Stereochemistry
Also known as: 3d chemistry
Stereochemistry is the study of how atoms are arranged in three-dimensional space within molecules and how that arrangement affects chemical and physical properties. Two molecules with identical connectivity but different spatial arrangements are stereoisomers, and they can behave very differently.
Isomers are compounds with the same molecular formula. Constitutional isomers differ in which atoms are bonded to which; stereoisomers have identical connectivity and differ only in how the atoms are oriented in space. Stereochemistry is the branch of chemistry concerned with that second kind of difference.
Most stereochemistry begins with chirality. A molecule is chiral if it is not superimposable on its mirror image, most commonly because it contains a stereocenter — a carbon bonded to four different groups. A chiral molecule and its mirror image are enantiomers; stereoisomers that are not mirror images are diastereomers. A molecule with n stereocenters has at most 2ⁿ stereoisomers, fewer when an internal mirror plane creates an achiral meso compound. Configuration at each stereocenter is assigned R or S by the Cahn-Ingold-Prelog rules: rank the substituents by atomic number, point the lowest-priority group away, and read whether the remaining three descend clockwise (R) or counterclockwise (S).
Restricted rotation produces a second family of stereoisomers. Because a carbon-carbon double bond cannot rotate freely, substituents are locked on the same side (cis) or opposite sides (trans), described more rigorously by the E/Z system, which again applies CIP priorities. Conformational isomers — staggered versus eclipsed, chair versus boat — differ only by rotation about single bonds and interconvert freely, so they are not separable stereoisomers.
The differences are not cosmetic. Enantiomers have identical melting points and solubilities in achiral environments but rotate plane-polarized light in equal and opposite directions, which is why chirality is measured by optical activity; a 50:50 racemic mixture shows no net rotation. Diastereomers have genuinely different physical properties and can be separated by ordinary means. In biological systems, where enzymes and receptors are themselves chiral, two enantiomers of a drug can differ sharply in potency and toxicity. The MCAT tests stereochemistry across general chemistry, organic chemistry, and biochemistry — expect to assign R and S configurations, classify a pair of structures as enantiomers, diastereomers, or identical, and connect chirality to amino acids and sugars.
Key takeaways
- Stereochemistry studies the three-dimensional arrangement of atoms in molecules with identical connectivity.
- A chiral molecule is not superimposable on its mirror image; a carbon with four different substituents is a stereocenter.
- Enantiomers are non-superimposable mirror images; diastereomers are stereoisomers that are not mirror images.
- Configurations are assigned R or S using Cahn-Ingold-Prelog priority rules; double bonds are described as cis/trans or E/Z.
- Enantiomers share physical properties but rotate polarized light oppositely and can differ greatly in biological activity.