Commercially important alcohols: methanol and ethanol

What are the commercially important alcohols?

In plain words: methanol, once “wood spirit”, now comes from a gas reaction at high pressure, and even a little of it can blind; ethanol comes from yeast working on sugar, or from ethene. Industrial ethanol gets a colour and a foul smell so that no one drinks it.

Methanol

• Name and old source. Methanol is also called wood spirit: it was produced by destructive distillation of wood (NCERT page 214).
• Manufacture today. Most methanol is made by catalytic hydrogenation of carbon monoxide at high pressure and temperature, over a ZnO–Cr₂O₃ catalyst (NCERT page 214):

• Why high pressure? At 573–673 K all three substances are gases, and 3 mol of gas give 1 mol, so Δn_g = 1 − 3 = −2. By Le Chatelier’s principle, raising the pressure shifts the equilibrium towards methanol (reasoned from NCERT Class 11 Chemistry, Unit 6, Section 6.8.2).
• Beyond the NCERT text: ZnO–Cr₂O₃ at a few hundred atmospheres is the original process, first run in Germany in 1923. Today the most widely used catalyst is a mixture of copper and zinc oxides on alumina, first used in 1966, at about 50–100 atm and 523 K (Wikipedia, “Methanol”). In exams, give NCERT’s catalyst and conditions.
• Properties. A colourless liquid, boiling at 337 K. It is highly poisonous: even small quantities can cause blindness, and large quantities cause death (NCERT page 214). In the body it is oxidised to methanal and then methanoic acid; a poisoned patient is given dilute ethanol intravenously (NCERT page 210).
• Uses. A solvent in paints and varnishes, and chiefly for making formaldehyde (methanal) (NCERT page 214).

Ethanol

Fermentation

Ethanol is obtained commercially by fermentation, and the oldest method starts from sugars, in molasses, sugarcane or fruits such as grapes (NCERT page 214). It goes in two steps, each needing an enzyme:

1. Invertase converts the sugar into glucose and fructose, both C₆H₁₂O₆.
2. Zymase, found in yeast, ferments glucose and fructose into ethanol and carbon dioxide.

Three conditions matter (NCERT page 214):

• No air. Fermentation takes place in anaerobic conditions, and carbon dioxide is released. If air gets in, its oxygen oxidises ethanol to ethanoic acid, which spoils the taste of alcoholic drinks.
• A ceiling of about 14 percent. The action of zymase is inhibited once the alcohol formed exceeds 14 percent, so fermentation alone cannot give stronger alcohol.
• Wine making. Grapes supply both the sugar and the yeast: sugar builds up as grapes ripen, yeast grows on the outer skin, and crushing brings the two together.

Properties, uses and denaturation

• Ethanol is a colourless liquid, boiling at 351 K. It is used as a solvent in the paint industry and to prepare many carbon compounds (NCERT page 214).
• Denaturation: commercial alcohol is made unfit for drinking with copper sulphate, for colour, and pyridine, for a foul smell (NCERT page 214).
• NCERT page 210 calls ethanol mixed with methanol “denatured alcohol” too. The two descriptions do not clash: page 214 names a colouring agent and a foul-smelling one, page 210 a poisonous one. Beyond the NCERT text, the main additive is usually about 10 percent methanol, hence the name “methylated spirits”; pyridine is another typical denaturant, and a dye is often added for colour (Wikipedia, “Denatured alcohol”). If asked what denaturation is, give page 214’s answer: copper sulphate and pyridine.
• Nowadays large quantities of ethanol come from the hydration of ethene (NCERT page 214).
• In the body: ethanol acts on the central nervous system. Moderate amounts affect judgement and lower inhibitions; higher amounts cause nausea and loss of consciousness; still higher amounts interfere with breathing and can be fatal (NCERT page 214, margin note).

