Isomerism in Organic chemistry

Isomerism

This is the existence of different compounds with the same molecular formulae but different structural formulae. Compounds with same molecular formulae but different structural formulae are known as Isomers. Isomerism is divided into two classes.

Structural Isomerism. 

This is the type of isomerism which shows the sequence in which the atoms in a molecule are bonded or linked. It is divided into three classes:

• Chain Isomerism.

This is the type of structural isomerism whereby the isomers differ in the carbon chain. The isomers usually have the same functional group and belong to the same homologous series. It can be divided into classes i.e. Straight chain and branch chain

Example included has molecular formula C₄H₁₀, write down possible chain structural formula of Q. Since the M.F. is C4H10 isomers are:

• Positional Isomerism. 

This is the isomerism where the substituent groups are in different positions in the same carbon skeleton. The isomers are chemically similar because they possess the same functional groups.

• Functional Isomerism.

This is the isomerism whereby the isomers in different homologous series have different functional groups.

Examples include:

A compound Q has Molecular formula C₂H₆O. The isomers are CH3CH2OH (from Alcohol) and CH3OCH3 (from ether). A compound W has molecular formulaC₃H₆O 

The isomers are CH₃COCH₃ (from Ketones) and CH₃CH₂CHO (from Aldehydes)

A compound Z has molecular formula C₄H₈O₂ 

Þ The isomers are CH₃CH₂CH₂COOH (from Carboxylic acid) and CH₃COOCH₂CH₃ (from Esthers)

Stereoisomerism.

This is the type of isomerism whereby the compounds have the same molecular formula and same structural formula but differ in the spatial arrangement of the bonds. Stereoisomerism is divided into two classes:

• Cis-trans Isomerism (Geometrical Isomerism). 

This occurs in compounds in which free rotation is prevented by the presence of multiple bonds, a ring or steric factors. 

Geometrical Isomers differ in spatial arrangement giving rise to Cis isomers which has identical atoms or groups on the same side of the multiple bonds and a Trans isomer which has the atoms or groups in opposite side. Examples include:

• Optical Isomerism

This occurs in compounds with four atoms or groups attached to the same carbon such that the isomers obtained are not super imposable on the mirror images. These isomers which are not super imposable are referred as to as Enantiomers and the structures are said to be chiral. 

Examples include;

Enantiomers have same physical properties but may be distinguished from each other by the ability they rotate polarized light due to optical activity.

If the rotation of the plane is in clockwise direction (to the right), the substance is dextrorotatory (with + sign) while if the rotation is in the anticlockwise (to the left), the substance is levorotatory (with – sign). The degree of rotation is determined by use of a polarimeter.