Topic 4 of 9

Alcohols: identify -OH and follow ethanol

Structure connects preparation, combustion and oxidation.

O-Level 6092 (2026) / SEC G3 K324 (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
Name	Condensed structure
Methanol	CH₃-OH
Ethanol	CH₃-CH₂-OH
Propan-1-ol	CH₃-CH₂-CH₂-OH
Propan-2-ol	CH₃-CH(OH)-CH₃

Pure only

Pure: C4 alcohol connectivities
Structure	Condensed representation
Butan-1-ol	CH₃-CH₂-CH₂-CH₂-OH
Butan-2-ol	CH₃-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

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

Displayed ethanol

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

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

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

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

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

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

(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

Feedstock
Glucose solution, obtainable from sugarcane sugars.
Conditions
Yeast supplies enzymes; keep warm and exclude oxygen. Excessive heat damages enzymes.
Reaction
C₆H₁₂O₆(aq) -> 2C₂H₅OH(aq) + 2CO₂(g).
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 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.