Stereoisomerism

The compounds we have looked at so far — chain isomers, position isomers, functional group isomers — all differ in the order in which atoms are connected. That is, they have different constitutions (different connectivity of atoms). But there is another, subtler kind of isomerism: two molecules can have the same constitution (the same sequence of covalent bonds) yet differ in the relative positions of their atoms or groups in space. Such compounds are called stereoisomers, and the phenomenon is stereoisomerism.

Think of it this way: if you have a set of Lego bricks connected in exactly the same order, you can still twist or rotate parts of the structure into different three-dimensional shapes. The connectivity is identical, but the spatial arrangement is not. That is the essence of stereoisomerism.

Stereoisomerism is further divided into two main types:

• Geometrical isomerism (also called cis-trans isomerism)
• Optical isomerism

Both arise from the fact that molecules are three-dimensional objects, not flat drawings on paper. The key difference between them lies in the kind of spatial restriction that creates the isomers. Geometrical isomerism usually arises from restricted rotation (e.g., around a double bond or in a ring), while optical isomerism arises from the presence of a chiral centre (an asymmetric carbon atom). Both are taken up in detail later — geometrical isomerism with the hydrocarbons (Unit 9) and optical isomerism in later classes. …