Methylidenecarbene

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.

Nomenclature

The systematic names λ²-ethene, and dihydrido-1κ²H-dicarbon(C—C), valid IUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively.

In appropriate contexts, methylidenecarbene can be viewed as ethene with two hydrogen atoms removed, or as ethane with four hydrogens removed; and as such, ethen-1-ylidene (or vinylidene) or ethane-1-diylidene, respectively, may be used as a context-specific systematic names, according to substitutive nomenclature. By default, these names pay no regard to the radicality of the methylidenecarbene molecule. In even more specific context, these can also name the non-radical singlet ground state, whereas the diradical state is named ethene-1,1-diyl, or ethane-1,1-diylylidene, and the long-lived, tetraradical triplet state^[1] is named ethane-1,1,1,2-tetrayl.

Ethenylidene (without the -1- locant) or vinylidene, is used, systematically, to refer to the substituent group (=C=CH
2). It is used, non-systematically, to refer to substituent group ethene-1,1-diyl (>C=CH
2). Care should be taken to avoid confusing the names of the groups for the context-specific names for methylidenecarbene given above.

Chemical properties

The 1λ²-ethenediyl group (>C=C) in 1λ²-alk-1-enes such as methylidenecarbene can accept or donate a pair of electrons by adduction. Because of this acceptance or donation of the electron pair, methylidenecarbene has Lewis-amphoteric character.

Structure

With a half-life on the order of hundreds of femtoseconds, free methylidenecarbene will spontaneously decay via tautomerisation to acetylene,^[2] with an energy barrier of between 4 and 21 kJ mol⁻¹.^[1] Besides tautomerisation, methylidenecarbene can also autopolymerise to form various oligomers, the simplest of which, is the cumulene butatriene. Calculations determine that the ground state of methylidenecarbene is a non-radical singlet state.^[1]

Although, free methylidenearbene is quickly destroyed by tautomerisation into acetylene, its adducts are quite stable. One such adduct ([Ru(Cl)
2(PCy
3)
2(CCH
2)]) is useful as a catalyst for the polymerisation of norbornene.^[3]

Production

Most methylidenecarbene is produced in an adduct form by transition metal ethynation:

ML
n + HC≡CH → M(C=CH
2)L
n

See also

• Methylene (compound)

References

1. Bruce, Michael I. (March 1991). "Organometallic chemistry of vinylidene and related unsaturated carbenes" (PDF). Chemical Reviews (American Chemical Society) 91 (2): 197–257. doi:10.1021/cr00002a005. Retrieved 2 October 2013.
2. Hayes, Robin L.; Fattal, Eyal; Govind, Niranjan; Carter, Emily A. (31 January 2001). "Long live vinylidene! A new view of the H₂C=C:  → HC≡CH rearrangement from ab initio molecular dynamics". Journal of the American Chemical Society 123 (4): 641–657. doi:10.1021/ja000907x.
3. Bruneau, Christian; Dixneuf, Pierre (18 December 1998). "Metal vinylidenes in catalysis". Accounts of Chemical Research (American Chemical Society) 32 (4): 311–323. doi:10.1021/ar980016i. Retrieved 6 October 2013.