AQA A Level Chemistry
AQA A Level Chemistry 3.3.7 Optical Isomerism Exam Questions
Free AQA A Level Chemistry exam questions for 3.3.7 Optical Isomerism, with a mark scheme for every pack. Covers chiral centres, enantiomers, plane-polarised light, racemic mixtures and formation of racemates through planar intermediates.
3.3.7 Optical Isomerism exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA A Level Chemistry 3.3.7 Optical Isomerism
Optical isomerism is a form of stereoisomerism studied in the A Level part of the AQA organic course. You need to identify chiral centres, draw pairs of enantiomers in 3D, explain how enantiomers affect plane-polarised light, and explain why some reactions produce a racemic mixture. The most important example is nucleophilic addition of cyanide to an aldehyde or unsymmetrical ketone, where the planar carbonyl group can be attacked from either side. This topic links to 3.3.1 isomerism, 3.3.8 Aldehydes and Ketones and 3.3.13 Amino Acids. Examiners test identification of chiral centres, clear 3D drawings and explanations of optical inactivity.
What you need to know
- Chiral centre A carbon atom bonded to four different atoms or groups. Mark it with an asterisk on structures.
- Enantiomers Optical isomers are non-superimposable mirror images. Draw them using wedge and dashed bonds around a tetrahedral carbon, with one drawing the mirror image of the other.
- Plane-polarised light Enantiomers rotate the plane of plane-polarised light by the same amount in opposite directions. They have identical physical properties otherwise and react the same way with non-chiral reagents.
- Racemic mixture A 50:50 mixture of two enantiomers. It is optically inactive because the rotations cancel out.
- Formation of racemates When a nucleophile attacks a planar group, such as the trigonal planar C=O in an aldehyde or unsymmetrical ketone, attack from above and below is equally likely. This gives equal amounts of both enantiomers.
Common mistakes in 3.3.7 Optical Isomerism
- Counting a carbon with two identical groups, such as two CH3 groups or two H atoms, as a chiral centre.
- Drawing two structures that are not true mirror images, or drawing them flat without 3D bonds.
- Saying enantiomers 'rotate light'. They rotate the plane of plane-polarised light, in opposite directions.
- Stating that a racemic mixture contains no chiral molecules. It contains equal amounts of both enantiomers, so the overall rotation is zero.
- Forgetting to say that the carbonyl group is planar and that attack from either side is equally likely when explaining a racemic product.
Try three 3.3.7 Optical Isomerism exam-style questions
1. Identify the chiral centre in butan-2-ol and state what is meant by the term chiral centre. (2 marks)
Show the mark scheme answer
Carbon 2 (the carbon bonded to OH); a carbon atom bonded to four different atoms or groups.
2. Propanal reacts with KCN followed by dilute acid to form 2-hydroxybutanenitrile. Explain why the product is optically inactive. (3 marks)
Show the mark scheme answer
The C=O group in propanal is planar; the cyanide ion attacks from above and below with equal likelihood; a racemic mixture (equal amounts of each enantiomer) forms and the rotations cancel out.
3. Describe how you could show that a sample contains a single enantiomer rather than a racemic mixture. (2 marks)
Show the mark scheme answer
Pass plane-polarised light through the sample (polarimeter); a single enantiomer rotates the plane of polarisation, whereas a racemic mixture causes no overall rotation.
3.3.7 Optical Isomerism FAQs
What does AQA A Level Chemistry 3.3.7 Optical Isomerism cover?
It covers chiral centres, enantiomers and their 3D representation, the effect of enantiomers on plane-polarised light, racemic mixtures, and why reactions at planar groups form racemates.
Which paper is 3.3.7 Optical Isomerism examined in?
It is examined in Paper 2, and Paper 3 can also include it.
How should I draw optical isomers in the exam?
Draw the chiral carbon with two bonds in the plane of the paper, one wedge and one dashed bond, then draw its reflection with a mirror line between them. Make sure all four groups are attached to the correct atoms.
