Difference between absolute and relative stereochemistry
Answers
The precise arrangement of substituents at a stereogenic center is known as the absolute configuration of the molecule.
If we have a drawn structure, then we can use the R- and S- notation system to assign the absolute configuration of that structure.
It is a more significant challenge to be able to assign the absolute configuration to an actual sample of a molecule (i.e. a compound in a bottle).
This is can be accomplished by solving the x-ray crystal structure of a molecule (a method that is not always readily available), by spectroscopic methods, or possibly by inference based on chemical reactions of known specific stereochemistry involving a compound whose absolute configuration is known.
Relative configuration can be applied within the same molecule (i.e. intramolecularly such as cis- or trans-) or it can be applied to intermolecularly (i.e. a pair of enantiomers have opposite configurations).
