Section 1 of 5The Alcohol Homologous Series
Alcohols are a homologous series of organic compounds that all contain the hydroxyl functional group, -OH, bonded to a carbon atom.
General formula
- General formula Alcohols have the general formula CₙH₂ₙ₊₁OH, so each member differs from the next by a CH₂ unit.
Functional group
- Functional group The -OH group gives alcohols their characteristic properties and is written at the end of the formula, as in C₂H₅OH.
Naming
- Naming The first four members are methanol, ethanol, propanol and butanol, named by the number of carbon atoms in the chain.
Solubility
- Solubility Short-chain alcohols such as methanol and ethanol mix with water, but solubility falls as the carbon chain gets longer.
Section 2 of 5Making Ethanol
Ethanol is made by two routes, which differ in raw materials, conditions and the purity of the product.
Fermentation
- Fermentation Sugar solution is broken down by yeast in the absence of oxygen, giving a dilute solution that needs separating.
Hydration
- Hydration Ethene from crude oil reacts with steam over a catalyst, giving a pure product in a continuous process.
Renewability
- Renewability Fermentation uses plant sugars, which can be replaced each season, whereas hydration relies on crude oil.
| Feature | Fermentation | Hydration of ethene |
|---|---|---|
| Raw materials | Glucose solution and yeast | Ethene and steam |
| Equation | C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g) | C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g) |
| Conditions | About 30 °C, without oxygen | About 300 °C, 60–70 atm, phosphoric acid catalyst |
| Type of process | Batch, from renewable sugar | Continuous, from crude oil |
Section 3 of 5Burning and Uses
The properties of ethanol explain its many uses.
1
Ethanol burns completely in plenty of oxygen to form carbon dioxide and water: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l).
2
Because it burns cleanly and releases energy, ethanol is used as a fuel and is blended with petrol in some countries.
3
Ethanol dissolves many substances that do not dissolve in water, so it is used as a solvent in perfumes and cleaning products.
4
Ethanol is the alcohol found in drinks, and its concentration is controlled by the fermentation conditions and by distillation.
Section 4 of 5Worked Example
Question
- Calculate the mass of carbon dioxide produced by the complete combustion of 46 g of ethanol, C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l).
- Use C = 12, H = 1 and O = 16.
- Mr of C₂H₅OH = (2 × 12) + (6 × 1) + 16 = 46
- Moles of ethanol = 46 ÷ 46 = 1 mol
- The equation shows 1 mol of ethanol gives 2 mol of CO₂
- Mr of CO₂ = 12 + (2 × 16) = 44, so the mass of CO₂ = 2 × 44 = 88 g
Answer 88 g of carbon dioxide is produced.
Section 5 of 5Exam Checklist & Tips
7.2.4 – Key Points for the Exam
- Homologous series A family of compounds with the same functional group and a general formula, differing by CH₂ between members.
- Hydroxyl group The -OH functional group that defines alcohols.
- Fermentation The anaerobic breakdown of glucose by yeast to produce ethanol and carbon dioxide.
- Hydration The addition of water across a C=C double bond, here producing ethanol from ethene.
- Anaerobic Taking place without oxygen being present.
Common Mistakes
- Calling the -OH group a hydroxide ion; alcohols contain covalently bonded -OH and are not alkaline.
- Saying fermentation needs oxygen; yeast respires anaerobically, so air must be excluded.
- Writing carbon monoxide as the main product of ethanol combustion;
- with plenty of oxygen it forms carbon dioxide and water.
- Forgetting that the hydration of ethene needs a catalyst, high temperature and high pressure.