Allene

An allene is a compound in which one carbon atom has double bonds with each of its two adjacent carbon centres. Allenes are classified as polyenes with cumulated dienes. The parent compound of allene is propadiene. Compounds with an allene-type structure but with more than three carbon atoms are called cumulenes. Allenes are much more reactive than most other alkenes. For example, their reactivity with gaseous chlorine is more like the reactivity of alkynes than that of alkenes.

Structure and bonding

Geometry

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. It can also be viewed as an "extended tetrahedral" with a similar shape to methane.

Symmetry

The symmetry and isomerism of allenes has long fascinated organic chemists. 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 D_2dpoint group. Because of the symmetry, an unsubstituted allene has no net dipole moment.