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AS · Practice questions · Cis-trans and optical isomerism

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Six original Cambridge-style questions on cis-trans and 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.
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Rigid C=C or four different groups.

01
Recall
[2 marks]

Define stereoisomers and state the two types.

  • Compounds with the same structural formula but a different arrangement of atoms in space. ✓
  • Cis-trans (geometric) isomers and optical isomers. ✓
02
Explain
[2 marks]

Explain why 1,2-dichloroethene has cis and trans isomers but 1,1-dichloroethene does not.

  • Rotation about C=C is restricted; in CHCl=CHCl each carbon has two different groups (H and Cl), so they can be on the same or opposite sides. ✓
  • In CH2=CCl2 each carbon has two identical groups, so swapping them gives the same molecule. ✓
03
Analysis
[2 marks]

Draw the cis and trans isomers of pent-2-ene.

  • Cis: CH3 and C2H5 on the same side of the C=C, both H on the other side. ✓
  • Trans: CH3 and C2H5 on opposite sides. ✓
04
Analysis
[2 marks]

Identify the chiral centre in 2-hydroxypropanoic acid and list the four groups attached to it.

  • The central carbon (C2). ✓
  • H, OH, CH3 and COOH. ✓
05
Explain
[2 marks]

State how a pair of enantiomers can be distinguished, and what is meant by a racemic mixture.

  • They rotate plane-polarised light by equal amounts in opposite directions. ✓
  • A racemic mixture contains equal amounts of the two enantiomers and has no net effect on plane-polarised light. ✓
06
Analysis
[2 marks]

State which of these have optical isomers: (a) 2-bromobutane, (b) 2-bromopropane, (c) butan-2-ol.

  • (a) yes: C2 has H, Br, CH3, C2H5; (c) yes: H, OH, CH3, C2H5. ✓
  • (b) no: C2 carries two CH3 groups. ✓

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.