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4.4.2Core Organic Chemistry

Reactions of Alcohols — OCR A Level Chemistry A H432 Revision Notes & Topical Questions

5 min read 5 parts Lesson 18 of 29

📚 OCR A Level Chemistry A H432 Hub / Topical Revision
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Part 1 of 5Combustion and Oxidation

4.4.2 Reactions of Alcohols

The oxygen added to the alcohol is written as [O] when balancing organic oxidation equations.

Combustion
  • Alcohols burn in excess oxygen to give carbon dioxide and water, as in C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
Oxidising agent
  • Acidified potassium dichromate(VI), K₂Cr₂O₇ with dilute H₂SO₄, oxidises primary and secondary alcohols.
Colour change
  • The orange dichromate ion, Cr₂O₇²⁻, is reduced to the green chromium(III) ion, Cr³⁺.
Tertiary
  • Tertiary alcohols are not oxidised under these conditions, so the mixture stays orange.

Part 2 of 5Products of Oxidation

Products of Oxidation
Primary to aldehyde
  • Gentle heating with distillation as the product forms gives the aldehyde: CH₃CH₂OH + [O] → CH₃CHO + H₂O.
Primary to acid
  • Heating under reflux with excess oxidant gives the carboxylic acid: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.
Secondary to ketone
  • A secondary alcohol is oxidised to a ketone: CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O.
No further oxidation
  • The ketone cannot be oxidised further under these conditions, so reflux makes no difference.
AlcoholMethodProduct
PrimaryDistillation, limited oxidantAldehyde
PrimaryReflux, excess oxidantCarboxylic acid
SecondaryReflux or heatingKetone
TertiaryAnyNo oxidation

Part 3 of 5Dehydration and Substitution

1
Dehydration is elimination of water from an alcohol, heating it with concentrated sulfuric or phosphoric acid as a catalyst.
2
Ethanol forms ethene by this route: C₂H₅OH → C₂H₄ + H₂O.
3
This can give an alternative way to make alkenes from renewable alcohols instead of crude oil.
4
Substitution of the –OH group occurs when an alcohol is heated with a hydrogen halide such as HBr, generated in situ from NaBr and H₂SO₄.
5
Ethanol forms bromoethane in this way: C₂H₅OH + HBr → C₂H₅Br + H₂O.

Part 4 of 5Practical Points

Distillation
  • The apparatus removes the aldehyde as it forms, since it boils at a lower temperature than the alcohol.
Reflux
  • Vapours condense and return to the flask, so the reaction mixture can be heated for a long time without loss.
Heating
  • Flammable liquids are heated using a water bath or electric heater, not a naked flame.
Anti-bumping granules
  • Granules in the flask let the liquid boil smoothly and prevent violent bumping.

Part 5 of 5Exam Checklist & Tips

4.4.2 – Key Points for the Exam

  • Oxidation An increase in oxygen content or loss of hydrogen from an organic molecule.
  • Dehydration The removal of water from a molecule, forming a double bond.
  • Reflux Heating a mixture with a condenser fitted vertically so vapours return to the flask.
  • Distillation Separating a liquid from a mixture by evaporation followed by condensation.

Common Mistakes

  • Stating that the dichromate turns green to orange on oxidation of an alcohol; it goes orange to green.
  • Using distillation when the aim is to make a carboxylic acid; reflux is needed to ensure full oxidation.
  • Suggesting a tertiary alcohol forms a ketone; it resists oxidation.
  • Forgetting that dehydration is an elimination reaction and gives an alkene.

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