Organic 7: Optical Isomerism
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What is optical isomerism?
A form of stereoisomerism Where there are four different substituents attached to one carbon atom Results in two isomers that are non-superimposable mirror images of each other But are not identical Each of the two enantiomers rotate a plane of polarised light in opposite directions
What is chirality?
A molecule can have a chiral centre, or an asymmetric carbon atom, if it has four different substituents attached Usually indicated by an * All amino acids except glycine have a chiral centre
How is optical rotation measured?
Using a polarimeter: 1) Polarised light (has been passed through a polaroid) passes through two solutions of the same concentration each containing a different optical isomer of the same substance 2) The (+) -isomer rotates the plane of polarisation clockwise 3) The (-) -isomer rotates the plane of polarisation anticlockwise
What is a racemic mixture?
A 50:50 mixture of two optical isomers The rotational effects of each of the two enantiomers cancel out So plane-polarised light is not rotated
Why are racemic mixtures formed?
E.g. in nucleophilic substitution, the cyanide ions have a 50/50 probability of adding above or below the plane So an equal amount of each enantiomer is formed
What properties do optical isomers share?
Solubility Melting points Boiling points (Only differ by rotation of plane-polarised light)
Why are racemic mixtures not optically active?
Because racemic mixtures are a 50/50 mixture of two enantiomers So the rotational effects in opposite directions cancel out Plane-polarised light is not rotated
Define the term “optically active.”
Capable of rotating plane-polarised light