A level Chemistry · Edexcel 9CH0 · Topic 17A
Chirality
Key points
- A chiral centre is a carbon atom with four different groups attached; the molecule is not superimposable on its mirror image.
- The two mirror-image forms are enantiomers (optical isomers); draw them as 3D structures reflected in a mirror line.
- Enantiomers rotate plane-polarised light by equal amounts in opposite directions; use a polarimeter to tell them apart.
- A racemic mixture contains equal amounts of both and has no overall effect on plane-polarised light.
- SN1 reactions give a racemic mixture because the planar carbocation can be attacked from either side; SN2 inverts configuration, so a single enantiomer gives a single optically active product.
- Nucleophilic addition to an aldehyde or unsymmetrical ketone with HCN forms a racemic mixture because the C=O is planar.
- Many drugs are chiral; often only one enantiomer is effective, which is why pure enantiomers are made.
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Some molecules exist as two forms that are mirror images of each other but cannot be superimposed, like a left and right hand. These optical isomers have identical chemical and physical properties except for how they affect polarised light…
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