Stereochemistry

Chirality and R/S Assignment

A stereocenter is a carbon with four different substituents. Assign Cahn-Ingold-Prelog priorities by atomic number (duplicate atoms for double bonds), point the lowest priority away, and trace 1 to 2 to 3: clockwise is $R$, counterclockwise is $S$. If the lowest priority faces you, reverse the answer. A molecule with $n$ stereocenters has at most $2^n$ stereoisomers.

Stereochemical Relationships

Enantiomers are nonsuperimposable mirror images: every stereocenter inverted, identical physical properties, opposite optical rotation, and different behavior only in chiral environments (enzymes, receptors, polarized light). Diastereomers differ at some but not all centers and have genuinely different physical properties. Meso compounds contain stereocenters yet are achiral because of an internal mirror plane. Cis/trans and E/Z alkene isomers are diastereomers too.

Optical Activity

A single enantiomer rotates plane-polarized light with specific rotation $[\alpha] = \frac{\alpha_{obs}}{l \cdot c}$ A racemic mixture shows zero net rotation. Enantiomeric excess measures how far a mixture is from racemic. Fischer projections put horizontal bonds toward the viewer; swapping any two groups inverts the configuration.

Key Takeaways

• $R/S$ assignment: prioritize by atomic number, lowest priority in back.
• Enantiomers differ only in chiral contexts; diastereomers differ physically everywhere.
• Meso compounds cut the stereoisomer count below $2^n$.
• Racemates and meso compounds are both optically inactive for different reasons.