Optical Isomerism

2 types of isomerism

• Structural

• Stereoisomerism

Define structural isomerism

• Occurs when molecules have the same molecular formula but different structural formula

3 types of structural isomerism and their definitions

• Chain→ Different organisation of carbon chain

• Position→ Functional group in different position

• Functional→ Functional group is different

Define stereoisomerism

• occurs when molecules have the same structural formula but different spatial arrangement of atoms.

2 types of stereoisomerism and their definitions

• E/Z isomers (geometrical)→ Double bond has restricted rotation

• Optical isomerism→ Non-superimposable mirror images

What is required for a molecule to show optical isomerism?

• A chiral/asymmetric carbon

• I.e. A carbon atom that has 4 different groups attached

What is meant by optically active?

• A substance/sample is optically active if it rotates plane polarised light anticlockwise or clockwise

How can optical activity be detected in a sample?

• Plane polarised light

• Optically active samples will rotate plane polarised light

What is a racemic mixture?

• A mixture where 50% of the enantiomers have rotated the plane polarised light clockwise

• And the other 50% have rotated it anti clockwise

• Racemic mixtures are NOT optically active

• Because the effects of both enantiomers have been cancelled out

• (A racemic mixture does contain optical isomers but the mixture it self is not optically active)

1 way racemic mixtures form

• Nucleophilic addition to carbonyls

• The carbon of the carbonyl group becomes the chiral centre

What is meant by a carbonyl group ?

• Carbonyl group contains a carbon atom double bonded to oxygen atom

How does nucleophilic addition to carbonyls make racemic mixtures?

• Carbonyl group is planar (flat)

• Nucleophile can attack carbon from either side (top or bottom) with equal probability

• Producing equal amounts of each enantiomer

2nd way a racemic mixture can be formed

• Electrophilic addition

How does electrophilic addition produce a racemic mixture?

• The intermediate carbocation carbon has 3 bonds around it

• Making the molecule planar

• Allowing the nucleophile to attack from either side with equal probability

• Producing equal amounts of each enantiomer

Why are optical isomers important?

• Can have different biological properties due to different spatial arrangements

Example of a molecule that is an optical isomer and the enantiomers have differences in smell

• Carvone

• 1 enantiomer smells like mint

• The other of caraway seeds

Example of an optical isomer where the enantiomers have different effects?

• Thalidomide

• 1 enantiomer alleviates nausea

• The other causes birth defect

• Important to use enantiomerically pure drugs→ Drug which is 100% 1 enantiomer to give the desired effect

What’s the significance of enzymes also having enantiomers?

• Enantiomers of enzymes have different active sites specific to different substrates

How would you decipher if a molecule has a chiral centre from its formula e.g. C6H12

• Assume it has 4 different groups attached and draw them out

• (Check notes for additional guidance)