OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.3.7 Optical Isomerism Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.3.7 Optical Isomerism: what you need to know, common mistakes and exam-style questions with answers. Covers chiral centres, enantiomers, plane-polarised light, racemic mixtures and why they are optically inactive.
Practise 3.3.7 Optical Isomerism with past papers
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About OxfordAQA International A Level Chemistry 3.3.7 Optical Isomerism
Optical isomerism is a short A2 organic topic examined in Unit 4. You need to identify chiral centres, which are carbon atoms bonded to four different atoms or groups, and draw the two enantiomers as non-superimposable mirror images using 3D representations. You need to know that enantiomers rotate plane-polarised light in opposite directions, and explain why a racemic mixture (equal amounts of each enantiomer) is optically inactive. The topic links to aldehydes and ketones, where nucleophilic addition of cyanide produces racemic mixtures, to amino acids and to transition metal complexes, which can also show optical isomerism.
What you need to know
- Chiral centre A chiral centre is a carbon atom bonded to four different atoms or groups. Molecules with one chiral centre exist as two optical isomers.
- Enantiomers Enantiomers are non-superimposable mirror images. Draw them with wedge and dash bonds around a tetrahedral carbon, one as the mirror image of the other.
- Optical activity Each enantiomer rotates the plane of plane-polarised light by the same amount but in opposite directions.
- Racemic mixtures A racemic mixture contains equal amounts of both enantiomers. The rotations cancel out, so it is optically inactive.
- Forming racemates When a planar intermediate such as a carbonyl group or a carbocation is attacked, attack from either side is equally likely, so a racemic mixture forms.
Common mistakes in 3.3.7 Optical Isomerism
- Calling a carbon with two identical groups chiral. All four groups must be different.
- Drawing two enantiomers that are actually the same molecule rotated. Check that one is the mirror image of the other.
- Saying a racemic mixture does not rotate light because the molecules are not chiral. The molecules are chiral, but equal amounts of each enantiomer cancel out.
- Saying enantiomers have different chemical properties. They react the same way except with other chiral molecules, such as enzymes.
Try three 3.3.7 Optical Isomerism exam-style questions
1. Identify the chiral centre in butan-2-ol and explain why it is chiral. (2 marks)
Show the mark scheme answer
Carbon 2 (the carbon bonded to OH); it is bonded to four different groups: H, OH, CH3 and C2H5.
2. Explain why the reaction of propanal with KCN followed by dilute acid gives an optically inactive product. (3 marks)
Show the mark scheme answer
The C=O group is planar; the cyanide ion can attack from either side with equal probability; equal amounts of each enantiomer form, giving a racemic mixture.
3. State how two enantiomers could be distinguished. (1 mark)
Show the mark scheme answer
They rotate plane-polarised light in opposite directions.
3.3.7 Optical Isomerism FAQs
What does OxfordAQA International A Level Chemistry 3.3.7 Optical Isomerism cover?
It covers chiral centres, enantiomers, plane-polarised light, racemic mixtures and why they are optically inactive.
Which unit is 3.3.7 Optical Isomerism examined in?
It is examined in Unit 4, an International A2 paper covering organic and physical chemistry. The Unit 5 practical and synoptic paper can also assess it.
How should I draw optical isomers?
Draw a central carbon with two bonds in the plane of the paper, one wedge and one dash, then draw the second isomer as its reflection across a mirror line. Make sure all four groups are clearly labelled.
