Allene

Propadiene, the simplest allene, is also known as allene.

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.

Geometry

3D view of allene.

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.

Symmetry

For allenes with four identical substituents, the compound will have a twofold axis of rotation through the center carbon. The axis is inclined at 45° to the CH₂ planes on either side of the molecule. The molecule can thus be thought of as a two-bladed propeller. Another 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 will have a D_2d 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.

Synthesis

Laboratory methods for the formation of allenes are:

• from geminal dihalocyclopropanes and organolithium compounds in the Skattebøl rearrangement.
• from reaction of certain terminal alkynes with formaldehyde copper(II) bromide and added base^[1]
• from dehydrohalogenation of certain dihalides.^[2]

See also

• 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.^[3]
• Propadiene is one of the two main components of MAPP gas.

References

Further reading

• International Union of Pure and Applied Chemistry. "allenes". Compendium of Chemical Terminology Internet edition.

External links

• Stereochemistry study guide
• Synthesis of allenes