Optical isomerism enantiomers diastereomers and optical activity of dissymmetric molecule.

What is optical isomerism ?

A compound which contains asymmetric carbon atom that  is, a carbon atom linked to four different groups or atoms, can exists in two isomeric forms which differ in their sign of rotation, such a compound exhibits optical isomerism. 

For example, lactic acid .

The 3D isomeric structure of lactic acid are as follows.

What is optical isomers ?

Optical isomers are characterized  by  compounds having the same structure but different configuration due to their molecular asymmetry.

These compounds rotate the plane of polarization of plane polarized  light , either clockwise or anti clockwise. 

For example, lactic acid have two optical isomers are as follows.

What is condition of optical activity ?

The necessary and sufficient conditions for a molecule to exhibit optical activity is,

( I) the molecule will not be super imposable on its mirror image and

(II) the molecule possess  none of the symmetry elements  such as plane of symmetry, centre of symmetry, or alternating axis of symmetry.

What is enantiomers ?

Optical isomers are mirror image to each other but not superimposable,are called enantiomers.

For example, 1-bromo-1-chloro ethanol have two enantiomeric forms are shown below.

What is diastereomers  ?

Optical isomers are not mirrorimage to each other and not superimposable are called diastereomers.

For example, tartaric acid have a pair of diastereomers.

What is dissymmetric molecule ?

If a molecule possess a simple or proper axis of symmetry but does not possess plane of symmetry, center of symmetry or alternating axis of symmetry, such a molecule is termed as dissymmetric molecule and they are also optically active. 

For example, 1,3 dimethyl allene or penta-2,3-diene.

Why allene exhibit optically activity ? 

 There are few compounds have no asymmetric carbon atom yet they exhibit optical isomerism. Because, as a whole the molecule becomes asymmetric due to restricted rotation and absence of any elements of symmetry.

 example of this type of compounds are given below.

Summary:
optical isomerism, optical isomers.
Enatiomers, diastereomers .
Dissymmetric molecule and optical activity of allene.

Practice problem:
Why spirans exhibit optically activity ?
What is the cause of optically activity of biphenyl ?

Click here: optical activity of spirans and biphenyls.