Chiral center: Carbon bonded to 4 different groups. Designated R or S using CIP rules + steering wheel method.

Types:

• Enantiomers: Non-superimposable mirror images. Same physical properties except optical rotation direction.
• Diastereomers: Stereoisomers that are NOT mirror images. Different physical properties (mp, bp, solubility).
• Meso compounds: Have chiral centers but possess internal plane of symmetry. Net rotation = 0. Example: meso-tartaric acid.
• Racemic mixture: Equal mixture of enantiomers, optically inactive.

Counting stereoisomers:

• No meso possible: 2^n (n = number of chiral centers)
• With meso (identical substitution): If n = even, total = 2^(n-1) + 2^($\frac{n}{2}$-1)