Optical Isomerism (A2 Only)
Chemistry: AQA A Level, topic 6.1.1
Optical Isomerism
Optical isomerism is a form of stereoisomerism. Optical isomers (also called enantiomers) are non-superimposable mirror images of each other.
Non-superimposable mirror images
- Much like your left and right hands, some molecules have mirror images.
- These mirror images are non-superimposable upon each other.
- This means that they have a different spatial structure.
- No matter how you twist your hands, you can’t make a left hand into a right hand.
Chiral centres
- Molecules that have four different groups bonded to one carbon atom have two mirror images which are non-superimposable.
- The carbon atom with four different attached groups is called a chiral center. Chiral centers lead to chirality in molecules.
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Drawing chiral centres
- Dashed bonds indicate an atom below the plane of the paper.
- Thick wedge bonds indicate an atom above the plane of the paper.
- The different optical isomers of bromohydroxyacetonitrile are above.
- Try rotating them to make them the same - you can’t!
Drawing Enantiomers
You need to be able to draw the enantiomers for chiral molecules.
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Step 1 - Find chiral centre
- The chiral centre is at the carbon atom that is bonded to four different atoms or groups of atoms.
- The molecule shown above has a chiral centre at the carbon atom which is circled.
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Step 2 - Draw one enantionmer
- Draw the four different atoms or groups of atoms in any order around the chiral centre.
- Wedged line - shows the bond that's coming out of the paper.
- Dotted line - shows the bond that's going beneath the paper.
- Normal filled lines - show the bonds that are in the plane of the paper.
Step 3 - Draw other enantiomer
- Draw a dotted line to the right of the first enantiomer.
- Draw the mirror image on the other side of the dotted line.
- You should now have two molecules on either side of the dotted line.
- The two molecules will be non-superimposable and they are enantiomers of each other!
Plane-Polarised Light
Optical isomers are called 'optical' because of how they interact with polarised light.
Plane-polarised light
- Light has a property called polarisation.
- If light is plane-polarised, this means that all the light waves have the same polarisation.
- You can think of plane-polarised light like the edge of a piece of paper held in the air. It can be horizontal, or vertical, or anything in between. Light is the same.
Optical activity of enantiomers
- Enantiomers will rotate plane-polarised light in different directions:
- Light will be rotated in an anticlockwise direction by one enantiomer.
- Light will be rotated in a clockwise direction by the other enantiomer.
Racemates
- Some reactions can produce one specific enantiomer, others will produce an equal mixture of enantiomers.
- An equal mixture of enantiomers is called a racemate.
- Racemates are optically inactive because the different optical activities of each enantiomer exactly cancel each other out.
Test yourself on Optical Isomerism (A2 Only)
Optical isomers are most like which body part?
If you draw the mirror image of a molecule with 0 chiral centres, how are the two molecules related?
How many enantiomers does a molecule with one chiral centre have?
The Process of Drawing Enantiomers
What is a unique feature of enantiomers?