Vinylidene
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.
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 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.
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