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A2 · Practice questions · Optical isomerism

Find every chiral centre.

Six original Cambridge-style questions on optical isomerism.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Four different groups, two mirror images.

01
Recall
[3 marks]

Define the terms (a) chiral centre, (b) enantiomers, (c) racemic mixture.

  • (a) A carbon atom bonded to four different atoms or groups. ✓
  • (b) A pair of stereoisomers that are non-superimposable mirror images. ✓
  • (c) An equimolar mixture of two enantiomers, which has no effect on plane-polarised light. ✓
02
Analysis
[2 marks]

Draw three-dimensional diagrams of the two enantiomers of butan-2-ol.

  • Central C with H, OH, CH3 and C2H5: two bonds in the plane, one wedge, one dashed bond. ✓
  • The second diagram is the exact mirror image of the first, drawn either side of a mirror line. ✓
03
Analysis
[3 marks]

Open-chain ribose is HOCH2CH(OH)CH(OH)CH(OH)CHO. Identify the chiral centres and state the maximum number of stereoisomers.

  • The CH2OH carbon has two H atoms and the CHO carbon has a C=O, so neither is chiral. ✓
  • The three CH(OH) carbons each carry four different groups: 3 chiral centres. ✓
  • Maximum stereoisomers = 23 = 8. ✓
04
Explain
[2 marks]

Explain how a polarimeter could distinguish a pure enantiomer from a racemic mixture of the same compound.

  • The pure enantiomer rotates the plane of plane-polarised light by a measurable angle. ✓
  • In the racemic mixture equal and opposite rotations cancel, so the observed rotation is zero. ✓
05
Explain
[3 marks]

Thalidomide was sold in the late 1950s as a racemic mixture. Explain why this caused problems and why drug companies now aim to make single enantiomers.

  • One enantiomer had the desired sedative effect; the other caused birth defects. ✓
  • Receptors in the body are chiral, so the two enantiomers bind differently. ✓
  • A single enantiomer gives a lower dose and fewer side effects, though it needs chiral synthesis or separation. ✓
06
Analysis
[2 marks]

Explain why propan-2-ol and aminoethanoic acid are not optically active but 2-aminopropanoic acid is.

  • Propan-2-ol has two CH3 groups on C2, and aminoethanoic acid has two H atoms on its central carbon, so neither has four different groups. ✓
  • In 2-aminopropanoic acid, C2 carries H, NH2, CH3 and COOH: a chiral centre, so it has optically active enantiomers. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.