Stereochemistry is the study of the relative arrangement of atoms or groups in a molecule in three dimensional space. Stereochemical isomers are molecules, which have the same chemical formula and bond connectivity but different relative arrangement in three-dimensional space. In contrast, constitutional isomers have same molecular formula but different bond connectivity. Thus, n-butane and isobutane are structural isomers while the isomers of limonene, the compound which gives different taste to lemon and orange are examples of stereochemical isomers (Figure 1).

Figure 1

To understand the difference between the two isomers of limonene, introduction to some new terms and concepts are required. The most important being the concept of chirality.

A chiral object is one that cannot be superposed on its mirror image. The term originates from the greek term for “hand”. As it is with human hands, the left hand cannot be superimposed on the right hand. It is the same with chiral molecules. They are non-superimposable mirror images of each other. Achiral objects, on the other hand, are easily superimposable on each other. A tennis racquet and a spoon are examples of achiral objects.

The next question that comes to the mind is how to determine whether a molecule is chiral or achiral. At times, it becomes extremely difficult to determine with increasing molecular complexity, to determine the non superimposibity of a compound with its mirror image. Thus,  a mathemathical concept known as group theory can be applied to determine the symmetry elements in a molecule. There are four symmetry elements which needs to be considered for this purpose: