Optical isomerism and Enantiomers

The light whose vibrations occur only in one plane is termed as plane polarised light . The device that brings polarisation in light is called a polariser. Some substances have the ability to rotate the plane polarised light either to right (clockwise) or to the left (anticlockwise). These substances are termed as optically active substance and the property is called optical activity.
The apparatus which measures the extent of rotation of the polarised light is called Polarimeter. 


Compounds having similar physical and chemical properties but differing only in the bahaviour towards polarised light are called optical isomers and this phenomenon as optical isomerism. 
The optical isomer which rotates the plane polarised light to the right direction (Clockwise) is known as dextroratatory isomer indicated by "d" or "+" sign
The optical isomer which rotates the plane polarised light to the left direction (Anti-Clockwise) is known as levorotatory isomer indicated by "l" or "-" sign.


In order to exhibit optical activity, an object must be chiral. Compounds which are mirror images of each other and are not superimposable are termed as enantiomers. 
The molecular dissymmetry or chirality is a necessary condition for the existence of enantiomers. A pair of enantiomers have identical physical and chemical properties but differ from each other in their  action on plane polarised light. They have identical structures but differ from each other in having different spatial disposition of atoms or groups constituting them. 


A carbon atom which is bonded to four different atoms or groups in a molecule is called chiral carbon or tetrahedral centre or asymmetric carbon atom or Chiral Centre.

Some special examples  of optical isomerism in compounds containing no chiral carbon atom :-
1. ALLENES : Allenes are the compounds with general formula Allenes are optically active when the two groups attached to each terminal carbon atom are different. Examples are
When one C-C double bond in allenes is replaced by one cyclohexyl ring,the compound is known as alkylidenes. 
1. Biphenyl is an asymmetric compound so it must not have plane of symmetry, centre of symmetry or alternate axis of symmetry to be optically active. The following types of the biphenyls are optically inactive as they have a plane of symmetry

2. In a normal chiral compound all the food groups must be different. However in biphenyl each ring must have two different orth-substituting groups but it's not necessary that “a” should be different from “a”   or “b” from “b”.
3. Van der Waals radii affects racemization
a. Smaller van der waal radii groups: The substituents F-, H-, -OCH3 etc have smaller van der waal radio. Therefore when the four orth-substituting groups are smaller, the rotation about the single bond is free and so the compound is not resolvable.
b. Medium sized groups: It contains at least amino groups, carboxyl group or one amino and one carbon and the remaining group may be F, OCH3
 In this situation biphenyl readily racemizes.
c. Bulky groups:  It contains two nitro groups or sulphonic groups at Ortho position and the remaining groups are F, OCH3 then the compound will be resolvable. 
The resonance affects the rate of racemization.

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