An allene is a hydrocarbon in which one atom of carbon is connected by double bonds with two other atoms of carbon. Allene also is the common name for the parent compound of this series, propadiene.
Such pair of bonds make allenes much more reactive than other alkenes. For example, their reactivity with gaseous chlorine is more like the reactivity of alkynes. Allenes can be chiral.
The central carbon of allene forms two sigma bonds and two pi bonds. The central carbon is sp-hybridized, and the two terminal carbons are sp2-hybridized. The bond angle formed by the three carbons is 180°, indicating linear geometry for the carbons of allene.
For allenes with four identical substituents, there exist two twofold axes of rotation through the center carbon, inclined at 45° to the CH2 planes at either end of the molecule. The molecule can thus be thought of as a two-bladed propeller. A third twofold axis of rotation passes through the C=C=C bonds, and there is a mirror plane passing through both CH2 planes. Thus this class of molecules belong to the D2d point group.
An allene with only two substituents, A and B will be chiral because there will no longer be any mirror planes. Where A has a greater priority than B according to the Cahn-Ingold-Prelog priority rule, the configuration can be determined by considering the top, then the bottom. For the bottom, only the group of higher priority need be considered.
Because of the symmetry, an unsubstituted allene has no dipole moment.
Laboratory methods for the formation of allenes are:
- Compounds with three or more adjacent carbon-carbon double bonds are called cumulenes.
- The allene motif is frequently encountered in carbomers.
- Using a suitable catalyst (e.g. Wilkinson's catalyst), it is possible to reduce just one of the double bonds of an allene.
- Propadiene is one of the two main components of MAPP gas.
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