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Chemistry · Practice book · Senior 5 · Unit 7 of 15

Carboxylic acids and acyl halides

Senior 5 Chemistry · 14 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 7 Carboxylic acids and acyl halides

Key unit competence: Relate the chemical and physical properties of carboxylic acids and acyl halides to their structures and uses.

Key ideas

  • Carboxylic acids contain –COOH (suffix -oic acid: methanoic, ethanoic, propanoic acid; ethanedioic acid). They are weak acids: CH3COOH ⇌ CH3COO− + H+; the carboxylate ion is stabilised by delocalisation of the negative charge over both O atoms.
  • Acid strength increases with electron-withdrawing groups near –COOH (chloroethanoic acid > ethanoic acid; trichloroethanoic acid is much stronger) and decreases with electron-donating alkyl groups (ethanoic acid is weaker than methanoic acid).
  • Physical properties: high boiling points because molecules form hydrogen-bonded dimers; small acids are water-soluble.
  • Preparation: oxidation of primary alcohols/aldehydes (reflux with acidified dichromate); hydrolysis of nitriles, esters or amides; oxidation of alkylbenzenes with KMnO4 (benzoic acid); carbonation of Grignard reagents.
  • Reactions: with metals (H2), bases (salts + water), carbonates (CO2: test); with alcohols (conc. H2SO4, reflux) → esters (esterification, reversible); with PCl5, PCl3 or SOCl2 → acyl chlorides; reduction with LiAlH4 → primary alcohols.
  • Acyl chlorides (RCOCl, ethanoyl chloride) are very reactive (C attached to two electronegative atoms): with water → acid + HCl (steamy fumes); with alcohols → esters; with ammonia → amides; with amines → N-substituted amides (nucleophilic addition–elimination). Uses: vinegar (ethanoic acid), citric and lactic acids in food, aspirin and polyester manufacture; methanoic acid in ant stings.

Common misconceptions

  • "Carboxylic acids are strong acids because they contain oxygen." No: They are weak acids; only a small fraction ionises in water.
  • "Esterification goes to completion." No: Esterification is reversible and reaches equilibrium; excess reactant or removal of water increases the yield.
  • "Acyl chlorides react slowly with water like halogenoalkanes." No: Acyl chlorides react violently with cold water, giving HCl fumes.

Worked examples

Example 1. Explain why carboxylic acids are acidic and why chloroethanoic acid is stronger than ethanoic acid.

Solution

Carboxylic acids release H+ from –COOH; the carboxylate ion is stabilised by delocalisation of the negative charge over two oxygen atoms.

In chloroethanoic acid, Cl is electronegative and withdraws electron density from the –COO− group.

This spreads and stabilises the negative charge further, so the acid ionises more: it is stronger.

Example 2. Describe the preparation of ethyl ethanoate in the laboratory.

Solution

Mix ethanol and ethanoic acid with a few drops of concentrated sulfuric acid (catalyst).

Heat under reflux for about 20 minutes.

Distil off the ester; wash with sodium carbonate solution to remove acid, then with water.

Dry with anhydrous calcium chloride and redistil (b.p. 77 °C); the ester smells fruity.

Practice questions

  1. Name CH3CH2CH2COOH.

    Show answer

    Butanoic acid.

    Common mistake: Writing propanoic acid.

  2. Write the equation for ethanoic acid with magnesium.

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    2CH3COOH + Mg → (CH3COO)2Mg + H2.

    Common mistake: Missing the 2.

  3. Why do carboxylic acids have higher boiling points than alcohols of similar mass?

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    They form two hydrogen bonds per pair (dimers).

    Common mistake: Saying they are ionic.

  4. Describe the test for a carboxylic acid.

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    Add sodium hydrogencarbonate: effervescence; gas turns limewater milky.

    Common mistake: Using Tollens' reagent.

  5. Name the catalyst for esterification.

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    Concentrated sulfuric acid.

    Common mistake: Saying nickel.

  6. Write the equation for ethanoic acid with SOCl2.

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    CH3COOH + SOCl2 → CH3COCl + SO2 + HCl.

    Common mistake: Writing chloroethane.

  7. Why is ethanoyl chloride stored away from moisture?

    Show answer

    It reacts with water to form HCl and ethanoic acid.

    Common mistake: Saying it is explosive.

  8. Explain why trichloroethanoic acid is a strong acid.

    Show answer

    Three Cl atoms strongly withdraw electrons, stabilising the anion.

    Common mistake: Saying it contains HCl.

  9. Name two uses of carboxylic acids.

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    Food preservatives (vinegar), flavourings, making esters, aspirin, polyesters (any two).

    Common mistake: Saying fuel.

  10. Name the product of ethanoyl chloride with phenol (in alkali).

    Show answer

    Phenyl ethanoate.

    Common mistake: Writing benzoic acid.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Vinegar contains:a. ethyl ethanoate and HCl
    2. Ethanoic acid + ethanol (conc. H2SO4) gives:b. –COOH
    3. The functional group of carboxylic acids is:c. ethyl ethanoate and water
    4. Ethanoyl chloride with ethanol gives:d. ethanoic acid
    Show answer

    1 → d (ethanoic acid) · 2 → c (ethyl ethanoate and water) · 3 → b (–COOH) · 4 → a (ethyl ethanoate and HCl)

    • About 5% ethanoic acid.
    • Esterification.
    • Carboxyl group.
    • Faster than acid + alcohol, not reversible.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    alkaline KMnO4, then acidifying  ·  carbon dioxide  ·  ethanoic acid and HCl  ·  oxygen
    1. Ethanoic acid reacts with sodium carbonate to give .
    2. Ethanoyl chloride reacts with water to give .
    3. Benzoic acid can be made by oxidising methylbenzene with .
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    1: carbon dioxide · 2: ethanoic acid and HCl · 3: alkaline KMnO4, then acidifying

    • Test for –COOH.
    • Vigorous hydrolysis.
    • Side-chain oxidation.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Carboxylic acids are prepared by oxidising primary alcohols under:a. ethanol
    2. Reduction of ethanoic acid with LiAlH4 gives:b. reflux with excess acidified dichromate
    3. Carboxylic acids have high boiling points because they form:c. Chloroethanoic acid
    4. Which acid is stronger?d. hydrogen-bonded dimers
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    1 → b (reflux with excess acidified dichromate) · 2 → a (ethanol) · 3 → d (hydrogen-bonded dimers) · 4 → c (Chloroethanoic acid)

    • Complete oxidation.
    • Strong reducing agent.
    • Two molecules pair up.
    • Cl withdraws electrons.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    ethanenitrile  ·  ethanamide  ·  sour milk and tired muscles  ·  ant and nettle stings
    1. Lactic acid is found in .
    2. Ethanoyl chloride reacts with ammonia to give .
    3. Methanoic acid is found in .
    Show answer

    1: sour milk and tired muscles · 2: ethanamide · 3: ant and nettle stings

    • 2-hydroxypropanoic acid.
    • Addition–elimination.
    • Formic acid.

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