Stereochemistry
The idea
Stereochemistry studies molecules that share the same connectivity but differ in 3D arrangement. The headline case is chirality: a carbon bearing four different groups (a stereocenter) makes a molecule non-superimposable on its mirror image, and the two mirror forms are enantiomers. Enantiomers match in melting point, boiling point, and solubility, yet rotate plane-polarized light in opposite directions and behave differently in chiral environments — which is why a drug and its mirror image can have entirely different physiological effects.
The Cahn–Ingold–Prelog system labels each stereocenter: rank the four substituents by atomic number at the first point of difference (working outward atom by atom when first atoms tie), orient the lowest-priority group away from you, and read the path 1 → 2 → 3. Clockwise is R, counterclockwise is S. Molecules with several stereocenters can also form diastereomers — stereoisomers that are not mirror images — which, unlike enantiomers, differ in ordinary physical properties.
The error to root out is conflating R/S with the sign of optical rotation. R and S come from an arbitrary ranking convention; (+) and (−) come from a polarimeter measurement. There is no shortcut from one to the other — an R compound can rotate light either way, and only experiment decides.
Worked example
A model of butan-2-ol is oriented so that the H on C2 points directly away from you. Looking at the front face you see the OH group at the top, the ethyl group at the lower right, and the methyl group at the lower left. Assign the configuration as R or S, and state the relationship between this molecule and its mirror image.
- Verify C2 is a stereocenter: it carries OH, CH₃, CH₂CH₃, and H — four different groups, so a configuration label applies.
- Rank by CIP rules: oxygen beats carbon, so OH is priority 1. Ethyl versus methyl is a tie at the first atom (both C), so look outward: ethyl's carbon is attached to (C, H, H) while methyl's is attached to (H, H, H), so ethyl is 2, methyl is 3, and H is 4.
- Confirm the viewing geometry: the lowest-priority H already points away from you, so you may read the front face directly with no mental rotation.
- Trace priorities 1 → 2 → 3: top (OH) to lower right (ethyl) to lower left (methyl) sweeps clockwise, so the configuration is R.
- Relate to the mirror image: reflecting swaps the handedness to S; the pair are enantiomers — identical in bp, mp, and density but opposite in optical rotation and distinguishable by chiral reagents.
Answer. The molecule is (R)-butan-2-ol, and its mirror image, (S)-butan-2-ol, is its enantiomer.
Check your understanding
- Why does swapping any two groups on a stereocenter invert the R/S label, and how can you use that fact to check assignments?
- What would you do differently in the assignment if the lowest-priority group pointed toward you instead of away?
- How do diastereomers differ from enantiomers in their physical properties, and why does that difference make them separable by ordinary distillation or chromatography?
- Why can two enantiomers smell different or act differently as drugs even though their bond energies are identical?
Build the foundations first
Stereochemistry builds on these concepts. If any feel shaky, start there.