Optical isomerism

What type of isomerism is optical isomerism?

Stereoisomerism

What is stereoisomerism?

Same structural formula but different arrangements of atoms in space

Give 2 features of optical isomers

Mirror images of each other

Have a chiral carbon atom

What is a chiral molecule?

Has 4 different groups attached to a carbon atom

We can arrange these groups in 2 different ways which forms 2 different molecules - enantiomers

Give 2 features of enantiomers

Mirror images of eachother

Non-superimposable (won’t overlap)

How can you identify an optically active compound?

It will rotate plane polarised light

How do optical isomers differ in how they rotate plane polarised light?

One enantiomer rotates light clockwise (dextrorotatory), the other will rotate it anticlockwise (laevorotatory)

How is light polarised?

By passing it through a polaroid filter so oscillations are only in one direction

What are polarimeters used for?

Allows you to identify which enantiomer is present by detecting optical activity and measuring the angle and direction of rotation

What is a racemic mixture?

When we have an equal amount of each enantiomer

Why are racemic mixtures formed?

Attack by reagents occurs equally from both sides of a planar intermediate, forming equal amounts of the 2 enantiomers

Do racemates rotate plane polarised light? Why?

No - the 2 enantiomers are equal in proportions and rotate light in opposite directions so they cancel out (net rotation=0)

How can you make a racemic mixture of a chiral product?

By reacting achiral substances together

Which molecules can make racemic products?

Molecules with planar profiles such as double bonds in C=C and C=O, as they can be attacked from either above or below forming 2 different enantiomers

Why is optical isomerism a problem for the drug industry?

• Biological systems (enzymes, receptors, DNA) are chiral so they often only recognise one enantiomer

• This means that only one enantiomer is effective

• The other may be inactive, less effective or even harmful

• It is hard to separate enantiomers

What are the options to resolve the issue of only one enantiomer being effective in the drug industry?

• Separate enantiomers - difficult and expensive

• Use alternative synthesis route that produces one enantiomer

Give 2 examples of optically active drugs

Ibuprofen

Thalidomide

Why is ibuprofen able to be sold as a racemate, even though only one isomer (S enantiomer) is needed to treat inflammation?

Due to chiral inversion - the body converts around half of the inactive R enantiomer into the S enantiomer