CBSE Class 12 Chemistry Notes: Haloalkanes and Haloarenes – Optical Isomerism

SOME BASIC CONCEPT OF OPTICAL ISOMERISM:-

           i. SN² reaction proceeds with inversion of configuration

ii.  SN¹ reaction proceeds with racemization.

• Plane Polarized Light:-

Ordinary light have e.m. waves in in different directions (e.m→ electromagnetic). Thus such a beam of light which has vibration of EM waves only in one plane is called Plane Polarized Light.

• Optical Activity/Optically Active Substances:-

Such substances which can rotate the plane polarized light are called optically active substances.

And the property of a substance to rotate the plane polarized light light is called the Optical Activity.

 

• Dextrorotatary:-

A substance which rotates plane of polarized light towards right in clockwise direction is called Dextrorotary (+)

• Leavorotatory:-

A substance which rotates the plane polarized light towards left is known as a Leavorotory substance (-)

• Optical Isomerism:-

The leavoratary (-)andDextrotatory (+) isomers of an aplocally active compound are called the optical isomers and phenomena is called as Optical Isomerism.

• Chiral Molecules:-

Objects or Molecules which are non superimposable on their mirror images are called Chiral Molecules and this properties is known as Chirality.

• Assymtrical Molecule:-

The molecule which have an Assymtric carbon atom (such that all the group or atom attached to carbon are different) is called Assymeterical molecule.

Example:-

 

SOME BASIC CONCEPT OF OPTICAL ISOMERISM-II

• Enantiomers:-

Optical isomers which are the non superimpossible mirror images of each other are called as the Enantiomers

Example:-

 

 

• Racemic Mixture:-

A mixture of the equal Amounts of the two enantiomers is called the Racemic Mixture or Racemic Modification.

• Racemization:-

The process of conversion of one entiomer (+ or -) into a Racemic Mixture is called Racemisation

• Retention:-

If in a chemical reaction the special arrangement of bonds at an assymteric centre remains preserve than it would be known as Retention of Configuration.

 

Example:-

 

 

SOME BASIC CONCEPT OF OPTICAL ISOMERISM-III

• Inversion:-

If the position of the substituent is get changed by 180^o than it would be known as inversion of mixture

Example:-

 

• Stereochemistry of SN² Reaction:-

SN² Reactions of optically active alkyl halide are always accompanied by inversion of configuration at the asymmetric centre.

Example:-

 

 

• Stereochemistry of SN¹ Reaction:-

In SN¹ reaction if the alkyl halide is optically active then the product will be us a racemic mixture. Thus SN¹reaction always accompanied Racemization.

 

  i.   Draw structures of possible enantiomers of 3 –methylpent – 1 –ene

Ans.     The enantiomers of 3 – methylpent – l – ene are

 

  ii.   Identify chiral and achiral molecules in each of following pairs of compounds:-

 

 

Ans.

a.(i) is the chiral molecule because it have assymetric carbon atom.

b.(I) molecule is chiral molecule because it have the assymetric carbon atom having all the four different substituents for C₂.

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