optical activity of spirans and biphenyl compounds.

What is spirans ?

If both double bonds in allene are replaced by ring systems, the resulting molecules are  called spirans. For example , spiro-2,2 pentane,  1-chloro spiro-5,3-nonane. 

3D structure of 1-chloro-spiro-5,3-nonane is as follows.

Why spirans are optically active ?

Examination of formula of spirans shows that the two rings are perpendicular to each other, and hence suitable substitution will produce molecules with no elements of symmetry. 

So the optical activity of the spirans are due to the asymmetry of the molecule as a whole, thereby giving rise to optically active forms.

For example, dilactone of benzophenone-2,2’24,4’-tetra carboxylic acid. In this molecule the upper portions are perpendicular to the lower portion , and consequently there are no elements of symmetry. Hence, it is optically active.

Another example of optically active spiran is 3,8 dimethyl-1,6-dioxaspiro [4.4] nonane-2,7 dione.

Why biphenyl  exhibits optically activity ?

Dipole moment  measurements  and X-ray diffraction have been shown that the two benzene ring in biphenyl are co-axial, that is, the two rings lie in different planes.

It was found that two conditions were necessary for biphenyl compounds to exhibit optical activity,

(I)neither ring must have a vertical plane of symmetry

(II) the substitutions in the ortho positions must have a large .

For example, 6-nitro diphenic acid, 6,6’ dinitro diphenic acid and 2,2’ diamino 6,6’ dimethyl biphenyl.

If the ortho substitutions are large enough, free rotation about the single bond is restricted due to steric and repulsive effects of the groups in the ortho positions.

Thus the two benzene rings in biphenyl derivatives can not be coplanar. Such a molecule is not superimpposable on its mirror image and hence it is optically active.

For example, 6,6’-dinitro-diphenic acid  is optically active because ,as a whole the molecules is asymmetric.

Why 6,6-difluro-diphenic  acid is optically inactive ?

Due to small size of F-atom the rings rotate more easily around the central bond. The asymmetry is lost  and racemisation take place, hence it  becomes optically inactive.

Summary: 

Optical activity of spirans and biphenyl compounds

Click here: optical isomerism