mirror image

Meaning: Reflect a molecule across a plane and you get its mirror image. If you cannot superimpose the reflection on the original, however you rotate it or twist single bonds, the molecule is chiral and the two forms are enantiomers. Chirality usually comes from a stereocenter, often a carbon bonded to four different groups. An enantiomer has the opposite configuration (R becomes S) at every stereocenter. If a molecule can be superimposed on its mirror image, it is achiral.

On the MCAT: You are given two drawn structures, often with wedges, dashes or Fischer projections, and asked whether they are enantiomers, diastereomers or identical. Enantiomers share melting point, boiling point and solubility in achiral solvents. They rotate plane-polarized light by equal amounts in opposite directions, and they can interact differently with other chiral molecules such as enzymes. A racemic mixture, with equal amounts of both enantiomers, shows no net rotation.

Common confusion: Having stereocenters does not guarantee a nonsuperimposable mirror image. A meso compound has stereocenters and an internal mirror plane, so it is achiral and optically inactive. Also, inverting only one of several stereocenters gives a diastereomer, not the mirror image. Finally, R and S labels do not tell you whether a compound rotates light clockwise (+) or counterclockwise (−). That must be measured.