resolution of enantiomers
You have a jar with equal amounts of two mirror-image molecules mixed together, and you want just one of them, pure. The frustrating catch is that enantiomers share almost every physical property — same boiling point, same solubility — so ordinary distillation and crystallization cannot tell them apart. Pulling them into separate pots is called resolution, and it requires a clever trick.
The classic trick is to recruit a chiral helper. You react the racemic mixture with a single pure enantiomer of some chiral reagent (a 'resolving agent'). Because the helper is itself one-handed, the two products are no longer mirror images of each other — they are diastereomers, which DO have different physical properties. Now ordinary separation works: you crystallize or chromatograph the diastereomers apart, then chemically remove the helper to recover each pure enantiomer. Modern alternatives skip the chemistry: chiral chromatography columns and chiral solvents act as a one-handed environment that grips the two enantiomers slightly differently, or one can simply synthesize a single enantiomer from the start using a chiral catalyst.
Resolution matters enormously in the drug industry, where regulators may require a single pure enantiomer because the other is inactive or harmful. It is also a chemist's everyday reminder of why diastereomers are precious: turning an impossible separation (enantiomers) into an easy one (diastereomers) by temporarily borrowing handedness is one of the most elegant moves in practical stereochemistry.
To resolve a racemic carboxylic acid, react it with a single enantiomer of a chiral amine to form two diastereomeric salts. These salts have different solubilities, so one crystallizes out preferentially; filtering, then freeing the acid with strong acid, gives one pure enantiomer. Pasteur's hand-sorting of tartrate crystals in 1848 was the very first resolution.
Borrow a chiral helper to turn enantiomers into separable diastereomers.
Resolution does not create one enantiomer from nothing — at best you recover half the material as the wanted enantiomer and discard (or recycle) the other half. To get more than 50% yield you must instead make the single enantiomer selectively (asymmetric synthesis) rather than separate a racemate.