Page "Allene" Paragraph 5

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 < sub > 2 </ sub > 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 < sub > 2 </ sub > planes.

Thus this class of molecules belong to the D < sub > 2d </ sub > point group.

Because of the symmetry, an unsubstituted allene has no net dipole moment.