STEREOCHEMISTRY

Stereochemistry involves the study of the relative spatial arrangement of atoms within molecules. It is also known as 3D chemistry because the prefix "stereo-" means "three-dimensionality.

Some basic concepts are;

Isomers are molecules that have the same molecular formula but different arrangements of their constituent atoms.
Stereoisomers are molecules with identical connectivity but different spatial arrangements of their constituent atoms that cannot be interconverted by bond rotation. 

In order to categorise stereoisomers it is necessary to prioritise different atomic substituents using the Cahn–Ingold and Prelog sequence rules.

Chirality (cheir, Greek for "hand") refers to objects which are related as non–superimposable mirror images and the term derives from the fact that left and right hands are examples of chiral objects.

An enantiomer is one of a pair of stereoisomers that are related as non–superimposable mirror images.
A solution of a single enantiomer will rotate the plane of plane–polarised light and is referred to as optically active; although this physical property cannot be directly related to absolute configuration of the molecule. An enantiomer is given the prefix (+)– if the rotation is clockwise (dextrorotatory) and (–)– if the rotation is anticlockwise (levorotatory).

An equal mixture of opposite enantiomers is a racemate and solutions of racemic mixtures do not rotate the plane of plane–polarized light.

Enantiomeric Excess = (%Enantiomer A –% Enantiomer B)% 


A chiral molecule is a type of molecule that lacks an internal plane of symmetry and has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom.

Diastereoisomers are stereoisomers with a different relative configuration and are not related as mirror images. They have different chemical and physical properties.

Cis-trans isomerism or geometric isomerism or configuration isomerism or E-Z isomerism is a form of stereoisomerism describing the orientation of functional groups within a molecule. In general, such isomers contain double bonds, which cannot rotate, but they can also arise from ring structures, wherein the rotation of bonds is greatly restricted.