Carboxylic acids
Cambridge A-Level Chemistry (9701) · Unit 33: Carboxylic acids and derivatives (A Level) · 8 flashcards
Carboxylic acids is topic 33.1 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 33 — Carboxylic acids and derivatives (A Level) , alongside Esters and Acyl chlorides. In one line: Benzoic acid is produced by the oxidation of alkylbenzenes, such as toluene, using hot alkaline potassium manganate(VII) (KMnO₄) as the oxidising agent. The alkyl group is oxidised to the carboxyl group.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 8 flashcards — 3 definitions, 4 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
How is benzoic acid commonly produced in the lab
Benzoic acid is produced by the oxidation of alkylbenzenes, such as toluene, using hot alkaline potassium manganate(VII) (KMnO₄) as the oxidising agent. The alkyl group is oxidised to the carboxyl group.
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Recall how benzoic acid is made: (a) by reacting an alkylbenzene, for example methylbenzene, with hot alkaline KMnO₄ and then with dilute acid.
- Describe how carboxylic acids are converted into acyl chlorides using PCl₃ with heat, PCl₅, or SOCl₂.
- Recognise that certain carboxylic acids undergo further oxidation: (a) methanoic acid, HCOOH, gives carbon dioxide and water with acidified K₂Cr₂O₇ or acidified KMnO₄, Tollens' reagent or Fehling's reagent; (b) ethanedioic acid, HOOCCOOH, gives carbon dioxide with warm acidified KMnO₄.
- Describe and explain how the acidity of alcohols, phenols and carboxylic acids compares.
- Describe and explain how the acidities of carboxylic acids with chlorine substituents compare.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
How is benzoic acid commonly produced in the lab?
Benzoic acid is produced by the oxidation of alkylbenzenes, such as toluene, using hot alkaline potassium manganate(VII) (KMnO₄) as the oxidising agent. The alkyl group is oxidised to the carboxyl group.
What products are formed when a carboxylic acid reacts with PCl₃ and heat?
A carboxylic acid heated with PCl₃ forms an acyl chloride and phosphonic acid, H₃PO₃ (also called phosphorous acid). No HCl is formed in this reaction. Equation: 3RCOOH + PCl₃ -> 3RCOCl + H₃PO₃.
Write the balanced equation for the oxidation of methanoic acid (HCOOH) with acidified potassium permanganate (KMnO₄).
2KMnO₄ + 5HCOOH + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 5CO₂ + 8H₂O. Methanoic acid is oxidised to carbon dioxide and water.
Describe the reaction of ethanedioic acid (HOOCCOOH) with warm acidified KMnO₄.
Ethanedioic acid is oxidised to carbon dioxide by warm, acidified potassium permanganate (KMnO₄). The purple colour of the KMnO₄ decolourises as MnO₄⁻ is reduced to Mn²⁺.
Explain the relative acidities of carboxylic acids, phenols, and alcohols.
Carboxylic acids are generally more acidic than phenols, which are more acidic than alcohols. This is because the negative charge on the carboxylate ion (RCOO⁻) is delocalised over two oxygen atoms, making it more stable than the phenoxide ion (delocalised over the benzene ring). Alkoxide ions are the least stable.
How does the presence of chlorine substituents affect the acidity of carboxylic acids? Explain why.
Chlorine substituents increase the acidity of carboxylic acids. This is because chlorine is electron-withdrawing, which stabilises the carboxylate anion by dispersing the negative charge, making the acid more likely to donate a proton.
Write the general equation for the reaction of a carboxylic acid with thionyl chloride (SOCl₂).
RCOOH + SOCl₂ → RCOCl + SO₂ + HCl. This reaction produces an acyl chloride, sulfur dioxide, and hydrogen chloride gas.
What is the product formed when ethanoic acid reacts with phosphorus pentachloride (PCl₅)?
The reaction of ethanoic acid (CH₃COOH) with phosphorus pentachloride (PCl₅) forms ethanoyl chloride (CH₃COCl), phosphorus oxychloride (POCl₃), and hydrochloric acid (HCl).
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