Methylidenecarbene (systematically named λ²-ethene and dihydrido-1κ²''H''-dicarbon(''C''—''C'')) is an

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

with the

chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...

(also written [] or ). It is a metastability, 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, a carbene is a molecule containing a neutral carbon atom with a Valence (chemistry), valence of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms. Th ...

Nomenclature

The systematic names ''λ²-ethene'', and ''dihydrido-1κ²H-dicarbon(C—C)'', valid

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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 In the nomenclature of organic chemistry, a locant is a term to indicate the position of a functional group or substituent within a molecule. Numeric locants The International Union of Pure and Applied Chemistry (IUPAC) recommends the use of n ...

) or ''vinylidene'', is used, systematically, to refer to the

substituent In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...

group (). It is used, non-systematically, to refer to substituent group ''ethene-1,1-diyl'' (). 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 In chemistry, an electron pair or Lewis pair consists of two electrons that occupy the same molecular orbital but have opposite spins. Gilbert N. Lewis introduced the concepts of both the electron pair and the covalent bond in a landmark paper ...

, methylidenecarbene has Lewis-amphoteric character.

Structure

With a half-life on the order of hundreds of

femtosecond A femtosecond is a unit of time in the International System of Units (SI) equal to 10 or of a second; that is, one quadrillionth, or one millionth of one billionth, of a second. A femtosecond is to a second, as a second is to approximately 31.6 ...

s, free methylidenecarbene will spontaneously decay via tautomerisation to acetylene, with an energy barrier of between 4 and 21 kJ mol⁻¹. Besides tautomerisation, methylidenecarbene can also autopolymerise to form various oligomers, the simplest of which, is the

cumulene A cumulene is a compound having three or more ''cumulative'' (consecutive) double bonds. They are analogous to allenes, only having a more extensive chain. The simplest molecule in this class is butatriene (), which is also called simply ''cumu ...

butatriene. Calculations determine that the ground state of methylidenecarbene is a non-radical singlet state. Although, free methylidenearbene is quickly destroyed by tautomerisation into acetylene, its adducts are quite stable. One such adduct ([]) is useful as a catalyst for the polymerisation of norbornene.

Production

Most methylidenecarbene is produced in an adduct form by transition metal ethynation: : + HC≡CH →

References

{{Reflist Alkenes Carbenes

Nomenclature

Chemical properties

Structure

Production

See also

References