naming and isomerism

• stereoisomers have the same structural formula but different arrangement of atoms in space
• types of stereoisomerism are E-Z and optical isomerism
• optical isomerism occurs in carbon compounds with 4 different functional groups attached to a carbon

  this is a chiral (asymmetrical) carbon
• optical isomer is non superimposable mirror images
• two compounds that are optical isomers of eachother are called enantiomers
• a mixture containing a 50/50 mixture of two isomers (enantiomers) is described as being a racemate or racemic mixture
• optical isomers have similar physical and chemical properties
• you can distinguish between enantiomers as they rotate plane polarised light in different directions

  -ve enantiomer rotates anticlockwise

  +ve enantiomer rotates clockwise
• racemic mixture will not rotate plane polarised light
• racemate formed in reaction mechanism where a reactant or intermediate has trigonal planar group and is equally likely to be attacked from either side by nucleophile

  this can be seen in nucleophilic addition of aldehyde and ketones
• racemate can also be formed in electrophilic addition of HBr with unsymmetrical alkene
• drug action may be determined by stereochemistry of molecule

  different optical isomers can have very different effects
• one enantiomer of thalidomide causes birth defects in unborn children

  other had useful sedative properties