But-1-ene (C₄H₈) is an alkene. Explain why but-1-ene is classified as an alkene, describe the general formula that links all members of the alkene homologous series, and explain how you could confirm that but-1-ene is unsaturated using a chemical test. Include the expected result of the test.

Edexcel A-Level Chemistry (9CH0) — 6.3 Alkenes · Explain · 5 marks · View as Markdown

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

But-1-ene is a colourless gas used in the manufacture of synthetic rubber. Its molecular formula is C₄H₈.

Model answer (5 marks)

But‑1‑ene contains a carbon–carbon double bond (C=C), so it is an alkene.
The general formula for all alkenes is CₙH₂ₙ, and C₄H₈ satisfies this formula.
To confirm that but‑1‑ene is unsaturated, add bromine water to a sample. The orange‑brown bromine water will decolourise to colourless, indicating addition across the C=C bond.

Examiner tips

  • State the C=C bond first to show classification. Give the general formula CₙH₂ₙ and check the given formula. Describe the bromine water test and the colour change to prove unsaturation.

Common mistakes

  • Saying the formula is CₙHₙ₊₂ (wrong for alkenes). Not mentioning the colour change of bromine water. Using the wrong test (e.g., iodine vapour) without noting the result.

Mark scheme (5 marks)

  1. But-1-ene contains a carbon-to-carbon double bond (C=C)
  2. But-1-ene is unsaturated (because of the C=C double bond)
  3. The general formula of alkenes is CₙH₂ₙ (which C₄H₈ fits)
  4. Add bromine water to but-1-ene
  5. Bromine water decolourises / turns from orange-brown to colourless

Key terms in this question

alkene · unsaturated · general formula · homologous series

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