Optical Isomerism

Optical isomers definition

Non-superimposable mirror images

What compounds express optical isomerism?

Compounds which contain a carbon atom which is bonded to 4 different groups. The asymmetric C is a chiral carbon

What are optical isomers also called?

Enantiomers - they have the same chemical properties as they have the same functional groups

What is the way that enantiomers differ?

• They differ in their effect on plane polarised light

• Optical isomers rotate plane polarised light (optically active) by an equal amount but in opposite directions

How to draw enantiomers?

Draw out the chiral carbon with tetrahedral shape of 4 different groups attached (if any of 4 groups are the same it does not exhibit optical isomerism)

Draw the mirror image of the structure you have drawn

Racemic mixture definition

A mixture of equal amounts of the two enantiomers

Why is a racemic mixture not optically active (what does it mean to be optically active?)

Optically active : the molecule will rotate plane polarised light

Racemic mixtures not optically active as one enantiomer rotates plane polarised light in one direction and the other rotates it in an equal amount in the other direction - the two cancel out

Why is a product formed in a reaction often not optically active?

• __ can attack the trigonal planar carbon from above or below with equal probability

• This leads to equal amounts of the two enantiomers

• This gives a racemic mixture

• The racemic mixture will not be optically active