Chirality, Enantiomers, and Stereochemical Nomenclature

Chiral molecules are non-superimposable mirror images of each other (like hands).
Enantiomers are pairs of chiral molecules that are mirror images.
A molecule is chiral if it lacks an internal plane of symmetry.
A carbon atom with four different substituents is a chiral center or stereocenter.

Observed physical property: Enantiomers rotate plane-polarized light in opposite directions.
Dextrorotatory (d or +)): Rotates light clockwise.
Levorotatory (l or -): Rotates light counter-clockwise.
This nomenclature is experimentally determined and cannot be predicted from structure alone.

Cahn-Ingold-Prelog (CIP) Rules assign absolute configuration (R or S) to chiral centers.
1. Prioritize substituents attached to the chiral center based on atomic number; higher atomic number gets higher priority.
  - If the first atoms are the same, move to the next atoms along the chain until a difference is found.
2. Orient the molecule so the lowest priority group (usually hydrogen) points away (on a dash).
3. Trace a path from priority 1 to 2 to 3.
  - If the path is clockwise, the configuration is R (Rectus).
  - If the path is counter-clockwise, the configuration is S (Sinister).
Important: If the lowest priority group is on a wedge (pointing towards you), assign R/S normally and then reverse the result (R becomes S, S becomes R).

A simplified 2D representation of 3D molecules, primarily used for carbohydrates.
Horizontal lines represent bonds coming out (wedges), vertical lines represent bonds going in (dashes).
Interchanging any two bonds on a Fischer projection inverts the stereochemistry (R becomes S, S becomes R).
Interchanging bonds twice (or an even number of times) returns to the original stereochemistry (R becomes S, then S becomes R).
To assign R/S configuration in Fischer projections, the lowest priority substituent should ideally be at the top or bottom position for easier assignment relative to standard rules.