Concept Of Isomerism In Organic Chemistry

In Organic chemistry, isomers are groups of molecules with the same molecular formula but differ in the arrangement of atoms within the molecule. The isomers will have different properties such as boiling point and melting point etc.

Classifications Of Isomers

Isomers are categorized according to the chemical structure of the compound as shown below.

Isomers are mainly classified into structural isomers and stereoisomers. The detailed explanation is given below.

Structural isomerism

Structural isomers are also called constitutional isomers. The molecules will have the same chemical formula but differ in the arrangement of atoms or functional groups within the molecules. The phenomena is known as structural isomerism. Structural isomers are further classified as,

1. Chain isomers

The molecules will have the same chemical formula but differ in the arrangement of carbon branching.

Example: Structure of pentane can also be written as isopentane and neopentane.

2. Position Isomers

The molecules will have the same chemical formula but differ in the arrangement of position of the functional group or substituted atoms.

Example: Position isomers of C₃H₇Br

3. Functional Isomers

The molecules will have the same chemical formula but they differ in the arrangement of functional groups in the compound.

Example: Functional isomers of the compound with chemical formula C₃H₆O

Stereoisomerism

The compounds will have the same molecular formula but differ in the special arrangement of the atoms or functional group within the molecule. Stereoisomerism is further classified as Geometrical isomerism and optical isomerism. The two terms to understand under stereoisomers are 

1. Enantiomers: Non- superimposable mirror images (Optical isomers).
2. Diastereomers: They are neither superimposable nor mirror images.

1. Geometrical isomerism

Geometrical isomerism is also known as cis-trans isomerism. The geometrical isomers will have the same chemical formula but differ in the position of atoms in three-dimensional space. 

Example: Cis – trans isomers of 2- Butene compound

2. Optical isomerism

The optical isomers will have the same chemical formula but differ in the spatial arrangement of atoms in three-dimensional space and the phenomena are known as optical isomerism. The compound which undergoes optical isomerism is known as a chiral molecule. 

The term optical isomerism is due to the reason that a plane-polarized light passes through the enantiomer (1) and it rotates. When the plane-polarized light passes through a pair of an enantiomer (1), it turns in the opposite direction with the same magnitude. 

The Important Concept Of Optical Isomerism

• There are simple compounds which demonstrate optical isomerism as two isomers known as enantiomers.
• One enantiomer solution rotates the polarizing plane in the clockwise direction. This enantiomer is called form (+).
• The other enantiomer’s solution rotates the polarisation plane in an anti-clockwise direction. That enantiomer is known as the (-) form.
• If the two solutions are of the same concentration, then the amount of rotation induced by the two isomers is the same but in a different direction. 
• When optically active compounds are produced in the laboratory, the two enantiomers often appear as a mixture of 50/50 of them. The mixture is called racemic or racemate mixture.
• The racemic mixture does not affect plane-polarized light.
• An alternative way of defining the (+) and(-) forms is by using the lowercase letters d- and l- respectively which indicates the dextrorotattion and laevorotation.

Examples 1: Enantiomers or optical isomers of alanine

Example 2: Optical isomers of glucose 

The chemical formula of the glucose is C₆H₁₂O₆ and the glucose structure is given below.

The number of Chiral carbon atoms in glucose are 4. So the number of optical isomers possible in the glucose structure is 16.

 2ⁿ = 2⁴ = 16

Example 3: Optical isomers of Butan-2-ol