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| {{About|the chemical class|the compound|Methylene (compound)}}
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| {{distinguish|carbine|carbyne}}
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| [[File:Carbene.png|thumb|60px|Methylene, the simplest carbene.]]
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| In [[chemistry]], a '''carbene''' is a [[molecule]] containing a neutral [[carbon]] atom with a [[Valence (chemistry)|valence]] of two and two unshared [[valence electron]]s. The general formula is R-(C''':''')-R' or R=C''':'''.
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| The term "carbene" may also refer to the specific compound H<sub>2</sub>C''':''', also called [[methylene radical|methylene]], the parent [[hydride]] from which all other carbene compounds are formally derived.<ref>{{Cite book|title=Molecular Orbitals of Transition Metal Complexes|isbn=0-19-853093-5|page=7|last=Hoffmann|first=Roald|authorlink=Roald Hoffmann|publisher=Oxford|year=2005|postscript=<!--None-->}}</ref><ref>{{GoldBookRef|title=carbenes|file=C00806}}</ref>
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| Carbenes are classified as either [[singlet state|singlets]] or [[triplet state|triplets]] depending upon their electronic structure. Most carbenes are very short lived, although [[persistent carbene]]s are known.
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| One well studied carbene is Cl<sub>2</sub>C''':''', or [[dichlorocarbene]], which can be generated ''[[in situ]]'' from [[chloroform]] and a strong [[base (chemistry)|base]].
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| == Structure and bonding ==
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| [[Image:carbenes.png|frame|right|Singlet and triplet carbenes]]
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| The two classes of carbenes are [[Diradical|singlet]] and [[diradical|triplet]] carbenes. Singlet carbenes are spin-paired. In the language of [[valence bond theory]], the molecule adopts an ''sp''<sup>2</sup> [[Orbital hybridisation|hybrid structure]]. Triplet carbenes have two unpaired electrons. They may be either linear or bent, i.e. ''sp'' or ''sp''<sup>2</sup> hybridized, respectively. Most carbenes have a nonlinear triplet ground state, except for those with nitrogen, oxygen, or sulfur atoms, and halides directly bonded to the divalent carbon.
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| Carbenes are called singlet or triplet depending on the electronic [[spin (physics)|spins]] they possess. Triplet carbenes are [[paramagnetic]] and may be observed by [[electron spin resonance spectroscopy]] if they persist long enough. The total spin of singlet carbenes is zero while that of triplet carbenes is one (in units of <math>\hbar</math>). Bond angles are 125-140° for triplet methylene and 102° for singlet methylene (as determined by [[Electron paramagnetic resonance|EPR]]). Triplet carbenes are generally stable in the gaseous state, while singlet carbenes occur more often in aqueous media.
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| For simple hydrocarbons, triplet carbenes usually have energies 8 [[kilocalorie|kcal]]/[[mole (unit)|mol]] (33 [[kilojoule|kJ]]/mol) lower than singlet carbenes (see also [[Hund's rule of maximum multiplicity]]), thus, in general, triplet is the more stable state (the [[ground state]]) and singlet is the [[excited state]] species. [[Substituent]]s that can donate [[electron pair]]s may stabilize the singlet state by delocalizing the pair into an empty p-orbital. If the energy of the singlet state is sufficiently reduced it will actually become the ground state.
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| No viable strategies exist for triplet stabilization. The carbene called 9-[[fluorenylidene]] has been shown to be a rapidly [[chemical equilibrium|equilibrating]] mixture of singlet and triplet states with an approximately 1.1 kcal/mol (4.6 kJ/mol) energy difference.<ref>{{Cite doi|10.1021/ja00361a014}}</ref> It is, however, debatable whether diaryl carbenes such as the [[fluorene]] carbene are true carbenes because the electrons can delocalize to such an extent that they become in fact [[biradical]]s. ''[[In silico]]'' experiments suggest that triplet carbenes can be [[thermodynamic]]ally stabilized with [[electropositive]] heteroatoms such as in [[silyl]] and [[silyloxy]] carbenes, especially trifluorosilyl carbenes.<ref name="nemirowski">{{cite journal | title=Electronic Stabilization of Ground State Triplet Carbenes | author=Nemirowski, A | author2=Schreiner, P. R. | journal=J. Org. Chem. |date=November 2007 | volume=72 | issue=25 | pages=9533–9540 | doi=10.1021/jo701615x}}</ref>
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| == Reactivity ==
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| [[image:singletriplet.png|right|frame|Carbene addition to alkenes]]
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| Singlet and triplet carbenes exhibit divergent reactivity. Singlet carbenes generally participate in [[cheletropic reaction]]s as either [[electrophile]]s or [[nucleophile]]s. Singlet carbenes with unfilled p-orbital should be electrophilic. Triplet carbenes can be considered to be [[free radical|diradicals]], and participate in stepwise radical additions. Triplet carbenes have to go through an [[reactive intermediate|intermediate]] with two unpaired electrons whereas singlet carbene can react in a single [[concerted reaction|concerted]] step.
