<P> 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 . </P> <P> The central carbon atom of allene forms two sigma bonds and two pi bonds . The central carbon is sp - hybridized, and the two terminal carbon atoms are sp - hybridized . The bond angle formed by the three carbon atoms is 180 °, indicating linear geometry for the carbon atoms of allene . It can also be viewed as an "extended tetrahedral" with a similar shape to methane . </P> <P> 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 CH 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 CH planes . Thus this class of molecules belong to the D point group . Because of the symmetry, an unsubstituted allene has no net dipole moment . </P>

The ccc bond angle in allene (h2ccch2) is