Optical Isomerism in Compounds without any Stereocentre | Organic Chemistry PDF Download

Optically activity in the absence of a chiral carbon/ sterecentre:

There are many cases where the Chirality is not because of the presence of one or more chiral centers, but it is because of the presence of other elements of Chirality.

Molecules belonging to the following category are chiral without chiral
(i) Allenes
(ii) Spiranes
(iii) Biphenyls
(iv) Cyclohexanes 

Optical activity due to a chiral axis:-

When four ligands attached to the central atom C are located on the corners of a regular tetrahedron, C is a chiral center of the molecule and is replaced by linear grouping of e.g. C—C or C=C=C.  The tetrahedron becomes elongated i.e. extended along the axis of grouping to produce a chiral axis XY. 

The minimum condit ion for Chiralit y of a elo ngated tetrahedron is that A should be different fro m B. i.e. pair of ligands at X end of the axis and pair of ligands at Y end of the axis are each constituted by different ligands.

Stereochemistry of Allenes:-

When three or more adjacent carbon atoms in a molecule are bonded by double bonds, the compound is called cumulene or is said to have cumulative double bonds. Allene is the simplest cumulene

The spatial arrangement of the cumulative double bond in allene is such that its terminal methylene groups are

⊥ to  each other. This is easily understandable because the two terminal carbon atoms in allene are in Sp² and the central carbon atom is in Sp hybrid state, thus central carbon forms two π bonds which are ⊥ to each other.
Thus allenes of the type:- abC=C=Cab (a≠b) are chiral.

• In allenes if double bonds are even there are two ⊥ planes. If there is no plane of symmetry than molecule is chiral. If there is any plane of symmetry (then the molecule is achiral).
• If cumulene has odd no of double bonds, then it is achiral due to mo lecular plane of symmetry or it shows geometrical isomerism. ·
• A 2-fold rotational axis is present in allene when a=a and b=b
• Since the replacement of a double bond in allene by a ring does not alter the basic geo metry of the molecule. ·
• Compound where Sp² carbon is replaced by nitrogen have also been obtained as enantiomers.

R-S configuration for allenes

Molecule is viewed from the point which is in plane. Substituents at this site are given priority 1 and 2 according to CIP Rule and thus other end is cited in the same way. Above shown figure shows how to assign configuration to allenes.
Problems:-

 Stereochemistry of Spirane :-If both the double bonds in allenes are replaced by a ring systems, the resulting molecules are known as spiranes or Spiro compounds.

For Chirality X≠Y

If X=Y – plane of symmetry is present and molecule becomes achiral.

R-S configuration in case of spiranes:-

Molecule is viewed from the point which is in plane. Substituents at this site are given priority 1 and 2 according to CIP Rule and thus other end is cited in the same way. Above shown figure shows how to assign configuration to spiranes.

Stereochemistry of biphenyls:-In the crystals, both benzene rings lie in the same plane. However, in solution and vapour phase the two rings are twisted with respect to each other by an angle 450 due to steric interactions between the 2,2’ and 6,6’ pairs of hydrogen’s. These interactions effects are further enhanced by ortho substituent’s larger than hydrogen so that

The rotation about the bond linking the two phenyl rings doesn’t occur due to steric strain (hindrance) between the bulky ortho substituent’s.

Biphenyls show enantiomerism when following two conditions are satisfied

• Each ring must be unsymmetrically subst ituted.

• Both rings must be substituted in ortho posit ions (minimum number of subst ituent’s should be two, one substituent in each ring) and the substituent must have a large size.

R-S configuration of biphenyls