Stereoisomerism

Stereoisomerism is defined as two molecules which have the same molecular formula but the atoms are arranged differently in space.  This is due to the restricted rotation of the carbon carbon double bond. There are two main types of stereoisomerism- Cis and Trans. Cis (or Z) isomers (Z standing for Zusammen, the German word for together) occurs when the same groups or groups of a high priority are both found on the same side of the restricted double bond. Trans (or E) isomers (E standing for Entgegen, the German word for opposite) occurs when the same groups or groups of a high priority are across the double bond and are so found on opposite sides.  

The other major form of stereoisomerism is optical isomerism.  Optical isomers arise when a chrial carbon is present, this is a carbon atom attached to four different groups.  Optical isomers are non superimposable mirror images.  They can be distinguised using plane polarised light, one enantiomer will polarise the light in a clockwise direction (known as the + form) and the other in an anti-clockwise direction (known as the - form).  Ensuring the correct enantiomer is present is critical in pharmaceuticals.  It was the presence of the incorrect enantiomer in Thalidomide that caused deformaties in the foetus to develop.