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Topic 3.3.7: Optical Isomerism - Exam Practice

AQA A-Level Chemistry • Organic Chemistry • Exam-Style Questions

Exam Practice

Topic 3.3.7: Optical Isomerism Exam Practice

Test your understanding of chiral centres, non-superimposable mirror images, optical activity, and racemic mixture generation with exam-style questions.

Optical Isomerism › Exam Practice

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📋 Structured Questions

Complete each question on paper, then check your answers against the mark scheme.

Question 1: Chiral Centres and Mirror Images

4 marks

Butan-2-ol exhibits optical isomerism.

(a) Draw 3D structures (using wedge/dash notation) of both enantiomers of butan-2-ol to show their relationship. [2]

(b) Identify the chiral carbon atom in butan-2-ol and explain why it is chiral. [2]

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(a)

  • Draw a central carbon atom with tetrahedral geometry (two bonds in the plane, one wedge pointing forward, one dash pointing away). [1]
  • The second enantiomer must be drawn as a non-superimposable mirror image of the first across a vertical plane. [1]

(b)

  • Carbon-2 (the second carbon atom in the chain). [1]
  • It is chiral because it is bonded to four different groups: a hydrogen atom (-H), a hydroxide group (-OH), a methyl group (-CH3), and an ethyl group (-CH2CH3). [1]
Examiner tip: When drawing enantiomers, make sure the mirror line relationship is clear. The same group (e.g. -OH) must face the mirror line in both drawings. Ensure you write out the ethyl group as -CH2CH3 or -C2H5, not just -C2H6.

Question 2: Planar Carbonyl Attack and Racemisation

5 marks

Ethanal reacts with hydrogen cyanide (HCN) to form 2-hydroxypropanenitrile.

(a) Name the mechanism for this reaction. [1]

(b) Explain, in terms of the geometry of the reactant, why the product is formed as a racemic mixture. [3]

(c) State the optical rotation of the resulting mixture. [1]

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(a)

  • Nucleophilic addition. [1]

(b)

  • The carbonyl group (C=O) of ethanal (specifically the carbonyl carbon atom) has a planar geometry. [1]
  • The nucleophile (:CN-) can attack the carbonyl carbon from either side of this plane (above or below). [1]
  • There is an equal probability of attack from either side, which results in the formation of equal amounts (a 50:50 ratio) of both enantiomers (a racemic mixture). [1]

(c)

  • 0 degrees / no rotation / optically inactive (the rotations of the two enantiomers cancel out). [1]
Examiner tip: Do not say the "molecule is planar". Only the carbonyl group/bond area is planar. Saying the whole molecule is planar will lose the mark. The term "equal probability" or "equal chance" of attack is essential.

Question 3: Pharmaceutical Relevance

3 marks

Many pharmaceutical drugs are chiral molecules. Explain why drugs are often synthesized or administered as a single enantiomer rather than a racemic mixture. Reference therapeutic effectiveness and side-effects. [3]

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  • Enantiomers have different shapes in 3D space. Receptors and enzymes in the body are chiral, so only one enantiomer will have the correct fit for the active site. [1]
  • The other enantiomer is often therapeutically inactive. Administering a racemate would halve the dose efficiency. [1]
  • More importantly, the inactive enantiomer may fit other receptors in the body, causing toxic or harmful side-effects (e.g. thalidomide, where one enantiomer cured morning sickness, while the other caused birth defects). [1]
Examiner tip: Standard examples like thalidomide or ibuprofen are great for explaining this context. Focus on the concept of stereospecific interactions with receptors.

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