological Interest">{{cite web|title=methanediyl (CHEBI:29357)|url=http://www.ebi.ac.uk/chebi/searchId.do?chebiId=CHEBI%3A29357|work=Chemical Entities of Biological Interest|publisher=European Bioinformatics Institute|access-date=2 January 2012|location=UK|date=14 January 2009|at=IUPAC Names}}</ref> | OtherNames = {{ubl |Dihydridocarbon |Carbene |Methene<ref name="Chemical Entities of Biological Interest" /> }} |Section1={{Chembox Identifiers | CASNo = 2465-56-7 | CASNo_Ref = | PubChem = 123164 | ChemSpiderID = 109779 | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | MeSHName = carbene | ChEBI = 29357 | ChEBI_Ref = {{ebicite|correct|EBI}} | Beilstein = 1696832 | Gmelin = 56 | SMILES = [CH2] | StdInChI = 1S/CH2/h1H2 | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = HZVOZRGWRWCICA-UHFFFAOYSA-N | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} }} |Section2={{Chembox Properties | Formula = {{Chem2|CH}}{{su|p=•|b=2|lh=1}} | C=1 |H=2 | Appearance = Colourless gas | Solubility = Reacts | ConjugateAcid = Methenium }} |Section3={{Chembox Thermochemistry | DeltaHf = {{val|386.39|u=kJ/mol}} | Entropy = {{val|193.93|u=J|up=(K⋅mol)}} }} |Section8={{Chembox Related | OtherCompounds = {{ubl |Methyl ({{chem2|CH3}}) |Methylidyne (CH) |Carbide (C) |Silylene ({{chem2|SiH2}}) }} }} }}
'''Methylene''' (IUPAC name: '''methylidene''', also called '''carbene''' or '''methene''') is an organic compound with the chemical formula {{chem2|CH2}} (also written {{chem2|[CH2]}} and not to be confused with compressed hydrogen, which is also denoted {{chem2|CH2}}). 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.<ref name="hoff">{{cite book |last1=Jean |first1=Yves |title=Molecular Orbitals of Transition Metal Complexes |date=2005 |publisher=Oxford University Press |location=Oxford |isbn=0-19-853093-5 |doi=10.1093/oso/9780198530930.001.0001 |page=7}}</ref><ref>{{GoldBookRef |title=carbenes |file=C00806}}</ref> It is usually detected only at very low temperatures or as a short-lived intermediate in chemical reactions.<ref name=demben>{{cite book |first1=W. B. |last1=DeMore |first2=S. W. |last2=Benson |editor-first1=David H. |editor-last1=Volman |editor-first2=George S. |editor-last2=Hammond |editor-first3=Klaus |editor-last3=Gollnick |orig-date=1964 |date=2009 |url=https://books.google.com/books?id=RSkziXqqPQgC&pg=PA219 |chapter=Preparation, Properties, and Reactivity of Methylene. |title=Advances in Photochemistry |publisher=John Wiley & Sons |page=219 |isbn=9780470133590}}</ref>
== Nomenclature == The trivial name carbene is the preferred IUPAC name.<ref name="Powell 2013 p1054" /> 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 ({{chem2|>(CH2)}}), and methylidene ({{chem2|\dCH2}}). 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 {{wikt-lang|grc|μέθυ}} (''methy'') "wine" and {{wikt-lang|grc|ὕλη}} (''hȳlē'') "forest, firewood, matter" with the intention of highlighting its origins, 'alcohol made from wood (substance)'.<ref>{{cite journal |first1=J. |last1=Dumas |first2=E. |last2=Péligot |year=1835 |title=Mémoire sur l'espirit de bois et sur les divers composés ethérés qui en proviennent |trans-title=Memoir on spirit of wood and on the various ethereal compounds that derive therefrom |journal=Annales de chimie et de physique |volume=58 |page=9 |url=https://books.google.com/books?id=94c5AAAAcAAJ&pg=PA9 |quote=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. |trans-quote=We will give the name "methylene" (1) to a radical … (1) methy, wine, and hulē, wood; that is, wine or spirit of wood. |language=fr}}</ref>
===Methylidene group=== {{Main|Methylidene group}} {{Short description|1=Chemical group (=CH2)}} [[File:Propene Methylidene Structural Formulae V.1.svg|thumb|240px|The blue part of this diagram of a propene molecule is a methylidene group.|left]]
A '''methylidene group''' is any part of a molecule that consists of a {{chem2|\dCH2}} group.<ref>{{cite web|url=https://old.iupac.org/reports/provisional/abstract04/BB-prs310305/CompleteDraft.pdf#page=314|title=methylidene (preferred IUPAC name)|page=314}}</ref>
In contrast, methylene is connected to the rest of the molecule by two single bonds.<ref>{{cite web|url=https://old.iupac.org/reports/provisional/abstract04/BB-prs310305/CompleteDraft.pdf#page=58|title=methylene (preferred IUPAC name)|page=58}}</ref> The distinction is often important, because the double bond is chemically different from two single bonds.
