CCEA A Level Chemistry
CCEA A Level Chemistry 4.6 Isomerism Exam Questions
Free CCEA A Level Chemistry revision for 4.6 Isomerism: what you need to know, common mistakes and exam-style questions with answers. Covers structural isomerism between families of compounds, chiral centres, optical isomers, plane-polarised light, racemic mixtures and stereospecific drug action.
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About CCEA A Level Chemistry 4.6 Isomerism
Section 4.6 of Unit A2 1 extends the isomerism you met in AS 2 (2.2). You need structural isomerism between different families, for example alcohols and ethers or aldehydes and ketones with the same molecular formula. The main new content is optical isomerism: a chiral centre is a carbon atom with four different groups, giving two non-superimposable mirror image isomers (enantiomers). You must draw them in 3D, explain that they rotate plane-polarised light in opposite directions, define optically active and explain why a racemic mixture is inactive. CCEA also asks how drug action depends on stereochemistry because receptor sites are stereospecific.
What you need to know
- Functional group isomers Compounds with the same molecular formula can belong to different homologous series, for example propanal and propanone (C3H6O), or ethanol and methoxymethane (C2H6O).
- Chiral centre A carbon atom bonded to four different atoms or groups. Mark it with an asterisk. Butan-2-ol has a chiral centre; propan-2-ol does not.
- Optical isomers Enantiomers are non-superimposable mirror images. Draw them with a tetrahedral arrangement using wedge and dashed bonds, reflected across a mirror line.
- Optical activity Each enantiomer rotates the plane of plane-polarised light by the same angle in opposite directions. An optically active substance rotates plane-polarised light.
- Racemic mixture A 50:50 mixture of enantiomers is optically inactive because the rotations cancel. Reactions forming a chiral centre from a planar intermediate often give racemic mixtures.
- Drugs Receptor sites in the body are stereospecific, so often only one enantiomer has the desired effect, and the other may be inactive or harmful. Drugs may need to be made as a single enantiomer.
Common mistakes in 4.6 Isomerism
- Drawing enantiomers that are actually identical. Check that the two 3D structures are true mirror images and cannot be superimposed.
- Saying optical isomers rotate light by different amounts. They rotate it by equal amounts in opposite directions.
- Marking a carbon in a C=C or CH2 group as chiral. A chiral carbon must have four different groups.
- Saying a racemic mixture does not rotate light because the molecules are not chiral. The molecules are chiral, but the equal and opposite rotations cancel.
- Confusing E/Z and optical isomerism. E/Z needs a C=C; optical isomerism needs a chiral centre.
Try three 4.6 Isomerism exam-style questions
1. Explain why 2-hydroxypropanoic acid shows optical isomerism. (2 marks)
Show the mark scheme answer
Has a carbon atom bonded to four different groups (H, OH, CH3, COOH); this is a chiral centre so it has non-superimposable mirror images.
2. Describe how the two enantiomers of a compound could be distinguished. (2 marks)
Show the mark scheme answer
Pass plane-polarised light through a solution of each; they rotate the plane by the same angle in opposite directions.
3. Explain why a drug may be produced as a single enantiomer. (2 marks)
Show the mark scheme answer
Receptor sites are stereospecific so only one enantiomer fits and is active; the other may be inactive or cause harmful side effects.
4.6 Isomerism FAQs
What does CCEA A Level Chemistry 4.6 Isomerism cover?
It covers structural isomerism between families, chiral centres, optical isomers, plane-polarised light, racemic mixtures and stereospecific drug action.
Which unit is 4.6 Isomerism examined in?
It is examined in Unit A2 1: Further Physical and Organic Chemistry, a 2 hour paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit A2 3.
How do I spot chiral centres quickly?
Look at each carbon with four single bonds and check whether all four groups differ. Ignore CH3, CH2 and any carbon in a double bond.
