Stereoisomerism is a type of structural isomerism in which molecules have the same molecular formula and the same connectivity of atoms but differ in the spatial arrangement of atoms or groups. Stereoisomers are molecules that are mirror images of each other but cannot be superimposed, similar to left and right hands. Stereoisomerism arises due to the presence of chiral centers or the presence of double bonds that restrict rotation.

There are two main types of stereoisomerism: enantiomerism and diastereomerism.

1. Enantiomerism: Enantiomers are a type of stereoisomer that are non-superimposable mirror images of each other. They have the same physical properties (e.g., melting point, boiling point) but interact differently with polarized light. Enantiomers are chiral molecules, meaning they lack an internal plane of symmetry. Enantiomerism arises in molecules with chiral centers (asymmetric carbon atoms) that are bonded to four different groups. Enantiomers have opposite configurations at all chiral centers. An example of enantiomers is L- and D-amino acids.
2. Diastereomerism: Diastereomers are stereoisomers that are not mirror images of each other and have different physical properties. They arise from molecules with more than one chiral center and result in different spatial arrangements due to the different configurations at some chiral centers. Diastereomers are not necessarily equal in number and can have distinct chemical and physical properties. An example of diastereomers is cis-trans isomerism in alkenes, where the relative positions of substituents on a double bond differ.

Key points about stereoisomerism:

• Configuration vs. Conformation: Configuration refers to the fixed spatial arrangement of atoms or groups in a molecule, while conformation refers to the different possible arrangements of atoms that arise from single-bond rotations.
• Chirality: Chiral molecules have no superimposable mirror image, while achiral molecules can be superimposed on their mirror images.
• Chiral Centers: Chiral centers are carbon atoms bonded to four different substituents. The presence of one or more chiral centers in a molecule leads to the potential for stereoisomerism.
• Optical Activity: Chiral molecules interact with polarized light, causing rotation of the plane of polarized light. This property is known as optical activity and is used to identify enantiomers.
• Racemic Mixture: A racemic mixture is an equimolar mixture of enantiomers. It does not exhibit optical activity because the effects of one enantiomer cancel out those of the other.

Stereoisomerism is an important concept in organic and biochemistry, as it plays a role in various biological processes, drug interactions, and chemical reactions.