Optical Isomerism | Stereochemistry Class 11/12

Optical Isomerism

Stereochemistry | Chirality & Isomer Types

1. Optical Activity & Chirality

                    Optical Activity: The ability of a compound to rotate the plane of plane-polarized light (PPL).
                    Dextrorotatory (d or +): Rotates light to the Right (Clockwise).
Levorotatory (l or -): Rotates light to the Left (Anti-clockwise).

                

Chirality (Handedness): An object or molecule is chiral if it is non-superimposable on its mirror image.
Chiral Carbon ($C^*$): A carbon atom bonded to four different groups.

Condition for Optical Activity: The molecule must lack any element of symmetry (Plane of Symmetry or Center of Symmetry). It must be Chiral.

2. Enantiomers

Stereoisomers which are non-superimposable mirror images of each other.

They have identical physical properties (BP, MP, density, refractive index).
They differ only in their interaction with PPL (rotate light in opposite directions) and with other chiral reagents.

3. Diastereomers

Stereoisomers which are NOT mirror images of each other.

They have different physical properties (different BP, MP, Solubility).
They can be separated by physical methods like fractional distillation or crystallization.
Example: Cis and Trans isomers are diastereomers.

4. Meso Compounds

Compounds that contain chiral centers but are optically inactive (achiral) due to the presence of a Plane of Symmetry (POS).

                    Internal Compensation: The rotation caused by one half of the molecule is cancelled by the equal and opposite rotation of the other half.
                

Example: Meso-Tartaric Acid.

5. Racemic Mixture (Modification)

An equimolar mixture (1:1) of two enantiomers (d and l).

                    External Compensation: The rotation caused by the 'd' isomer is exactly cancelled by the 'l' isomer, making the mixture optically inactive ($\alpha = 0$).
                

Separation of a racemic mixture into pure enantiomers is called Resolution.

6. Number of Stereoisomers

If a molecule has '$n$' chiral centers:

Case A: Molecule is Unsymmetrical

$$ \text{Total Optical Isomers} = 2^n $$ $$ \text{Meso forms} = 0 $$

Case B: Molecule is Symmetrical (n is even)

$$ \text{Total} = 2^{n-1} + 2^{\frac{n}{2}-1} $$ $$ \text{Enantiomers} = 2^{n-1} $$ $$ \text{Meso} = 2^{\frac{n}{2}-1} $$

Practice Quiz

Test your logic on Stereoisomers.

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