chiral

Meaning: A molecule is chiral if it is not superimposable on its mirror image, like left and right hands. At MCAT level the usual cause is a stereocenter, typically an sp³ carbon bonded to four different groups. The molecule and its mirror image are enantiomers. Enantiomers have the same melting point, boiling point and solubility in achiral settings. They differ in the direction they rotate plane-polarized light (equal angles, opposite signs) and in how they interact with other chiral things, such as enzymes and receptors. A 50:50 mixture of enantiomers (racemic mixture) shows no net rotation.

Equation: maximum number of stereoisomers = 2ⁿ, where n is the number of stereocenters (a dimensionless count). The true number is lower when a meso compound is possible.

On the MCAT: You may be asked to find the stereocenters in a drawn structure, assign R or S, and decide whether two structures are identical, enantiomers or diastereomers. Inverting every stereocenter gives the enantiomer. Inverting only some gives a diastereomer. Passages also link chirality to enzyme specificity and to SN2 inversion or racemization in SN1.

Common confusion: A stereocenter does not guarantee a chiral molecule. A meso compound has stereocenters but an internal mirror plane, so it is achiral and optically inactive. Also, R or S does not tell you whether a compound rotates light (+) or (−). Only measurement shows that.