Stereoisomers are molecules that have the same molecular formula and sequence of bonded atoms (constitution), but differ in the 3D orientations of their atoms in space. This concept is fundamental in medicinal chemistry and biochemistry.
Occurs when molecules contain Chiral Centers—atoms bonded to four different groups. These isomers (enantiomers) rotate plane-polarized light in opposite directions.
Often found in alkenes or cyclic compounds where restricted rotation leads to cis/trans or E/Z configurations.
For advanced exams like JEE and NEET, the following mathematical relationships are used to determine the number of stereoisomers based on symmetry:
A meso compound is an achiral stereoisomer of a set of stereoisomers that contains chiral centers but is optically inactive due to an internal plane of symmetry.
Each double bond capable of GI multiplies the stereoisomeric possibilities by a factor of 2, assuming the chiral centers and bonds are independent ($Total = Optical \times 2^m$).
Technically, every molecule has at least one "isomer" (itself). If $n=0$ and $m=0$, the total count is 1.
While both describe the 3D arrangement (configuration) of atoms, they use different criteria:
| Feature | D/L System (Relative) | R/S System (Absolute) |
|---|---|---|
| Basis | Glyceraldehyde reference | Cahn-Ingold-Prelog (CIP) Priority |
| Usage | Sugars and Amino Acids | All chiral organic molecules |
| Determination | Position of $-OH$ or $-NH_2$ | Atomic number of substituents |
Note: There is no direct mathematical correlation between D/L and (+)/(−) optical rotation. A 'D' isomer can be either dextrorotatory (+) or levorotatory (−).