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| Due to these two modes of reactivity, reactions of singlet methylene are [[stereospecific]] whereas those of triplet methylene are [[stereoselective]]. This difference can be used to probe the nature of a carbene. For example, the reaction of methylene generated from [[photolysis]] of [[diazomethane]] with ''cis''-[[2-butene]] or with ''trans''-[[2-butene]] each give a single diastereomer of the 1,2-dimethylcyclopropane product: ''cis'' from ''cis'' and ''trans'' from ''trans'', which proves that the methylene is a singlet.<ref>{{Cite doi|10.1021/ja01598a087}}</ref> If the methylene were a triplet, one would not expect the product to depend upon the starting alkene geometry, but rather a nearly identical mixture in each case.
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| Reactivity of a particular carbene depends on the [[substituent]] groups. Their reactivity can be affected by [[metal]]s. Some of the reactions carbenes can do are [[Carbene C-H insertion|insertions into C-H bonds]], skeletal rearrangements, and additions to double bonds. Carbenes can be classified as nucleophilic, electrophilic, or ambiphilic. For example, if a substituent is able to donate a pair of electrons, most likely carbene will not be electrophilic. [[Alkyl]] carbenes insert much more selectively than methylene, which does not differentiate between primary, secondary, and tertiary C-H bonds.
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| ===Cyclopropanation===
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| {{main|Cyclopropanation}}
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| [[image:cyclopropanation.png|right|frame|Carbene cyclopropanation]]
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| Carbenes add to double bonds to form [[Cyclopropane#Cyclopropanes|cyclopropanes]]. A concerted mechanism is available for singlet carbenes. Triplet carbenes do not retain [[stereochemistry]] in the product molecule. Addition reactions are commonly very fast and [[exothermic]]. The slow step in most instances is generation of carbene. A well-known reagent employed for alkene-to-cyclopropane reactions is [[Simmons-Smith reagent]]. This reagent is a system of [[copper]], [[zinc]], and [[iodine]], where the active reagent is believed to be [[iodomethylzinc iodide]]. Reagent is complexed by [[Hydroxyl|hydroxy]] groups such that addition commonly happens [[syn]] to such group.
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| ===C—H insertion===
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| {{main|Carbene C−H insertion}}
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| [[image:insertion.png|thumb|frame|Carbene insertion]]
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| [[Carbene C-H insertion|Insertions]] are another common type of carbene reactions. The carbene basically interposes itself into an existing bond. The order of preference is commonly: 1. X–H bonds where X is not carbon 2. C–H bond 3. C–C bond. Insertions may or may not occur in single step.
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| [[Intramolecular]] insertion reactions present new synthetic solutions. Generally, rigid structures favor such insertions to happen. When an intramolecular insertion is possible, no [[intermolecular]] insertions are seen. In flexible structures, five-membered ring formation is preferred to six-membered ring formation. Both inter- and intramolecular insertions are amendable to asymmetric induction by choosing chiral ligands on metal centers.
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| :[[image:carbene intra.png|left|frame|Carbene intramolecular reaction]]
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| {{clear}}
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| :[[image:carbene inter.png|left|frame|Carbene intermolecular reaction]]
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| {{clear}}
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| Alkylidene carbenes are alluring in that they offer formation of [[cyclopentene]] moieties. To generate an alkylidene carbene a ketone can be exposed to [[trimethylsilyl]] [[diazomethane]].
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| :[[image:alkylidene carbene.png|left|frame|Alkylidene carbene]]
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| {{clear}}
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| ===Carbene dimerization===
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| {{main|Carbene dimerization}}
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| [[image:Wanzlick equilibrium lemal Hahn 1999.svg|right|frame|Wanzlick equilibrium]]
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| Carbenes and [[carbenoid]] precursors can undergo [[dimerization]] reactions to form [[alkene]]s. While this is often an unwanted side reaction, it can be employed as a synthetic tool and a direct metal carbene dimerization has been used in the synthesis of polyalkynylethenes.
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| Persistent carbenes exist in equilibrium with their respective dimers. This is known as the [[Wanzlick equilibrium]].
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| == Carbene ligands in organometallic chemistry ==
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| In [[organometallic chemistry|organometallic]] species, metal complexes with the formulae L<sub>n</sub>MCRR' are often described as carbene complexes. Such species do not however react like free carbenes and are rarely generated from carbene precursors, except for the persistent carbenes. The [[transition metal carbene complex]]es can be classified according to their reactivity, with the first two classes being the most clearly defined:
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| *[[Fischer carbene]]s, in which the carbene is bonded to a metal that bears an electron-withdrawing group (usually a carbonyl). In such cases the carbenoid carbon is mildly electrophilic.
