Isomerism

Compounds having the same molecular formula but different physical and chemical properties are called isomers and the phenomenon is known as isomerism. It is classified as follows.

Structural Isomerism

Compounds which possess the same molecular formula but differ in bonding arrangement of atoms (or) groups within the molecule i.e. differ in structure are structural isomers and this phenomenon is known as structural isomerism. Structural isomerism is also known as constitutional isomerism. It is of six types which discussed below.

Chain Isomerism

Chain isomerism is is also known as skeletal isomerism. Isomers differ in the arrangement of their carbon chains, which might be branched or straight is called chain isomerism. The molecular formula C₅H₁₂ has three isomers: n-pentane, 2-methyl butane or isopentane, and 2,2-dimethylpropane or neopentane.

Positional Isomerism

Isomers with the same functional groups but in different positions on the same carbon chain are also known as position isomerism. The molecular formula C₃H₈O has two isomers: 1-propanol and 2-propanol.

Functional Isomerism

Compounds having the same molecular formula but differ in the functional group is called functional isomerism. The molecular formula C₃H₆O can be arranged as a ketone or an aldehyde.

Metamerism

It arises when different alkyl chains are joined with the same divalent functional group present in the molecule. Ethers, thioethers, secondary amines, ketones, esters etc. can show metamerism.

Tautomerism

Tautomerism is a special type of functional isomerism in which the isomers are readily interchangeable and maintain a dynamic equilibrium with each other. Both the isomers represent a single substance. Reversibility of change is due to mobility of a group or atom which can move from one position to another in the molecule, often with rearrangement of a double bond. These two forms are called tautomers and tautomerides of each other.

Structural Requirement of Tautomerism
• Compounds contain polar molecules and weakly acidic group (functional groups).
• It involves the change in position of an atom.
• It has no effects on bond length or such features.
• Generally, it occurs in planar or non-planar molecules.

Ring-Chain Isomerism

In chain isomerism, two isomers have the same molecular formula but one isomer has an open-chain structure, while the other has a ring structure. They usually contain a variable number of π bonds. C₃H₆ isomers is an excellent example of ring-chain isomerism which exist as propene and cyclopropane. Other examples are 2-butene and cyclobutane, 1,3-butadiene and cyclobutene.

Stereoisomerism

Stereoisomers have identical molecular formula and same connectivity of the constituent atoms but the relative three dimensional arrangements of the atoms are different. As a result they possess different properties. Enantiomers and Diastereomers are the two major classes of stereoisomers. Enantiomers have non-superimposable mirror image relationship whereas Diastereomers do not have mirror image relationship. Learn more about Stereochemistry