Explain why pent-2-ene can exist as cis and trans isomers but pent-1-ene cannot.

WJEC A-Level Chemistry (Wales) — 4.1 Stereoisomerism · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Stereoisomers are molecules that have the same molecular formula and the same structural formula but differ in the arrangement of atoms in space. Cis-trans isomerism is one type of stereoisomerism found in some alkenes.

Model answer (5 marks)

Cis‑trans isomerism requires a double bond that cannot rotate and that has two different groups on each of the double‑bonded carbons.

In pent‑2‑ene the double bond is between C2 and C3. C2 carries a hydrogen and a methyl group; C3 carries a hydrogen and an ethyl group – each carbon has two different substituents, so the relative positions of the hydrogens can be either on the same side (cis) or on opposite sides (trans).

In pent‑1‑ene the double bond is between C1 and C2. C1 has two hydrogens attached – the two groups are identical. Because one carbon of the double bond has identical groups, the terms cis and trans cannot be defined, so no cis‑trans isomerism occurs.

Examiner tips

  • Show the requirement of two different groups on each double‑bonded carbon
  • Explain the specific groups on C2 and C3 of pent‑2‑ene
  • Point out the two hydrogens on C1 in pent‑1‑ene and why this precludes cis‑trans

Common mistakes

  • Confusing the positions of the double bond (e.g. saying pent‑1‑ene has a double bond at C2‑C3)
  • Claiming that rotation is possible in alkenes
  • Forgetting to mention that identical groups on a double‑bonded carbon prevent cis‑trans distinction

Mark scheme (5 marks)

  1. Cis-trans isomerism requires restricted rotation about the carbon–carbon double bond
  2. For cis-trans isomerism to occur, each carbon of the double bond must have two different groups attached to it
  3. In pent-2-ene, one carbon of the double bond has a hydrogen and a methyl group attached, and the other carbon has a hydrogen and an ethyl group attached — both carbons have two different groups
  4. In pent-1-ene, one carbon of the double bond (C1) has two hydrogen atoms attached — both groups are the same
  5. Because one carbon of the double bond in pent-1-ene has two identical groups, cis and trans arrangements cannot be distinguished / no cis-trans isomerism is possible

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