Methylidenecarbene (systematically named λ
2-ethene and dihydrido-1κ
2''H''-dicarbon(''C''—''C'')) is an
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...
with the
chemical formula
In chemistry, 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, ...
(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 of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms.
The term "carbene" ma ...
.
Nomenclature
The systematic names ''λ
2-ethene'', and ''dihydrido-1κ
2H-dicarbon(C—C)'', valid
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
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
A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as ''side ...
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 1λ
2-ethenediyl group (>C=C) in 1λ
2-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. For context, a femtosecond is to a second as a second is to about 31. ...
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
In organic chemistry, 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 ...
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 →
See also
*
Methylene (compound)
Methylene (systematically named methylidene and dihydridocarbon; also called carbene) is an organic compound with the chemical formula (also written ). It is a colourless gas that fluoresces in the mid-infrared range, and only persists in dilut ...
References
{{Reflist
Alkenes
Carbenes