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, 1,2-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.

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Geometry

  The central carbon of allene forms two sigma bonds and two pi bonds. The central carbon is sp-hybridised, and the two terminal carbons are sp2-hybridised. The bond angle formed by the three carbons is 180 degrees, 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.
Allene has no dipole moments

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.

References