Ethanol: preparation, properties and uses
Ethanol (C₂H₅OH) is made by fermenting molasses with yeast. It reacts with sodium, burns, oxidises to ethanoic acid and forms esters.
Manufacture from molasses
**Molasses** is the dark, thick liquid left after sugar crystals are taken out of sugarcane juice. It still contains about 30% sucrose.
- 1Dilution: molasses is diluted with water to bring the sugar down to about 8 to 10%.
- 2Nitrogen source: if molasses lacks nitrogen, ammonium sulphate or ammonium phosphate is added as food for yeast.
- 3Adding yeast: yeast is added and the mixture is kept at about 303 K for a few days. Yeast enzymes turn the sugar into ethanol.
- 4★C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose)Invertase
- 5★C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂↑Zymase
- 6Distillation of wash: the fermented liquid (wash) is fractionally distilled. The main fraction is rectified spirit (95.5% ethanol).
- 7Absolute alcohol: rectified spirit is heated under reflux with quicklime (CaO) for 5 to 6 hours, then distilled to give 100% ethanol.
★ key step
Physical properties
- A colourless liquid with a pleasant smell and a burning taste.
- Boils at 351 K (78 °C), lower than water, so it is volatile.
- Mixes with water in all proportions.
- Neutral to litmus.
- A good solvent: it dissolves many organic compounds that water cannot.
Chemical properties (each equation is balanced)
- 1★C₂H₅OH → C₂H₄ + H₂OExcess conc. H₂SO₄, 443 K · Dehydration: ethanol loses water to give ethene
- 22C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑With sodium: sodium ethoxide and hydrogen gas
- 3★CH₃CH₂OH + 2[O] → CH₃COOH + H₂OAlkaline KMnO₄ or acidified K₂Cr₂O₇ · Oxidation: first ethanal, then ethanoic acid
- 4CH₃CH₂OH → CH₃CHO + H₂Hot copper, 573 K · Dehydrogenation: gives ethanal
- 5★C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂OConc. H₂SO₄ · Esterification: ethyl ethanoate, a fruity-smelling ester
- 6C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + heatCombustion: burns with a clean, pale blue flame
★ key step
The oxidation ladder
Acidified potassium dichromate turns from orange to green when it oxidises ethanol. This colour change is a test for an alcohol.
Uses of ethanol
- Medicine: an antiseptic (in hand sanitisers) and a solvent in tonics, cough syrups and tinctures.
- Fuel: mixed with petrol (ethanol-blended petrol, also called power alcohol).
- Solvent for paints, varnishes, dyes, perfumes and inks.
- Antifreeze in vehicle radiators, and a preservative for biological specimens.
- Methylated spirit (ethanol with a little poisonous methanol) and denatured spirit (ethanol with pyridine) are made unfit to drink and are used in industry.
- It is the alcohol in alcoholic drinks. Drinking it harms the liver and brain, and illicit liquor mixed with methanol can cause blindness or death.
Ethanol is the alcohol in hand sanitisers, and Indian sugar mills make it from molasses to blend with petrol. A by-product of the sugar industry thus becomes a fuel.
2 marks: “What is absolute alcohol? How is it obtained?” 100% ethanol. Rectified spirit (95.5%) is heated under reflux with quicklime for 5 to 6 hours and then distilled.
4 marks: “Write the equations for the action of ethanol on (i) sodium (ii) excess conc. H₂SO₄ at 443 K. Name the products.” 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑ (sodium ethoxide, hydrogen); C₂H₅OH → C₂H₄ + H₂O (ethene). State the conditions.
7 marks: “How is ethanol manufactured from molasses? Write any three chemical properties of ethanol.” Dilution, nitrogen source, yeast at 303 K, invertase and zymase equations, distillation to rectified spirit and absolute alcohol. Then dehydration, reaction with sodium and oxidation, with equations.
Making ethanol: 'Do Not Yell, Distil': Dilute, add Nitrogen food, add Yeast, Distil. Invertase comes In first; Zymase finishes (Z is the last letter).