AlcoholsCambridge IGCSE Chemistry: Revision notes
Section 1
What Defines an Alcohol?
Alcohols are a homologous series with the general formula , defined by the -OH (hydroxyl) functional group.
You should be able to name and draw the structural and displayed formulae of unbranched alcohols up to four carbons: propan-1-ol, propan-2-ol, butan-1-ol and butan-2-ol — the number shows which carbon the -OH group is attached to.
Section 2
How Is Ethanol Manufactured?
Ethanol, the most important alcohol at this level, can be manufactured by two different routes:
- Fermentation — aqueous glucose is fermented at 25–35 °C, in the presence of yeast, and in the absence of oxygen
- Catalytic addition of steam to ethene — carried out at 300 °C and 6000 kPa/60 atm, in the presence of an acid catalyst
Both routes produce the same product, ethanol, but by very different chemistry.
Fermentation: glucose -yeast, no oxygen-> ethanol + carbon dioxide. Catalytic addition: ethene + steam -acid catalyst, 300C, 60atm-> ethanol.
Section 3
Fermentation vs Catalytic Addition — Which Is Better?
Each manufacturing route has advantages and disadvantages:
| Fermentation | Catalytic addition of steam | |
|---|---|---|
| Raw material | Renewable (glucose from plants) | Non-renewable (ethene from crude oil) |
| Rate of reaction | Slow (batch process) | Fast (continuous process) |
| Purity of product | Produces a dilute, impure solution (needs further purification) | Produces a pure, concentrated product directly |
| Energy demand | Low temperature needed | High temperature and pressure needed (energy-intensive) |
The choice between the two often depends on the cost and availability of raw materials, and how quickly and purely the ethanol is needed.
When asked to 'evaluate' or compare these two methods, always weigh renewability of the raw material against reaction rate and product purity — exam mark schemes reward both sides of the comparison.
Section 4
How Does Ethanol Behave as a Fuel and Solvent?
Ethanol undergoes combustion, releasing energy — this is why it is used as a fuel (sometimes blended with petrol). It is also widely used as a solvent, for example in perfumes, inks and cleaning products, because it can dissolve substances that water cannot.
Must Know
- Alcohols have general formula with the -OH functional group
- Ethanol is made by fermentation (glucose, yeast, 25-35°C, no oxygen) or catalytic addition of steam to ethene (acid catalyst, 300°C, 60 atm)
- Fermentation uses a renewable raw material but is slower and gives a dilute/impure product
- Catalytic addition uses a non-renewable raw material but is fast and gives a pure product directly
- Ethanol combusts to release energy and is used as a fuel and a solvent
- Name unbranched alcohols with numbers showing the -OH position: propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol
That's the notes covered.
Carry on to the next subtopic.