Key structures for this topic — drawn live.
Start with the definition of stereoisomers
Stereoisomers share the same formula and connectivity but differ in 3-D arrangement; the only question that splits them is are the two mirror images?
Enantiomers: nonsuperimposable mirror images
Enantiomers are mirror-image pairs that won't superimpose — like your hands — with every stereocenter inverted ((2R,3R) ↔ (2S,3S)).
They have identical physical properties, differing only here:
- They rotate plane-polarized light equally in opposite directions ((+) vs (–)).
- They behave differently toward other chiral things — reagents, catalysts, receptors — so one drug enantiomer can heal while its mirror is inactive.
Diastereomers: stereoisomers that are NOT mirror images
Diastereomers have some centers the same and at least one different — e.g. (2R,3R) vs (2R,3S).
Being genuinely different compounds, they have different physical properties (mp, bp, solubility, NMR), so ordinary distillation, crystallization, or chromatography separates them — unlike enantiomers.
Cis/trans and E/Z isomers count too: cis- and trans-2-butene are diastereomers.
Counting stereoisomers: the 2ⁿ rule
With n stereocenters there are up to 2n isomers: for two centers, (R,R)/(S,S) are one enantiomer pair, (R,S)/(S,R) another, and across the pairs they are diastereomers (fewer if a meso compound exists).
A quick decision method
- Confirm same connectivity — otherwise they're constitutional isomers.
- Assign R/S to every stereocenter in both.
- All centers inverted → enantiomers.
- Only some differ → diastereomers.
- Every center identical → same compound.
Worked example
- Enantiomers = non-superimposable mirror images; identical physical properties except optical rotation (and behaviour in chiral environments).
- Diastereomers = stereoisomers that are not mirror images; they have different physical properties (mp, solubility, etc.).
- A meso form is a diastereomer of the chiral (±) pair.
Answer. Mirror images → enantiomers; non-mirror-image stereoisomers → diastereomers.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.