7.0 Optically active compounds having no asymmetric carbon

  Isomerism

1. Allenes: An sp-hybridized carbon atom possesses one electron in each of two mutually perpendicular p orbitals. When it is joined to two $s{p^2}$-hybridized carbon atoms, as in allene, two mutually perpendicular $\pi $ -bonds are formed and consequently the $\pi $-bonds to the $s{p^2}$-carbons are in perpendicular planes.

Allenes of the type $abC = C = Cab$ $(a \ne b)$ are therefore not superimposable on their mirror images and despite the absence of any asymmetric atoms, exist as enantiomers and several optically active compounds have been obtained.$\left( {e.g.{\text{ }}a{\text{ }} = {\text{ }}phenyl,{\text{ }}b{\text{ }} = {\text{ }}1 - napthyl} \right)$

2. Any molecule containing an atom that has four bonds pointing to the corners of a tetrahedron will be optically active if the groups are different.