Methanol and ethanol are both members of the alcohol homologous series. State two structural features that all alcohols share, and explain why methanol is considered more toxic to humans than ethanol.

Cambridge International IGCSE Chemistry (0620) — 11.6 Alcohols · State and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

All alcohols contain an –OH (hydroxyl) functional group
All alcohols contain a carbon chain bonded to the –OH group
Methanol is oxidised in the body to formaldehyde and formic acid, which are highly toxic and damage the optic nerve, potentially causing blindness
Ethanol is metabolised to acetaldehyde and acetic acid, which are less harmful; therefore methanol causes greater harm at equivalent doses

Examiner tips

  • Use the exact terms – ‘hydroxyl group’ and ‘carbon chain’ Explain the metabolic pathway of methanol and contrast it with ethanol Show the link between the toxic metabolites and visual damage

Common mistakes

  • Confusing the carbon skeleton with the entire molecule Using vague terms like ‘alcohol group’ instead of ‘hydroxyl group’ Failing to mention the optic nerve damage or the specific metabolites

Mark scheme (4 marks)

  1. All alcohols contain an –OH (hydroxyl) functional group
  2. All alcohols contain a carbon chain / carbon skeleton / hydrocarbon chain bonded to the –OH group
  3. Methanol is oxidised / broken down in the body to methanal (formaldehyde) / methanoic acid, which is toxic / damages the optic nerve / can cause blindness
  4. Ethanol is metabolised to ethanal / ethanoic acid, which is less harmful / less toxic than the products from methanol metabolism; so methanol causes greater harm at equivalent doses

Key terms in this question

homologous series · toxic

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