Stereoisomers are molecules that have the same molecular formula and bonding arrangement; however, they differ in how their atoms are positioned in 3-dimensional space (spatial orientation) with respect to each other.
These molecules, which differ in orientation while still having the same molecular formula, are also known as spatial isomers.
In the above image, for example, the carboxylic acid group (-COOH) is on the opposite side of the double bond in Fumaric acid. Whereas the -COOH group is on the same side of the double bond in Maleic acid. As is evident, such a small difference in how the one bond is oriented has a profound influence on the molecules’ physical properties. The melting point of Maleic acid is significantly higher than that of Fumaric acid, showing the impact of the shape.
Related Reading – How the shape of molecules impacts the melting point, such that one arrangement is superior to another, is covered in the Chapter, Physical Properties.
Stereoisomers are of three types: conformational, geometric, and enantiomers.
1. Conformational isomers refer to one compound capable of producing an enormous, infinite number of variations in its structure due to rotations around the single bonds. The relative positions of the groups continually change with respect to one another. However, the most effective rotations responsible for isomerism are around the carbon-carbon (C-C) single bonds. Since the molecules are easily interconvertible by simple bond rotations, isomers are also known as rotamers.
2. Geometrical isomers have fixed double bonds, and the relative position of groups varies across the double bonds, producing isomers.
3. Enantiomers are optical isomers that use the optical property of reflection. Compounds that are mirror images of each other, however, are non-superimposable when stacked; such compounds are called enantiomers.
A simple analogy is that of our left and right hands. When placed side by side, they are like mirror images of each other. However, when the hands are stacked on top of each other, only the middle fingers match; the rest don’t.
In the example below, imagine rotating the carbon-carbon single bond on the molecule on the right to match the molecule on the left. No amount of rotation would give the actual structure.
