optical isomerism (3.3.7)
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what is optical isomerism?
a form of stereoisomerism
why does optical isomerism occur?
as a result of chirality in molecules
what is a chiral carbon atom?
one which is asymmetric (bonded to 4 different groups)
what do chiral carbon atoms do?
give rise to optical isomers (enantiomers) which exist as non super-imposable mirror images and differ in their effect on plane polarised light
what are enantiomers?
two compounds that are optical isomers of each other
what is plane polarised light?
light that travels in one direction
what is a racemic mixture/racemate?
a mixture of equal amounts of enantiomers (50/50)
what comparisons can be made between optical isomers?
• they have identical physical and chemical properties • they differ in their smells, tastes and effects on plane polarised light
how can separate samples of two optical isomers for a particular molecule be distinguished from each other?
• when plane polarised light passes through enantiomer, the light will be rotated clockwise and the other enantiomer will rotate the light by the same angle anticlockwise • the angle of rotation can be detected by passing the rotated light through a rotatable polaroid that is then measured by an observer
what will the effect of a racemic mixture be when distinguishing separate sample of a particular molecule from each other?
• a racemic mixture is a 50:50 mixture of the two optical isomers of a molecule and it will have no effect on plane polarised light • the clockwise rotation caused by one enantiomer will be cancelled out by the anticlockwise rotation caused by the other enantiomer
what is an advantage of using a racemic mixture in medicine?
it is cheaper as there is no cost of separating two optical isomers from each other
what are disadvantages of using a racemic mixture in medicine?
• one of the enantiomers may be ineffective so you would need to take double the dose to have the same effect • one of the enantiomers may have negative side effects