A patient poisoned with methanol is treated by giving intravenous infusions of diluted ethanol. How does this treatment help?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The enzyme that oxidises methanal to methanoic acid is swamped, which allows time for the kidneys to excrete methanol.
In the body, methanol is oxidised first to methanal and then to methanoic acid, which may cause blindness and death:
\( \displaystyle \ce{CH3OH -> HCHO -> HCOOH} \)
Ethanol is oxidised in the body by the same route. When diluted ethanol is infused, the enzyme that oxidises the aldehyde to the acid is kept busy (swamped), so little methanoic acid is formed, and the kidneys get time to excrete methanol.
Why the other options are incorrect
Option (1) Ethanol oxidises methanol directly to carbon dioxide and water. — Incorrect. Ethanol does not oxidise methanol; it is itself oxidised in the body. The treatment works on the enzyme, not by any reaction between the two alcohols.
Option (2) Ethanol reacts with the methanoic acid formed and converts it into an ester, which is excreted. — Incorrect. The treatment acts on the enzyme, so that methanol is not turned into methanoic acid while the kidneys remove it; no ester is made in the body.
Option (4) Ethanol reduces methanal back to methanol, which is less harmful than methanoic acid. — Incorrect. The treatment does not turn methanal back into methanol. It slows the oxidation, and the methanol is removed by the kidneys.
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “Biological oxidation of methanol and ethanol in the body produces the corresponding aldehyde followed by the acid. … In the body, methanol is oxidised first to methanal and then to methanoic acid, which may cause blindness and death. A methanol poisoned patient is treated by giving intravenous infusions of diluted ethanol. The enzyme responsible for oxidation of aldehyde (HCHO) to acid is swamped allowing time for kidneys to excrete methanol.”