L7 - Optical Isomerism

objectives

What are Optical Isomers?

• another form of stereoisomerism
• occurs when compounds have non-superimposable mirror images
• They form when Carbons have 4 DIFFERENT GROUPS attached to them
• These Carbons are called CHIRAL CARBONS and can also be called CHIRAL CENTRES
• Chiral Carbons are termed Asymmetric

Spotting Chiral Centres

• Look at each carbon atom in the chain and see what is attached to it.
• For a chiral centre you need an asymmetric carbon with four different atoms/groups) arranged tetrahedrally around it.
• IF A CARBON HAS MORE THAN ONE OF ANY ATOM/GROUP ATTACHED, IT CAN’T BE CHIRAL

Spatial differences between isomers

• two forms exist which are NON-SUPERIMPOSABLE MIRROR IMAGES of each other
• non-superimposable means you you can’t stack one form exactly on top of the other

• Some common objects are:
• mirror images and superimposable spoons
• superimposable but not mirror images books
• non-superimposable mirror images hands

• NB For optical isomerism in molecules, both conditions must apply...

they must be mirror images AND be non-superimposable

OPTICAL ISOMERS - DIFFERENCE

• Optical isomers differ in their reaction to plane-polarised light
• plane polarised light vibrates in one direction only
• one isomer rotates light to the right, the other to the left
• rotation of light is measured using a polarimeter
• rotation is measured by observing the polarised light coming out towards the observer

Racemate or a Racemic Mixture

• a 50-50 mixture of the two enantiomers (dl) or (±) is a racemic mixture.
• The opposite optical effects of each isomer cancel each other out
• A 50-50 mixture us the same as an equimolar mixture

Examples of Optical Activity

• Optical activity is common in biochemistry and pharmaceuticals
• Most amino acids exhibit optical activity
• many drugs must be made of one optical isomer to be effective:

- need smaller doses (safer and cost effective)

- get reduced side effects

- improved pharmacological activity

The Polarimeter

Used to measure optical activity

A Light source produces light vibrating in all directions

B Polarising filter only allows through light vibrating in one direction

C Plane polarised light passes through sample

D If substance is optically active it rotates the plane polarised light

E Analysing filter is turned so that light reaches a maximum

F Direction of rotation is measured coming towards the observer

Producing Optical Isomers - the Walden Inversion

• In SN2 reactions the nucleophile can attack from above or below the plane
• This can give rise to a racemic mixture of optical isomers
• The Walden Inversion is a name given to the ability of a chiral carbon undergoing SN2 reactions
• In these reactions:
  • A chiral carbon reacts with the nucleophile
  • This chiral carbon will be one enantiomer and so rotate plane polarised light in one direction
  • After SN2 reaction an enantiomer that rotates plane polarised light in the opposite direction will be produced
  • As the chiral carbon on the enantiomer has inverted this called an inversion reaction.