8 min readConcept sheet1-mark MCQ and assertion-reason on manufacture and poisoning; 2-mark questions on fermentation and denatured alcohol; sometimes a case-based question

Commercially important alcohols: methanol and ethanol

Methanol (wood spirit) is made from CO and H₂ and is poisonous. Ethanol is made by fermenting sugars with yeast; denatured alcohol is made unfit to drink.

Methanol manufactureCO + 2H₂ → CH₃OH; ZnO-Cr₂O₃ catalyst, 573 to 673 K, 200 to 300 atm

Fermentation enzymesInvertase: sucrose → glucose + fructose. Zymase (in yeast): glucose → ethanol + CO₂

Methanol poisoningSmall amounts can cause blindness; large amounts can cause death

Denatured alcoholEthanol made unfit to drink by adding copper sulphate (colour) and pyridine (foul smell)

Manufacture of methanol

1★
CO + 2H₂ → CH₃OH
Catalytic hydrogenation of carbon monoxide: ZnO-Cr₂O₃ catalyst, 573 to 673 K, 200 to 300 atm
2
Earlier, methanol came from the destructive distillation of wood, which is why it is called wood spirit.

★ key step

Ethanol by fermentation

1
C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose)
Invertase · The sugar comes from molasses, sugarcane or fruits such as grapes
2★
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Zymase (found in yeast) · Takes place without air (anaerobic); carbon dioxide is given off
3
Zymase stops working once the alcohol content rises above about 14 percent.
4
If air gets in, oxygen oxidises ethanol to ethanoic acid, which spoils the taste.

★ key step

Methanol vs ethanol

Methanol, CH₃OHWood spirit. Colourless liquid, b.p. 337 K. Highly poisonous. Used as a solvent in paints and varnishes, and mainly to make methanal (formaldehyde).

Ethanol, C₂H₅OHColourless liquid, b.p. 351 K. Made by fermentation, and today also in large amounts by hydration of ethene. Used as a solvent in the paint industry and to make many carbon compounds.

Denatured alcohol and methanol poisoning

Commercial ethanol is made unfit for drinking by adding copper sulphate (to give it a colour) and pyridine (a foul-smelling liquid). This is called denaturation. If methanol is drunk, the body oxidises it first to methanal and then to methanoic acid, which can cause blindness and death. Doctors give a methanol-poisoned patient a drip of dilute ethanol. The ethanol keeps the body's oxidising enzymes busy, so less methanol is turned into poisonous products, and the kidneys get time to excrete it.

In real life

News reports of spurious liquor deaths in India are usually cases of methanol poisoning. India also blends ethanol made from sugarcane molasses and grains into petrol, to cut its use of imported oil.

Exam angle

2 marks: “How is methanol manufactured? Why is it dangerous to drink?” CO + 2H₂ → CH₃OH (ZnO-Cr₂O₃, 573 to 673 K, 200 to 300 atm). The body oxidises it to methanal and methanoic acid, which can cause blindness or death.

2 marks: “Write the reactions of the fermentation of cane sugar, naming the enzymes.” C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆ (invertase); C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (zymase, from yeast), without air.

1 marks: “What is denatured alcohol?” Ethanol made unfit for drinking by adding copper sulphate and pyridine.

Remember it

Invertase inverts (splits) the sugar; zymase makes the spirit. Methanol: wood, poison, blindness.