CDS (I) 2024 General Knowledge · Question 36 of 120

Correct — B, methanol gets oxidised to methanal in liver which coagulates the protoplasm. The chemistry of the first step is the whole answer, and NCERT states it directly: 'In the body, methanol is oxidised first to methanal and then to methanoic acid, which may cause blindness and death.' Methanol has a single carbon atom, so the only aldehyde it can give is methanal, better known as formaldehyde, and the enzyme alcohol dehydrogenase in the liver performs that oxidation. Formaldehyde reacts rapidly with cell components, and the older textbook account describes that damage as coagulation of the protoplasm — the wording the paper has carried over. NCERT adds the clinical corollary: a methanol-poisoned patient is treated with intravenous infusions of dilute ethanol, which occupies the same enzyme and buys the kidneys time to excrete methanol unchanged.

• (a)methanol coagulates the protoplasm — Skips the metabolism. Methanol itself is a relatively mild intoxicant; the damage is done by what the liver turns it into, which is why blocking that conversion with ethanol is an effective treatment.
• (c)methanol gets oxidised to acetic acid in liver which coagulates the protoplasm — Impossible on carbon count. Methanol has one carbon and acetic acid has two, so no oxidation of methanol can yield acetic acid; it yields methanoic acid, that is formic acid.
• (d)methanol gets oxidised to CO in liver which coagulates the protoplasm — Carbon monoxide is not a metabolite of methanol. The oxidation sequence in the liver runs methanol to methanal to methanoic acid.

Alcohols are oxidised in the body along a fixed sequence — alcohol to aldehyde to carboxylic acid — carried out by alcohol dehydrogenase and then aldehyde dehydrogenase. Ethanol goes to ethanal and then to ethanoic acid, both of which the body handles routinely. Methanol goes to methanal, that is formaldehyde, and then to methanoic acid, that is formate, and neither is handled well. That single difference in the first product is why one alcohol is a drink and the other a poison.

The item is answered by carbon counting rather than by pharmacology: one carbon in, one carbon out, so methanal is the only possible aldehyde and acetic acid and carbon monoxide can both be struck out at once. It is worth being straight with students about how the mechanism is understood today, because the answer's phrasing is old. Modern toxicology attributes the blindness and the severe metabolic acidosis chiefly to formate, the next product in the chain, which 'is toxic because it inhibits mitochondrial cytochrome c oxidase, causing hypoxia at the cellular level' in tissues including the optic nerve — not to a general coagulation of protoplasm by formaldehyde. NCERT's own sentence points the same way, naming methanoic acid as what 'may cause blindness and death'. The exam has kept an older textbook formulation, and the card is authored to the official key; the step it asserts, oxidation to methanal in the liver, is correct in any account. On treatment, ethanol and the modern antidote fomepizole work identically, both reducing the action of alcohol dehydrogenase on methanol by competitive inhibition.

• NCERT: 'In the body, methanol is oxidised first to methanal and then to methanoic acid, which may cause blindness and death.'
• Methanal is formaldehyde; methanoic acid is formic acid, present in the body as formate.
• Alcohol dehydrogenase converts methanol to formaldehyde and aldehyde dehydrogenase converts formaldehyde to formic acid.
• Formate inhibits mitochondrial cytochrome c oxidase, causing cellular hypoxia — the modern explanation of the blindness.
• Ethanol and fomepizole both act by competitive inhibition of alcohol dehydrogenase, letting the kidneys excrete methanol unchanged.

• Choosing acetic acid because it is the familiar carboxylic acid; it has two carbons and cannot come from methanol.
• Assuming the parent alcohol itself is the poison, which would make the ethanol antidote meaningless.
• Reading 'CO' as a plausible oxidation product because it is a known poison; it is not on the methanol pathway.

Asked as a because-clause item where the four options differ only in the metabolite named, so the discrimination is purely chemical.

No directly related past PYQ was found.

• practice — not a real PYQ

  A patient poisoned by methanol is given intravenous ethanol because ethanol

  • (a)neutralises formic acid in the blood
  • (b)competitively inhibits alcohol dehydrogenase and prevents methanol from being oxidised
  • (c)converts methanol directly into carbon dioxide
  • (d)increases the rate of formaldehyde production

  Answer(b) competitively inhibits alcohol dehydrogenase and prevents methanol from being oxidised — the kidneys then excrete methanol unchanged.

• practice — not a real PYQ

  Oxidation of a primary alcohol first yields

  • (a)a ketone
  • (b)an aldehyde
  • (c)an ether
  • (d)an ester

  Answer(b) an aldehyde — which on further oxidation gives the corresponding carboxylic acid.