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Organic Chemistry

Topic 4 of 7

Alcohols: identify -OH and follow ethanol

Structure connects preparation, combustion and oxidation.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

Alcohols: identify -OH and follow ethanol

Structure connects preparation, combustion and oxidation.

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

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.
Make ethanol by fermentation
  1. Feedstock

    Glucose solution, obtainable from sugarcane sugars.

  2. Conditions

    Yeast supplies enzymes; keep warm and exclude oxygen. Excessive heat damages enzymes.

  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.

Check your understandingWhy does fermentation exclude oxygen while oxidation to ethanoic acid involves an oxidising agent?Think it through, then reveal the answer
They are different conversions. Fermentation uses glucose to make ethanol under oxygen-limited conditions; oxidation changes ethanol into ethanoic acid by supplying oxygen through air or another oxidant.