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A molecule is chiral if it is non-superimposable on its mirror image; the most common source is a tetrahedral carbon bonded to four different groups (a stereocenter). Enantiomers are pairs of chiral molecules that are non-superimposable mirror images. The Cahn–Ingold–Prelog (CIP) rules assign R or S configuration to each stereocenter: rank the four substituents by atomic number rules, orient the lowest-priority group away, and read 1→2→3 clockwise (R) or counterclockwise (S). Enantiomers have identical physical properties except for opposite rotations of plane-polarized light and different interactions with other chiral environments (e.g., enzymes).
Practice CIP ranking on simple cases before complex ones. Use 3D models to confirm assignments. Work through examples where the stereocenter is inside a ring or bears isotopic substituents to stress-test your understanding of the priority rules.
Chirality is a geometric property: a molecule is chiral if it cannot be superimposed on its own mirror image — just as a left hand and a right hand are mirror images but cannot be overlaid. The most common source of chirality in organic molecules is a tetrahedral carbon bonded to four different groups, called a stereocenter (or chiral center). When such a carbon exists, the molecule and its mirror image are non-superimposable: they are a pair of enantiomers. If all four groups were identical — or even if just two were the same — the mirror image would be superimposable, and no chirality would exist.
The Cahn–Ingold–Prelog (CIP) system provides a rigorous method for naming the configuration at each stereocenter. The procedure has three steps: (1) rank the four substituents by atomic number (higher atomic number = higher priority; break ties by going to the next atom out), (2) orient the molecule so the lowest-priority group (group 4) points away from you, and (3) read the remaining three groups from highest to lowest priority (1→2→3). Clockwise rotation is R (from Latin rectus, right); counterclockwise is S (sinister, left). The most common error is forgetting step 2 — if group 4 is pointing toward you, you must invert your conclusion.
A critical conceptual trap: the R/S designation and the (+)/(−) optical rotation are two completely separate systems. R/S is assigned by priority rules. Optical rotation is measured experimentally — you shine plane-polarized light through a sample and observe which direction it rotates. There is no reliable way to predict the sign of optical rotation from the R/S designation alone. Many students assume R means (+), but this is false. You must know the specific molecule to know its optical rotation.
Enantiomers are nearly chemically identical in achiral environments: same melting point, boiling point, solubility, and reactivity with achiral reagents. The difference emerges in chiral environments — especially in biology, where enzymes are chiral and interact differently with each enantiomer. Many drugs are chiral, and often only one enantiomer is biologically active; the other may be inactive or even harmful. This is why the pharmaceutical industry cares enormously about stereochemical purity.
Finally, having stereocenters does not guarantee chirality. Meso compounds have two or more stereocenters but contain an internal plane of symmetry that makes the molecule superimposable on its mirror image. Meso-tartaric acid (R at one carbon, S at the other) is the textbook example. Understanding meso compounds drives home the point that chirality is a property of the whole molecule, not just the individual stereocenters.