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| *[[Schrock carbene]]s, in which the carbene is bonded to a metal that bears an electron-donating group. In such cases the carbenoid carbon is nucleophilic and resembles Wittig reagent (which are not considered carbene derivatives).
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| *[[Persistent carbene]]s, also known as [[Anthony Joseph Arduengo III|Arduengo]] or [[Wanzlick equilibrium|Wanzlick]] carbenes. These include the class of ''N''-heterocyclic carbenes (NHCs) and are often are used as [[Ligand|ancillary ligand]]s in organometallic chemistry. Such carbenes are spectator ligands of low reactivity.
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| == Generation of carbenes ==
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| *A method that is broadly applicable to organic synthesis is induced elimination of [[halides]] from gem-dihalides employing [[organolithium reagent]]s. It remains uncertain if under these conditions free carbenes are formed or metal-carbene complex. Nevertheless, these metallocarbenes (or carbenoids) give the expected organic products.
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| :R<sub>2</sub>CBr<sub>2</sub> + BuLi → R<sub>2</sub>CLi(Br) + BuBr | |
| :R<sub>2</sub>CLi(Br) → R<sub>2</sub>C + LiBr
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| *For cyclopropanations, zinc is employed in the [[Simmons–Smith reaction]]. In a specialized but instructive case, alpha-halomercury compounds can be isolated and separately thermolyzed. For example, the "Seyferth reagent" releases CCl<sub>2</sub> upon heating.
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| :C<sub>6</sub>H<sub>5</sub>HgCCl<sub>3</sub> → CCl<sub>2</sub> + C<sub>6</sub>H<sub>5</sub>HgCl
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| *Most commonly, carbenes are generated from [[diazoalkane]]s, via [[photolysis|photolytic]], thermal, or [[transition metal]]-catalyzed routes. Catalysts typically feature [[rhodium]] and [[copper]]. The [[Bamford-Stevens reaction]] gives carbenes in [[aprotic solvent]]s and carbenium ions in protic solvents.
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| * Base-induced elimination HX from haloforms (CHX<sub>3</sub>) with under [[phase-transfer catalyst|phase-transfer conditions]].
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| * [[Photolysis]] of [[diazirine]]s and [[epoxide]]s can also be employed. Diazirines are cyclic forms of diazoalkanes. The strain of the small ring makes [[photoexcitation]] easy. Photolysis of epoxides gives [[carbonyl]] compounds as side products. With [[asymmetric synthesis|asymmetric]] epoxides, two different carbonyl compounds can potentially form. The nature of substituents usually favors formation of one over the other. One of the C-O bonds will have a greater double bond character and thus will be stronger and less likely to break. Resonance structures can be drawn to determine which part will contribute more to the formation of carbonyl. When one substituent is [[alkyl]] and another [[aryl]], the aryl-substituted carbon is usually released as a carbene fragment.
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| * Carbenes are intermediates in the [[Wolff rearrangement]]
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| ==Applications of carbenes==
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| A large scale application of carbenes is the industrial production of [[tetrafluoroethylene]], the precursor to [[Teflon]]. Tetrafluoroethylene is generated via the intermediacy of [[difluorocarbene]]:<ref name=William>{{Cite doi|10.1002/0471238961.0914201802011026.a01.pub2}}</ref>
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| : CHClF<sub>2</sub> → CF<sub>2</sub> + HCl
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| :2 CF<sub>2</sub> → F<sub>2</sub>C=CF<sub>2</sub>
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| ==History==
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| Carbenes had first been postulated by [[Eduard Buchner]] in 1903 in [[cyclopropanation]] studies of [[ethyl diazoacetate]] with toluene.<ref>{{cite DOI|10.1002/cber.190303603139}}</ref> In 1912 [[Hermann Staudinger]] <ref>{{cite DOI|10.1002/cber.19120450174}}</ref> also converted alkenes to cyclopropanes with [[diazomethane]] and CH<sub>2</sub> as an intermediate. [[William von Eggers Doering|Doering]] in 1954 demonstrated with [[dichlorocarbene]] synthetic utility.<ref>{{cite DOI|10.1021/ja01652a087}}</ref>
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| ==See also==
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| *[[Transition metal carbene complex]]es, also known as [[carbenoids]]
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| *[[Atomic carbon]] a single carbon atom with the chemical formula :C:, in effect a twofold carbene. Also has been used to make "true carbenes" in situ.
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| *[[Foiled carbene]]s derive their stability from proximity of a double bond (i.e. their ability to form conjugated systems).
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| *[[Carbene analogs]]
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| *[[Carbenium ion]]s, protonated carbenes
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| * [[Ring opening metathesis polymerization]]
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| ==References==
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| <references/>
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| [[Category:Carbenes]]
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| [[Category:Reactive intermediates]]
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| [[Category:Functional groups]]
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| [[Category:Organic compounds]]
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| [[pt:Carbeno]]
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