Methanol and ethanol are both alcohols, yet ingesting methanol in very small amounts can be fatal while ethanol, though harmful in excess, is not immediately lethal in dilute amounts. What accounts for the difference in their toxicity to humans?
Generated by claude-sonnet-4-6 · 2026-06-26 01:11 · grounding rag
Model Answer
Both methanol and ethanol belong to the same homologous series, but they differ in toxicity due to their metabolic products.
- Methanol is oxidised in the liver to methanal (formaldehyde), which reacts rapidly with cell components. It coagulates the protoplasm (similar to how heat coagulates egg white) and also damages the optic nerve, causing blindness and even death in very small quantities.
- Ethanol, when consumed in excess, slows metabolic processes and depresses the central nervous system, but it does not produce such immediately lethal metabolites in dilute amounts.
Thus, the extreme toxicity of methanol is due to its oxidation product — methanal — which is highly destructive to cells.
Source: Chapter 4, Section 4.4.1 (Do You Know? — How do alcohols affect living beings?)
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Explanation
Examiners look for three key points here (1 mark each):
- Methanol is oxidised to methanal in the liver.
- Methanal coagulates protoplasm / reacts with cell components.
- It also damages the optic nerve, causing blindness/death.
Do not write a general comparison of physical properties — the question is specifically about toxicity mechanism. Always anchor your answer to the metabolic/chemical difference, not just "methanol is poisonous."