Solved examples

Common mistakes to avoid

Quick revision summary

• Methanol (wood spirit): CO + 2H₂ → CH₃OH over ZnO–Cr₂O₃ at 200–300 atm and 573–673 K; b.p. 337 K; highly poisonous (blindness, death); solvent; chiefly for formaldehyde.
• Ethanol: fermentation of sugars (invertase, then zymase in yeast), anaerobic; stops above 14 percent alcohol; air turns it into ethanoic acid; also from hydration of ethene; b.p. 351 K.
• Denaturation: copper sulphate (colour) and pyridine (foul smell) make commercial alcohol unfit for drinking.

Questions students ask

Why is methanol called wood spirit?

Methanol is called wood spirit because it was once made by the destructive distillation of wood (NCERT page 214), that is, by heating wood strongly out of contact with air. Beyond the NCERT text, Robert Boyle obtained pure methanol from boxwood in 1661, and in 1834 Dumas and Péligot coined the word “methylène” from the Greek methy, an alcoholic drink, and hȳlē, wood; “methyl” was formed from it around 1840, and the -yl of ethyl, propyl and every other alkyl group comes from methyl (Wikipedia, “Methanol”). Today most methanol is made from carbon monoxide and hydrogen instead (NCERT page 214).

How is methanol made from water gas?

Methanol is made from water gas, a route beyond the NCERT text, by turning part of its carbon monoxide into extra hydrogen and then hydrogenating the rest over a ZnO–Cr₂O₃ catalyst, CO + 2H₂ → CH₃OH, the step NCERT gives (page 214). Steam over red-hot coke gives water gas, C + H₂O → CO + H₂, a 1 : 1 mixture, and the water–gas shift, CO + H₂O ⇌ CO₂ + H₂, makes up the hydrogen (Wikipedia, “Water gas”): shifting 1 mol of CO in every 3 leaves 2 mol of CO with 4 mol of H₂, the 1 : 2 that methanol needs (reasoned).

What is the difference between methanol and ethanol?

Methanol, CH₃OH, and ethanol, C₂H₅OH, are the first two members of the alcohol homologous series, one CH₂ apart (reasoned), and they differ most in source and danger: methanol is made from carbon monoxide and hydrogen and even a little can cause blindness, while ethanol comes from fermenting sugars or hydrating ethene and is the alcohol in drinks (NCERT page 214). Beyond the NCERT text, the two look and smell so alike that they are hard to tell apart without a chemical test, which is how methanol-tainted liquor poisons people (Wikipedia, “Methanol toxicity”).

Why is methanol so much more poisonous than ethanol?

Methanol is far more poisonous than ethanol because of what the body makes of it, methanal and then methanoic acid (NCERT page 210), whose formate ion, beyond the NCERT text, builds up, stops cells using oxygen, acidifies the blood and can destroy the optic nerve (Wikipedia, “Methanol toxicity”; NIOSH). Ethanol gives ethanal and then ethanoate, which the body turns into carbon dioxide and water in the citric acid cycle (Wikipedia, “Pharmacology of ethanol”). Symptoms may take 1 to 72 hours to show, and the antidote is fomepizole or ethanol (NIOSH).

Why is methanol added to ethanol?

Methanol is added to ethanol to make it poisonous and so unfit to drink: NCERT calls ethanol mixed with methanol denatured alcohol (page 210), and even a little methanol can cause blindness (page 214). Beyond the NCERT text, methanol is the usual choice because it is toxic and boils close to ethanol, 337 K against 351 K (NCERT page 214), so the two are hard to separate by simple distillation (Wikipedia, “Denatured alcohol”). Denaturing lets industry use ethanol as a solvent or fuel without paying the heavy taxes that many countries put on drinkable alcohol (Wikipedia, “Denatured alcohol”).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Name the two enzymes in the fermentation of cane sugar and say what each does.

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   Invertase converts the sugar into glucose and fructose; zymase, found in yeast, converts them into ethanol and carbon dioxide (NCERT page 214).

2. Q2. What is denaturation of alcohol, and why is it done?

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   Mixing commercial alcohol with some copper sulphate, to give it a colour, and pyridine, a foul-smelling liquid, so that it is unfit for drinking (NCERT page 214).