Constitutional and Stereoisomers

Isomers

Isomers are molecules that have the same molecular formula (same number and types of atoms) but differ in their structural arrangements. This difference in arrangement can lead to distinct physical and chemical properties.

There are two primary categories of isomers: constitutional isomers and stereoisomers.

Constitutional Isomers

Constitutional isomers possess the same molecular formula but exhibit different connectivity of atoms. In other words, they have distinct structural arrangements.

• Example: Butane (C₄H₁₀) has two constitutional isomers:
  • n-Butane: A linear arrangement of four carbon atoms.
    
    
  • Isobutane: A branched chain with three carbon atoms and one methyl group attached to the second carbon.
    

Stereoisomers

Stereoisomers share the same molecular formula and atomic connectivity but differ in the spatial arrangement of their atoms. This variation arises from different three-dimensional orientations.

• Example: 2-Butene (C₄H₈) has two stereoisomers:
  • cis-2-Butene: The two methyl groups are positioned on the same side of the double bond.
    
    
  • trans-2-Butene: The two methyl groups are situated on opposite sides of the double bond.
    
    
• Example: 2-Butanol: 2-Butanol is a chiral molecule (i.e has handedness), characterized by a stereocenter (a carbon atom bonded to four unique groups). Owing to this chirality, 2-butanol exists as two stereoisomers:
  • (R)-2-Butanol: The hydroxyl group is positioned on the right side of the stereocenter.
    
    
  • (S)-2-Butanol: The hydroxyl group is positioned on the left side of the stereocenter.
    
    

These two stereoisomers of 2-butanol are referred to as enantiomers and are mirror images of each other but as we will see they are non-superimposable. While they possess identical physical properties, they exhibit distinct interactions with plane-polarized light.