The Mirror-Image Problem
Your left and right hands are mirror images that cannot be superimposed — this geometric property, called chirality, pervades organic chemistry. When a carbon atom bears four distinct substituents, it creates two non-superimposable mirror-image arrangements (enantiomers) with identical physical properties except for the direction they rotate plane-polarized light and how they interact with other chiral molecules.
The CIP Priority System
Cahn, Ingold, and Prelog developed a universal system to name each enantiomer. Each substituent receives a priority based on the atomic number of the atom directly attached to the stereocenter: bromine (Z=35) outranks chlorine (Z=17), which outranks oxygen (Z=8), which outranks hydrogen (Z=1). When there is a tie, you follow the chain outward until you find a difference. This simulation lets you experiment with different substituent combinations to see how priority assignment works.
Assigning R or S
Once priorities 1-4 are assigned, orient the molecule so that priority 4 (lowest) points away from you. Then trace an arc from priority 1 to 2 to 3: if the arc is clockwise, the center is R (Latin rectus, right); if counterclockwise, it is S (Latin sinister, left). The visualization below shows this rotation in real time, highlighting the priority sequence and the resulting configuration as you adjust substituent atomic numbers.
Biological Consequences
Chirality has life-or-death consequences. All natural amino acids are L-configured, all natural sugars are D-configured, and enzymes distinguish enantiomers with exquisite selectivity. The thalidomide tragedy demonstrated that one enantiomer can be therapeutic while its mirror image is teratogenic. Modern asymmetric synthesis and chiral chromatography techniques now allow chemists to produce and verify single-enantiomer drugs with high stereochemical purity.