[[File:Methylenecyclopropene.svg|thumb|right|240px|3-Methylidenecycloprop-1-ene is named as a cyclopropene with a methylidene substituent.]]
The same name (methylidene) was used for the distinct molecule {{chem2|CH2}}, also known as carbene.<ref>{{cite web|url=https://old.iupac.org/reports/provisional/abstract04/BB-prs310305/CompleteDraft.pdf#page=921|title=methylidene (preferred IUPAC name)|page=921}}</ref> 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) ({{chem2|CH2\dCO}}), diazomethane (linear {{chem2|CH2\dN2}}), diazirine (cyclic {{chem2|[\sCH2\sN\dN\s]}}) and diiodomethane ({{chem2|I\sCH2\sI}}). The decomposition can be effected by photolysis, photosensitized reagents (such as benzophenone), or thermal decomposition.<ref name=demben/><ref name=shav/> Methylene can be produced by photolysis of diazomethane.<ref>{{cite journal | last1 = Herzberg | first1 = G. | last2 = Shoosmith | first2 = J. | year = 1959 | title = Spectrum and Structure of the Free Methylene Radical | journal = Nature | volume = 183 | issue = 4678 |pages = 1801–1802 | doi = 10.1038/1831801a0 | bibcode = 1959Natur.183.1801H | s2cid = 4272040 }}</ref> In its ultraviolet spectrum, gaseous methylene absorbs at around 141.5 nm. It was shown to have a bond angle of about 140°.<ref>{{cite conference |last1=Bunker |first1=Philip R. |title=The Spectrum, Structure and Singlet-Triplet Splitting in Methylene (CH<sub>2</sub>) |conference=Comparison of Ab Initio Quantum Chemistry with Experiment for Small Molecules |editor-last=Bartlett |editor-first=Rodney J. |publisher=Reidel Dordrecht |location=Netherlands |date=1985 |pages=141–170 |doi=10.1007/978-94-009-5474-8_6}}</ref>
The reactions of methylene were also studied around 1960 by infrared spectroscopy using matrix isolation experiments.<ref name=dempri>{{cite journal |doi = 10.1021/ja01531a008|title = Photochemical Experiments in Rigid Media at Low Temperatures. II. The Reactions of Methylene, Cyclopentadienylene and Diphenylmethylene|journal = Journal of the American Chemical Society|volume = 81|issue = 22|pages = 5874|year = 1959|last1 = Demore|first1 = William B|last2 = Pritchard|first2 = H. O|last3 = Davidson|first3 = Norman | bibcode=1959JAChS..81.5874D }}</ref><ref name=jacox>{{cite journal |doi=10.1021/ja00886a006 |title=Infrared Study of the Reactions of CH2and NH with C2H2and C2H4in Solid Argon |journal=Journal of the American Chemical Society |volume=85 |issue=3 |pages=278 |year=1963 |last1=Jacox |first1 =Marilyn E. |last2=Milligan |first2=Dolphus E. | bibcode=1963JAChS..85..278J }}</ref>
== 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 ({{overset|~|''X''|lh=0.6em}}{{sup|3}}''B''{{sub|1}}), and the first excited state is a singlet non-radical (''ã''{{sup|1}}''A''{{sub|1}}). With the singlet non-radical only {{val|38|u=kJ}} above the ground state, a sample of methylene exists as a mixture of electronic states even at room temperature, giving rise to complex reactions.<ref name=shav>{{cite journal | doi = 10.1016/S0040-4020(01)96393-8 | title = Geometry and singlet-triplet energy gap in methylene: A critical review of experimental and theoretical determinations | date = 1985 | last1 = Shavitt | first1 = Isaiah | journal = Tetrahedron | volume = 41 | issue = 8 | pages = 1531–1542 }}</ref> 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. :{{chem2|[CH2]^{2•} }}({{overset|~|''X''|lh=0.6em}}{{chem2|^{3}''B''_{1})+ + H2O -> [CH3]^{•} + [HO]^{•} }} :{{chem2|[CH2](''ã''^{1}''A''_{1}) + H2O -> H2CO + H2 (or H3COH)}}
The singlet state is also more stereospecific than the triplet.<ref name=shav/>
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.<ref name=shav/> :{{chem2|2 CH2 -> H2CCH}}{{su|b=2|p=•|lh=1}}{{chem2| -> HCCH + 2 H}}
Unsolvated, excited methylene will form stable ground state oligomers. :{{chem2|2 CH}}{{su|b=2|p=•|lh=1}}{{chem2| -> H2CCH2}}
