3 - sterochemistry

• isomers are molecules that have the same molecular formula but a different structural formula
• there are two ways atoms can be arranged in isomers

  1. structural isomers
  2. stereoisomers
• structural isomers

  the atoms are bonded together in a different order
• stereoisomers

  the atoms are bonded in the same order but the arrangement of the atoms in space is different
• there are two different types of stereoisomers

  1. geometric isomers
  2. optical isomers
• geometric isomers must contain a carbon to carbon double bond, which is why alkenes exhibit this type of isomerism

  the pi bond restricts rotation within the molecule
• geometric isomers can either be 'cis' or 'trans'

  cis means the substituent groups are on the same side of the carbon to carbon double bond

  trans means that these groups are on opposite sides of the double bond
• geometric isomers have different physical and chemical properties, e.g. the melting point and boiling point
• geometric isomers can only occur in organic molecules that have two different groups attached to each of the carbon atoms of the double bond
• geometric isomerism can also be found in saturated rings where rotation around the C-C bond is restricted
• optical isomers are asymmetrical, have a chiral carbon centre and have non-superimposable mirror images
• chiral molecules are those that have 4 different atoms bonded to it
• chirality arises from a lack of symmetry
• optical isomers are identical in their physical properties except the opposite rotation of polarised light
• if you have a mixture of equimolar optical isomers it would have no effect on plane-polarised light since the rotational effect on one isomer would be cancelled out by the opposite rotational effect of the other

  this mix is optically inactive and is known as a racemic mix