enantiomers

Meaning: Enantiomers are stereoisomers that are non-superimposable mirror images. Each stereocenter is inverted, so R becomes S and S becomes R. They share melting point, boiling point, density and solubility in achiral solvents. They differ in rotating plane-polarized light by equal amounts in opposite directions, and in how they interact with chiral molecules such as enzymes.

Equation: [α] = α / (l × c). [α] is specific rotation, α is observed rotation (degrees), l is path length (dm; 1 dm = 0.1 m), and c is concentration (g/mL). Enantiomers have [α] of equal size and opposite sign.

On the MCAT: Expect questions that ask you to pick the enantiomer of a drawn structure, assign R/S, or predict that a 50:50 racemic mixture shows no net rotation. Passages may show an enzyme acting on one enantiomer selectively, or a separation that needs a chiral environment.

Common confusion: Inverting only some stereocenters gives diastereomers, which do differ in physical properties. R/S does not tell you (+) or (−); the sign of rotation must be measured. Meso compounds are superimposable on their mirror images, so they have no enantiomer.