=== Structure ===
The ground state of methylene has an ionisation energy of {{val|10.396|ul=eV}}. It has a bent configuration, with {{chem2|H\sC\sH}} angle of 133.84°, and is thus paramagnetic. The correct prediction of this angle was an early success of ab initio quantum chemistry methods. Conversion to a linear configuration requires only {{val|5.5|ul=kcal/mol}}.<ref name=shav/>
The singlet state has a slightly higher energy (by about {{val|9|u=kcal/mol}}) than the triplet state, and its {{chem2|H\sC\sH}} angle is smaller, about 102°. In dilute mixtures with an inert gas, the two states will convert to each other until reaching an equilibrium.<ref name=shav/>
=== 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 {{Chem2|CH3L+}}. 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.<ref name="lazar">{{cite book |last1=Lazár |first1=Milan |title=Free radicals in chemistry and biology |date=1989 |publisher=CRC Press |location=Boca Raton, Fla |isbn=0-8493-5387-4}}</ref>{{page needed|date=April 2026}}
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 ({{chem|CH|2|•−}}), which can be obtained as the tetramethylammonium ({{chem2|(CH3)4N+}}) salt by the reaction of phenyl sodium ({{chem2|C6H5Na}}) with tetramethylammonium bromide ({{chem2|(CH3)4N+Br-}}).<ref name=demben/> The ion has bent geometry, with a {{chem2|H\sC\sH}} angle of about 103°.<ref name=shav/>
==== Reactions with inorganic compounds ==== Methylene is also a common ligand in coordination compounds, such as copper methylene {{chem2|CuCH2}}.<ref name=chang>{{cite journal | doi = 10.1021/ja00249a013 | title = Isolation and characterization of copper methylene (CuCH2) via FTIR matrix isolation spectroscopy | date = 1987 | last1 = Chang | first1 = Sou Chan | last2 = Kafafi | first2 = Zakya H. | last3 = Hauge | first3 = Robert H. | last4 = Billups | first4 = W. Edward | last5 = Margrave | first5 = John L. | journal = Journal of the American Chemical Society | volume = 109 | issue = 15 | pages = 4508–4513 | bibcode = 1987JAChS.109.4508C }}</ref>
Methylene can bond as a terminal ligand, which is called ''methylidene'', or as a bridging ligand, which is called ''methanediyl''.
== In popular culture ==
The formula of the methylene molecule ({{chem2|CH2}}) was mentioned as part of a Disney comic by the Donald Duck character in a 1944 story called ''The Mad Chemist'' by Carl Barks, in a humorous vein. In the same spirit, the comic was eventually cited in the scientific literature by Peter Gaspar and George S. Hammond.<ref>{{cite book|title=Carbene Chemistry|last1=Gaspar|first1=Peter P.|last2=Hammond|first2=George S.|author-link2=George S. Hammond|chapter=Chapter 12: The Spin States of Carbenes|pages=235–274|year=1964|volume=1|editor-last=Kirmse|editor-first=Wolfgang|publisher=Academic Press|location=New York|oclc= 543711|quote=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''.]}}<br></ref><ref>{{cite comic |title=Walt Disney's Comics and Stories |issue=44 |year=1944 |quote=If I mix CH<sub>2</sub> with NH<sub>4</sub> and boil the atoms in osmotic fog, I should get speckled nitrogen.}}</ref> The comic has been cited in other sources since, including a widely adopted textbook in organic chemistry by Robert Morrison and Robert Boyd.<ref>{{Cite book |last1=Morrison |first1=Robert Thornton |url=https://books.google.com/books?id=uYqyAAAAIAAJ&q=duck |title=Organic Chemistry |last2=Boyd |first2=Robert Neilson |date=1973 |publisher=Allyn and Bacon |isbn=978-0-205-04136-7 |language=en}}</ref>
== See also == *Methyl radical *Methylidyne radical *Atomic carbon *Alkene *Methylene group *Dichlorocarbene
==References== {{reflist|colwidth=30em}} {{Hydrides by group}} {{molecules detected in outer space}}
Category:Carbenes Category:Organic chemistry Category:Free radicals