Stereochemistry

Some compounds differ only in their shape or orientation in space. Compounds that have the same molecular formula are called isomers. Stereoisomers are isomers (i.e., they have the same molecular weight and formula) but that differ in their orientation in space. No matter how a stereoisomer is rotated it presents a different picture than its stereoisomer counterpart. Most importantly, stereoisomers are not superimposable.

Enantiomers are stereoisomers that are mirror images, that is, they can map onto one another (if the molecules were two dimensional we would say that the molecules, just like human hands, could not be laid on top or superimposed upon each other.

Stereoisomers that rotate polarized light are called optical isomers. With the help of an instrument called a polarimeter, molecules are assigned a sign or rotation, either (+) for dextrorotatory molecules that rotate a plane of polarized light to the right, or (-) for levorotatory molecules that rotate a plane of polarized light to the left. Enantiomers differ in the direction that they rotate a plane of polarized light and in the rate that they react with other chiral molecules. Racemic mixtures of compounds contain equal amounts of enantiomers.