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K324 / 2027
Organic Chemistry

Lesson 4 of 9

Alcohols: identify -OH and follow ethanol

Structure connects preparation, combustion and oxidation.

O-Level 6092 (2026) / SEC G3 K324 (2027)

  • I can recognise alcohol series.Syllabus

    Syllabus K324, 11.3(a). Recognise alcohol series

  • I can describe alcohol reactions.Syllabus

    Syllabus K324, 11.3(c). Describe alcohol reactions

  • I can draw and name alcohols.Syllabus

    Syllabus K324, 11.3(b). Draw and name alcohols

  • I can describe ethanol production.Syllabus

    Syllabus K324, 11.3(d). Describe ethanol production

  • I can explain ethanol oxidation.Syllabus

    Syllabus K324, 11.3(h). Explain ethanol oxidation

Alcohols: identify -OH and follow ethanol

Structure connects preparation, combustion and oxidation.

Make a guess. You are not marked.

Yeast is fermenting a glucose solution at about 37 degrees C. The mixture is heated to 80 degrees C. What happens to fermentation?

  1. It stops, as the yeast enzymes are denatured
  2. It carries on at the same rate as before
  3. It speeds up a lot, as particles move faster
Show the answer

It stops, as the yeast enzymes are denatured

The enzymes in yeast are denatured at high temperature, so they can no longer catalyse the reaction. Fermentation needs warm, not hot, conditions.

Alcohols form a homologous series containing the -OH functional group. The group is covalently attached within a molecule; it is not a free hydroxide ion, so the presence of -OH does not make ethanol an alkali.

Unbranched C1-C3 alcohol structures
NameCondensed structure
MethanolCH3-OH
EthanolCH3-CH2-OH
Propan-1-olCH3-CH2-CH2-OH
Propan-2-olCH3-CH(OH)-CH3
Pure only
Pure: C4 alcohol connectivities
StructureCondensed representation
Butan-1-olCH3-CH2-CH2-CH2-OH
Butan-2-olCH3-CH(OH)-CH2-CH3
Branched, terminal -OH(CH3)2CH-CH2-OH
Branched, central -OH(CH3)3C-OH

Expand condensed structures by showing each bond to H separately and checking C has four bonds and O two. Moving -OH along the carbon chain changes the structure, even when the carbon skeleton remains unbranched.

Displayed methanol

Complete displayed structure with 1 carbon, 4 hydrogen and 1 oxygen atoms: CH4O. Every bond and hydrogen is shown.

CH4O. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.

Displayed ethanol

Complete displayed structure with 2 carbon, 6 hydrogen and 1 oxygen atoms: C2H6O. Every bond and hydrogen is shown.

C2H6O. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.

Displayed propan-1-ol

Complete displayed structure with 3 carbon, 8 hydrogen and 1 oxygen atoms: C3H8O. Every bond and hydrogen is shown.

C3H8O. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.

Displayed propan-2-ol

Complete displayed structure with 3 carbon, 8 hydrogen and 1 oxygen atoms: C3H8O. Every bond and hydrogen is shown.

C3H8O. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.
Pure only

Displayed butan-1-ol

Complete displayed structure with 4 carbon, 10 hydrogen and 1 oxygen atoms: C4H10O. Every bond and hydrogen is shown.

C4H10O. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.
Pure only

Displayed butan-2-ol

Complete displayed structure with 4 carbon, 10 hydrogen and 1 oxygen atoms: C4H10O. Every bond and hydrogen is shown.

C4H10O. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.
Pure only

Displayed branched C4 alcohol: terminal OH

Complete displayed structure with 4 carbon, 10 hydrogen and 1 oxygen atoms: C4H10O. Every bond and hydrogen is shown.

C4H10O. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.
Pure only

Displayed branched C4 alcohol: central OH

Complete displayed structure with 4 carbon, 10 hydrogen and 1 oxygen atoms: C4H10O. Every bond and hydrogen is shown.

(CH3)3C-OH. Three methyl groups and oxygen use all four bonds of the central carbon; do not add a hydrogen to that carbon.
Make ethanol by fermentation
  1. Feedstock

    Glucose solution, obtainable from sugarcane sugars.

  2. Conditions

    Add yeast, which supplies the enzymes. Keep the mixture at about 37 °C, warm but not hot. Keep oxygen out, for example with an air lock. If the mixture gets too hot, the enzymes in yeast are denatured and fermentation stops.

  3. Reaction

    C6H12O6(aq) -> 2C2H5OH(aq) + 2CO2(g).

  4. Recovery

    Fractional distillation separates ethanol-rich liquid from the fermentation mixture.

Ethanol burns in sufficient oxygen: C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O(l), after cooling. It can also be oxidised to ethanoic acid by atmospheric oxygen, or by warming with acidified potassium manganate(VII). In shorthand, CH3CH2OH + 2[O] -> CH3COOH + H2O; [O] denotes oxygen supplied by an oxidising agent.

Use ethanol as the representative alcohol-to-acid conversion here. Do not assume that every possible alcohol structure gives a carboxylic acid: the position of -OH matters, and detailed alternative oxidation pathways are beyond these Secondary outcomes.

Pure only

Pure: ethanol can alternatively be made by catalytic addition of steam to ethene under the conditions given on the alkene page. Fermentation uses renewable plant sugars but gives a dilute mixture; the ethene route can operate continuously with a more concentrated product but commonly uses a fossil-derived feedstock.

Check your understanding

Fermentation is kept free of oxygen, yet turning ethanol into ethanoic acid needs oxygen. Why?

  1. They are different changes: fermentation makes ethanol, and oxidation turns it into acid.
  2. Oxygen would kill the yeast at once, but ethanoic acid forms without any yeast.
  3. They are the same change: oxygen is kept out at first and added at the end.
Show the answer

They are different changes: fermentation makes ethanol, and oxidation turns it into acid.

Yeast ferments glucose to ethanol without oxygen. With oxygen (or another oxidising agent), ethanol is oxidised to ethanoic acid, which is why wine left open turns sour.

Exam-style question

A student makes ethanol from a glucose solution. State the conditions needed, and explain why the flask is not heated to 100 °C. [3]

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