Alcohols
The −OH group, manufacture of ethanol by fermentation and hydration, and combustion.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 11.6.1Describe the manufacture of ethanol by fermentation and by catalytic addition of steam to ethene.
5 minute read
Alcohols
Alcohols are defined by a single functional group, the −OH group, and the whole topic at this level really centres on ethanol: how it's made two different ways, and what it does when it burns or is oxidised.
Two routes to ethanol
- Fermentation: yeast enzymes convert glucose into ethanol and carbon dioxide, glucose → ethanol + carbon dioxide, at 25–35 °C (warm enough for the enzymes to work quickly, not so hot that they're destroyed), in the absence of oxygen (anaerobic — oxygen would let the yeast respire aerobically instead, or let the ethanol oxidise). It uses a renewable raw material (plant sugars) but gives a dilute, impure product that needs distilling to concentrate.
- Catalytic addition of steam to ethene: ethene + steam ⇌ ethanol, using a phosphoric acid catalyst at about 300 °C and high pressure. It is fast, continuous, and gives a pure, concentrated product directly — but ethene comes from cracking crude oil, a non-renewable resource.
Reactions of ethanol
Ethanol burns in air (combustion) to give carbon dioxide and water, releasing energy — this is why it's used as a biofuel. It can also be oxidised (by an oxidising agent, or slowly by microorganisms in the air) to ethanoic acid, the carboxylic acid responsible for the sour taste of wine that has been left open too long.
Think of it like this
Fermentation vs. the ethene route is a classic slow-artisanal-bakery vs. fast-factory trade-off: fermentation is slower and needs simple equipment but uses a renewable ingredient, while the ethene route is quick and industrial but depends on crude oil.
Worked examples
Method, step by step
State the conditions needed for the industrial production of ethanol from ethene and steam, and write the equation.
- 1Catalyst: phosphoric acid.
- 2Conditions: about 300 °C and high pressure.
- 3Equation: C₂H₄ + H₂O ⇌ C₂H₅OH.
Phosphoric acid catalyst, ~300 °C, high pressure: C₂H₄ + H₂O ⇌ C₂H₅OH.
Common misconceptions
- Thinking fermentation happens best with lots of oxygen around — it specifically needs the *absence* of oxygen (anaerobic conditions); oxygen would let other reactions (including oxidation to ethanoic acid) compete.
- Assuming both manufacturing routes give the same product purity — fermentation gives a dilute aqueous solution that must be distilled, while the ethene/steam route gives pure ethanol directly.
In the exam
- Comparison questions on the two ethanol-manufacture routes expect a genuine trade-off — one advantage AND one disadvantage of each, not just a fact about one method.
- Remember the reversible-arrow (⇌) in the ethene + steam equation — it is an equilibrium reaction, unlike fermentation which is treated as one-way.