Stereochemistry

• Molecules that have the same molecular formula but different structural formulae are called isomers.
• Structural isomers occur when the atoms are bonded together in a different order in each isomer.
• Stereoisomers occur when the order of the bonding in the atoms is the same but the spatial arrangement of the atoms is different in each isomer.
• There are two types of stereoisomers:

  • Geometric
  • Optical
• Geometric isomers can occur when there is restricted rotation around a carbon-carbon double bond or a carbon-carbon single bond in a cyclic compound.
• Geometric isomers must have two different groups attached to each of the carbon atoms that make up the bond with restricted rotation
• Geometric isomers can be labelled cis or trans according to whether the substituent groups are on the same side (cis) or on different sides (trans) of the bond with restricted rotation.
• Geometric isomers have differences in physical properties, such as melting point and boiling point and can have differences in chemical properties.
• Optical isomers occur in compounds in which four different groups are arranged tetrahedrally around a central carbon atom (chiral carbon or chiral centre)
• Optical isomers are asymmetric
• Optical isomers can be described as enantiomers and are non-superimposable mirror images of each other.
• Optical isomers have identical physical properties, except for their effect on plane-polarised light, they also have identical chemical properties, except when in a chiral environment such as that found in biological systems (only one optical isomer is usually present).
• Optical isomers rotate plane-polarised light by the same amount but in opposite directions and so are optically active. When they are mixed in equal amounts they are optically inactive because the rotational effect of the plane-polarised light cancels out. This is called a racemic mixture.