3.3 Advanced Organic Chemistry · Year 13
3.3.7 Optical Isomerism
Recognise chirality and explain why enantiomers behave differently in chiral environments.
What you need to know
Open a line for a quick recap. If it feels obvious, move straight to the linked practice.
3.3.7 Recognise optical isomerism as stereoisomerism caused by chirality; AQA limits this point to molecules with one chiral centre. Quick revision
For the one-chiral-centre cases AQA uses, look for a tetrahedral carbon bonded to four different groups. If you have that arrangement, you can get two mirror-image forms that cannot be superimposed.
A carbon with two identical substituents is not chiral. Mark the four attachments explicitly before deciding that a centre gives optical isomerism.
3.3.7 Describe enantiomers as non-superimposable mirror-image molecules arising from a chiral carbon and having opposite effects on plane-polarised light. Quick revision
Enantiomers are non-superimposable mirror images. In an achiral environment they have essentially the same ordinary physical properties, but they rotate plane-polarised light by equal amounts in opposite directions.
If you are checking a proposed pair, the molecular formula and connectivity should be identical. The difference is the three-dimensional arrangement at the chiral centre, and the two forms rotate plane-polarised light in opposite directions.
3.3.7 Recognise a 1:1 mixture of two enantiomers as a racemic mixture (racemate). Quick revision
If you have a 50:50 mixture of the two enantiomers, you have a racemic mixture. Their rotations of plane-polarised light are equal and opposite, so the effects cancel and the mixture is optically inactive overall.
Optically inactive does not mean the molecules are achiral. A racemate can contain two fully chiral enantiomers whose effects simply cancel.
3.3.7 Draw displayed/structural representations that clearly show a pair of enantiomers. Quick revision
To draw an enantiomeric pair, keep the same connectivity and reflect the three-dimensional arrangement at the chiral carbon. Wedge/dash bonds are useful because they make the opposite spatial arrangements explicit.
Check that the two drawings really are mirror images and that the central carbon still has the same four groups. Rotating one drawing on the page does not make a new enantiomer.
3.3.7 Explain why common achiral synthesis of a single-chiral-centre product can give a racemate and why the mixture shows no net optical rotation. Quick revision
If an achiral reagent attacks a planar intermediate or functional group equally from either face and creates one chiral centre, the two enantiomers form in equal amounts. The product is then a racemate and shows no net optical rotation.
Picture the two faces of the planar site as equivalent in an achiral environment. Attack from either face is equally likely, so neither enantiomer is favoured and the 1:1 mixture has no net rotation.