Edexcel International A Level Chemistry
Edexcel International A Level Chemistry Chirality Exam Questions
Free Edexcel International A Level Chemistry exam questions for Chirality, with a mark scheme for every pack. Covers optical isomerism, chiral centres, enantiomers, plane-polarised light, racemic mixtures and linking optical activity to SN1, SN2 and nucleophilic addition mechanisms.
Chirality 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.
StemUp mark scheme flashcards
A-Level Chemistry Flashcards for 2027 Onwards
Every answer is written in mark scheme wording, so you learn exactly what examiners give marks for. Then test yourself with the questions on this page.
From £10.99View flashcards
About Edexcel International A Level Chemistry Chirality
Chirality in Unit 4 of Edexcel International A Level Chemistry introduces optical isomerism, a form of stereoisomerism that results from a carbon atom bonded to four different groups. You learn to identify chiral centres, draw enantiomers as three-dimensional mirror images and describe how they rotate plane-polarised light in opposite directions. A key skill is linking mechanisms to the optical activity of products: SN2 gives a single enantiomer with inverted configuration, while SN1 and nucleophilic addition to carbonyls give racemic mixtures. Examiners like asking you to mark chiral centres, draw 3D diagrams and explain why a product is optically inactive.
What you need to know
- Chiral centre A carbon atom attached to four different atoms or groups is a chiral centre. Molecules with one chiral centre exist as two non-superimposable mirror images called enantiomers.
- Drawing enantiomers Draw the chiral carbon tetrahedrally using wedges and dashed bonds, then draw its mirror image with two groups swapped.
- Optical activity Enantiomers rotate the plane of plane-polarised light by equal amounts in opposite directions. They have identical physical properties apart from this, and react differently only with other chiral molecules.
- Racemic mixtures An equimolar mixture of two enantiomers is racemic. It is optically inactive because the rotations cancel.
- SN1 and SN2 SN1 goes through a planar carbocation that can be attacked from either side, giving a racemic mixture. SN2 is a one-step attack from the opposite side to the leaving group, giving inversion and a single optically active product.
- Nucleophilic addition The C=O group is planar, so CN- attacks from either side with equal probability, giving a racemic hydroxynitrile.
Common mistakes in Chirality
- Marking a carbon as chiral when two of its groups are identical, such as a CH3 and another CH3, or when it is part of a C=C double bond.
- Saying enantiomers rotate light by different amounts. They rotate it by the same amount in opposite directions.
- Writing that a racemic mixture contains no chiral molecules. Every molecule is chiral, but the rotations cancel.
- Drawing mirror images that are actually identical when rotated. Check that two groups have swapped positions.
- Linking SN2 with a racemic product. SN2 inverts configuration; SN1 gives racemisation.
Try three Chirality exam-style questions
1. Explain what is meant by a chiral centre and identify the chiral centre in butan-2-ol. (2 marks)
Show the mark scheme answer
A carbon atom bonded to four different atoms or groups; carbon 2, bonded to H, OH, CH3 and C2H5.
2. Lactic acid, CH3CH(OH)COOH, exists as two optical isomers. Draw 3D diagrams of both and state how they could be distinguished. (3 marks)
Show the mark scheme answer
Two tetrahedral structures as mirror images with H, OH, CH3 and COOH around the central carbon, correct wedges and dashes; they rotate plane-polarised light in opposite directions (by the same angle).
3. A single enantiomer of 2-bromobutane is hydrolysed. The product is optically inactive. Explain what this suggests about the mechanism. (3 marks)
Show the mark scheme answer
The reaction proceeds by SN1; a planar carbocation intermediate forms; the nucleophile attacks equally from both sides, giving a racemic mixture with no net rotation.
Chirality FAQs
What does Edexcel International A Level Chemistry Chirality cover?
It covers chiral centres, enantiomers, optical activity with plane-polarised light, racemic mixtures and how SN1, SN2 and nucleophilic addition determine whether products are optically active.
Which unit is Chirality examined in?
Chirality is part of Unit 4, and you may need it again in Unit 5 when considering amino acids and synthesis of drugs.
How can I quickly find chiral centres?
Look only at sp3 carbon atoms with one or no hydrogens, then check whether all four groups attached are different, looking along each branch rather than just at the first atom.
