Cumulene
A cumulene is a hydrocarbon with two or more cumulative (consecutive) double bonds. A member of this compound class is butatriene (which is also called simply cumulene), H2C=C=C=CH2. A chain with two consecutive double bonds is called an allene. Unlike most alkanes and alkenes, cumulenes tend to be rigid, which makes them appealing for molecular nanotechnology. Polyynes are another kind of rigid carbon chains. Cumulenes are found in regions of space where hydrogen is rare (see astrochemistry). Cumulenes containing heteroatoms are called heterocumulenes; an example is carbon suboxide.
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 dihalovinylidene. Tetraphenylbutatriene was reported synthesized in 1977 by homocoupling of 2,2-diphenyl-1,1,1-tribromoethane with elemental copper in dimethylformamide.
The rigidity of cumulenes arises from the fact that the internal carbon atoms carry two double bonds. Their sp hybridisation results in two bonds separated by 90°. Cumulenes with (1) an even number of consecutive double bonds and (2) dissimilar substituents on either end can be chiral even though they lack a classical stereocenter. On the other hand, odd numbers of double bonds in a cumulated system with proper substituents show cis–trans isomerism.
The reactions of cumulene are those of the isolated double bond. Although this molecule possesses two π-bonds in very close vicinity, in essence these act as isolated double bonds. This is because the two π-bonds on the central carbon atom are formed by the non-hybridised p-orbitals. As these orbitals are perpendicular towards each other and occupy each other's nodal planes, the two bonds are in essence isolated.
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