In
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J ...
, 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 ''cumulene''. Unlike most
alkane
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in wh ...
s and
alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic ...
s, cumulenes tend to be rigid, comparable to
polyynes. Cumulene carbenes for ''n'' from 3 to 6 have been observed in interstellar
molecular clouds
and in laboratory experiments
by using microwave and infrared spectroscopy. (The more stable cumulenes are difficult to detect optically because they lack an electric dipole moment.) Cumulenes containing
heteroatoms are called
heterocumulenes; an example is
carbon suboxide
Carbon suboxide, or tricarbon dioxide, is an organic, oxygen-containing chemical compound with formula and structure . Its four cumulative double bonds make it a cumulene. It is one of the stable members of the series of linear oxocarbons ...
.
Synthesis
The first reported synthesis of a butatriene is that of tetraphenylbutatriene in 1921. The most common synthetic method for butatriene synthesis is based on
reductive coupling of a
geminal dihalo vinylidene. Tetraphenylbutatriene was reported synthesized in 1977 by homocoupling of 2,2-diphenyl-1,1,1-tribromoethane with elemental
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pink ...
in
dimethylformamide
Dimethylformamide is an organic compound with the formula ( CH3)2NC(O)H. Commonly abbreviated as DMF (although this initialism is sometimes used for dimethylfuran, or dimethyl fumarate), this colourless liquid is miscible with water and the majo ...
.
Structure
The rigidity of cumulenes arises from the fact that the internal carbon atoms carry two double bonds. Their sp hybridisation results in two
π bonds, one to each neighbor, which are perpendicular to each other. This bonding reinforces a
linear geometry of the carbon chain.
Cumulenes with non-equivalent substituents on each end exhibit
isomerism. If the number of consecutive double bonds is odd, there is
''cis''–''trans'' isomerism as for alkenes. If the number of consecutive double bonds is even, there is
axial chirality
Axial may refer to:
* one of the anatomical directions describing relationships in an animal body
* In geometry:
:* a geometric term of location
:* an axis of rotation
* In chemistry, referring to an axial bond
* a type of modal frame, in music ...
as for allenes.
Transition metal cumulenes
The first reported
complex containing a vinylidene ligand was (Ph
2C
2Fe
2(CO)
8, derived from the reaction of
diphenylketene and
Fe(CO)5 Structurally, this molecule resembles
Fe2(CO)9, wherein one ''μ''-CO ligand is replaced by 1,1-diphenylvinylidene, Ph
2C
2. The first monometallic vinylidene complex was (
C5H5)Mo(
P(C6H5)3)(CO)
2 2">=C(CN)2l.
See also
*
Cyclopropatriene
Cyclopropatriene is a hypothetical compound () which is an allotrope of carbon. It was once proposed as a candidate for a spectroscopically observed tricarbon species. It is a cyclic cumulene
In organic chemistry, a cumulene is a compound ha ...
and
cyclohexahexaene, cyclic cumulenes
References
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