In the study of organic chemistry, very often we come across different organic compounds which have same molecular formula. Like dimethyl ether and ethyl alcohol have same molecular formula, but their properties are quite different. Such compounds are called as isomers and the phenomenon is referred as isomerism. Isomerism may thus be defined as the phenomenon by virtue of which different organic compounds can be represented by same molecular formula. Isomerism can broadly be classified into two categories, namely; Structural isomerism and Stereoisomerism.



In structural isomerism, the isomers differ from one another in their structural arrangement. In other words, the structural isomers do have same molecular formula but they differ in the way in which the atoms are bonded to each other. For example, in dimethyl ether Oxygen atom is bonded to two Carbon atoms whereas in ethyl alcohol, the Oxygen atom is bonded to Carbon and Hydrogen atoms. In stereoisomerism, the isomers have same structural arrangement, i.e., they have same way in which the atoms are bonded to one another, but, the spatial orientations of various atoms/groups are different. For example, in both the geometrical isomers of 2-butene, the double bonded Carbon atoms are bonded to CH3- group and H-atom but the orientations of ?CH3 and H atoms in the two isomers are different.

The branch of chemistry which deals with the study of molecular structures in three dimensional space is called as stereochemistry. In 1848, a French Chemist Louis Pasteur made certain observations which later on became the foundations of stereochemistry. Working on crystals of optically inactive compound sodium ammonium tartrate; he observed two different kinds of crystals of the compound which were mirror images of each other. He separated these two types of crystals and further investigations revealed that each set of crystals is optically active. The specific rotation of two sets of crystals was equal but opposite in sign, which implied that one set of crystals rotates the plane of polarized light towards left whereas the other set of crystals rotates the plane towards right to the equal extent.

Pasture concluded that difference in specific rotation was not a certain characteristic of crystal but pertains to individual molecules. Such molecules were called enantiomers. This is a simplest kind of isomerism in which the isomers have different spatial arrangements of atoms or groups. Enantiomers are the isomers which bear object-image relationship but do not superimpose on each other. They rotate the plane of polarized in opposite directions to the equal extent.