Isomers have the same molecular formula. Constitutional isomers have different arrangements of the atoms. Stereoisomers are molecules with the same atoms and connectivity, but different spatial arrangement. Stereochemistry is the study of how atoms in a molecule are arranged in space.
Chiral and Achiral
A chiral object is not superimposable on its mirror image. Objects that are superimposible on their mirror images are called achiral objects. The achiral objects contain at least one plane of symmetry. Carbons with four different groups attached are chiral carbons. They are not superimposible on their mirror images. Chiral molecules contain one or more chiral centers.
A molecule and its mirror image are called enantiomers. They contain chiral carbons and are not superimposable.
When plane-polarized light is passed through a solution containing one enantiomer, it is rotated to a certain degree. A dextrorotatory (+) enantiomer gives a positive optical rotation. A levorotatory (-) enantiomer gives a negative optical rotation
R and S
R and S configurations can be assigned to chiral centers. R is rictus meaning clockwise and S is sinister meaning counterclockwise. Enantiomers are assigned either R or S based on the spatial arrangement of the groups on the stereogenic carbon. If one enantiomer is R, it=s mirror image will be S. The method of naming R and S involves determining the priority of the groups bound to carbon. Priority is assigned to the different groups on carbon. The higher atomic mass/number has the higher priority. More branching (i.e. iPr vs. Pr) is a higher priority. Orient the molecule so that the lowest priority group is facing away from you. Rotate molecule so priority 4 group is in back. Trace from priority 1 group through 2 to 3. If it is clockwise, assign R configuration. If counterclockwise, assign S.
Molecules with more than one stereocenter give rise to non-enantiomers called diastereomers. Diastereomers are non-mirror image stereoisomers. n chiral centers give rise to 2n sets of stereoisomers.
In Fischer projections, horizontal bonds are in front of the plane and vertical bonds are behind the plane.
Meso molecules have more than one stereogenic or chiral center and are achiral (they give a superimposable mirror image). Meso compounds can be recognized by containing a mirror plane of symmetry and each chiral center is the mirror image of the other.