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4.8.2Unit 4. Rates, Equilibria and Further Organic Chemistry

Preparation of Carboxylic Acids — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 4 parts Lesson 46 of 56 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
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Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
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Part 1 of 4Oxidation Routes

4.8.2 Preparation of Carboxylic Acids

Primary alcohols and aldehydes can be oxidised to carboxylic acids.

Reagent
  • Acidified potassium dichromate(VI), K₂Cr₂O₇ with dilute H₂SO₄, is the oxidising agent used.
Conditions
  • The mixture is heated under reflux with excess oxidising agent so that oxidation continues beyond the aldehyde stage.
Equation
  • CH₃CH₂OH + 2[O] → CH₃COOH + H₂O, with [O] representing oxygen from the oxidising agent.
Colour change
  • The orange dichromate(VI) ion is reduced to green chromium(III) ions as the reaction proceeds.
Why reflux
  • Reflux allows the mixture to boil for a long time without losing volatile reactants, so the oxidation is driven to completion.

Part 2 of 4Hydrolysis of Nitriles

A nitrile contains the C≡N group and can be converted into an acid.

1
A halogenoalkane such as bromoethane reacts with KCN in ethanol to make a nitrile, adding one carbon to the chain.
2
The nitrile is then heated under reflux with dilute hydrochloric acid to hydrolyse it.
3
The overall change is CH₃CH₂CN + 2H₂O + HCl → CH₃CH₂COOH + NH₄Cl.
4
If alkali is used instead, the carboxylate salt forms and the mixture must be acidified to release the free acid.

Part 3 of 4Choosing a Route

Choosing a Route
Same carbon number
  • Oxidation of a primary alcohol gives an acid with the same number of carbon atoms.
Longer chain
  • The nitrile route gives an acid with one more carbon than the starting halogenoalkane.
Distillation versus reflux
  • Distilling off the product gives the aldehyde, whereas reflux gives the carboxylic acid.
Tertiary alcohols
  • Tertiary alcohols resist oxidation, so they cannot be used to make carboxylic acids this way.

Part 4 of 4Exam Checklist & Tips

4.8.2 – Key Points for the Exam

  • Reflux Heating with a condenser so that vapours return to the flask.
  • Oxidation Loss of electrons or gain of oxygen.
  • Nitrile Organic compound containing the C≡N group.
  • Hydrolysis Breakdown of a compound by reaction with water.

Common Mistakes

  • Distilling rather than refluxing when an acid is required.
  • Forgetting the acid used in the hydrolysis of the nitrile.
  • Writing the oxidation equation without balancing water.
  • Using a tertiary alcohol as the starting material.

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