DONATSciences and Maths
Chemistry · Practice book · Senior 5 · Unit 8 of 15

Esters, acid anhydrides, amides and nitriles

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

Unit 8 Esters, acid anhydrides, amides and nitriles

Key unit competence: Relate the structures and properties of esters, acid anhydrides, amides and nitriles to their uses.

Key ideas

  • Esters (RCOOR'): named alkyl alkanoate (ethyl ethanoate). Sweet, fruity smells; volatile (no hydrogen bonding between molecules); used in flavourings, perfumes and solvents. Fats and oils are esters of glycerol with long-chain fatty acids (triglycerides).
  • Ester reactions: acid hydrolysis (reversible) → acid + alcohol; alkaline hydrolysis (irreversible) → carboxylate salt + alcohol. Saponification: fat + NaOH → glycerol + sodium salts of fatty acids (soap). Soap molecules have a polar 'head' (–COO−) and non-polar 'tail' that surround grease (micelles).
  • Acid anhydrides ((RCO)2O, ethanoic anhydride) are made from acyl chloride + carboxylate or by dehydration of acids. They react like acyl chlorides but more gently (by-product is a carboxylic acid, not HCl): with water → acid; with alcohols/phenols → esters (aspirin from salicylic acid); with ammonia → amides.
  • Amides (RCONH2, ethanamide): high melting points (hydrogen bonding), almost neutral. Prepared from acyl chlorides/anhydrides with ammonia or by heating ammonium salts. Reactions: hydrolysis (acid or alkali) → acid/salt + ammonia; dehydration with P4O10 → nitrile; Hofmann degradation (Br2/NaOH) → amine with one less carbon; reduction (LiAlH4) → amine.
  • Nitriles (R–C≡N, ethanenitrile): prepared from halogenoalkanes + KCN (ethanol) or by dehydrating amides. Hydrolysis → carboxylic acid; reduction (LiAlH4 or H2/Ni) → primary amine. Useful for adding a carbon atom.
  • Uses: esters (flavours, solvents, polyesters such as PET), anhydrides (aspirin and paracetamol manufacture, cellulose acetate), amides (nylon, proteins have amide/peptide links, paracetamol), nitriles (acrylic fibres from propenenitrile, synthesis intermediates).

Common misconceptions

  • "Alkaline hydrolysis of an ester gives a carboxylic acid." No: It gives the carboxylate SALT; acidifying afterwards gives the acid.
  • "Amides are basic like amines." No: Amides are almost neutral because the N lone pair is delocalised into the C=O group.
  • "Soap and detergent are the same." No: Soaps are carboxylate salts (form scum in hard water); synthetic detergents are usually sulfonates that do not.

Worked examples

Example 1. Describe the manufacture of soap from palm oil and explain how soap cleans.

Solution

Boil palm oil (a triglyceride) with concentrated sodium hydroxide: fat + 3NaOH → glycerol + 3RCOONa (saponification).

Add salt to precipitate the soap (salting out), then filter and mould it.

Soap anions have an ionic –COO− head (hydrophilic) and a long hydrocarbon tail (hydrophobic).

Tails dissolve in grease, heads face the water: micelles form and grease is washed away.

Example 2. Write equations for the conversion of ethanenitrile into (a) ethanoic acid and (b) ethylamine.

Solution

(a) Acid hydrolysis under reflux: CH3CN + 2H2O + H+ → CH3COOH + NH4+.

(b) Reduction with LiAlH4 in dry ether (or H2/Ni): CH3CN + 4[H] → CH3CH2NH2.

Both keep the same number of carbon atoms as the nitrile.

Practice questions

  1. Name CH3CH2COOCH2CH3.

    Show answer

    Ethyl propanoate.

    Common mistake: Writing propyl ethanoate.

  2. Write the equation for acid hydrolysis of ethyl ethanoate.

    Show answer

    CH3COOC2H5 + H2O ⇌ CH3COOH + C2H5OH.

    Common mistake: Using a one-way arrow.

  3. What is a triglyceride?

    Show answer

    An ester of glycerol with three fatty-acid molecules.

    Common mistake: Saying a polymer of glucose.

  4. Name the product of ethanoic anhydride with ammonia.

    Show answer

    Ethanamide (and ammonium ethanoate).

    Common mistake: Writing ethylamine.

  5. Write the formula of propanenitrile.

    Show answer

    CH3CH2CN.

    Common mistake: Writing CH3CH2CH2CN.

  6. Why does soap form scum in hard water?

    Show answer

    Ca2+/Mg2+ form insoluble calcium/magnesium salts of fatty acids.

    Common mistake: Saying soap reacts with chlorine.

  7. Give one use each of esters and amides.

    Show answer

    Esters: flavourings/solvents/polyesters; amides: nylon/paracetamol.

    Common mistake: Swapping the uses.

  8. What is Hofmann degradation used for?

    Show answer

    Converting an amide into an amine with one fewer carbon.

    Common mistake: Saying it adds carbon.

  9. Write the dehydration of ethanamide.

    Show answer

    CH3CONH2 → CH3CN + H2O (P4O10).

    Common mistake: Writing ethylamine.

  10. Explain why esters have lower boiling points than carboxylic acids of similar mass.

    Show answer

    Esters cannot form hydrogen bonds between their molecules.

    Common mistake: Saying esters are ionic.

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. Hydrolysis of a nitrile with acid gives:a. potassium cyanide
    2. Fats and oils are:b. triesters of glycerol
    3. Ethanenitrile is prepared from chloromethane and:c. a carboxylic acid
    4. Alkaline hydrolysis of fats to make soap is called:d. saponification
    Show answer

    1 → c (a carboxylic acid) · 2 → b (triesters of glycerol) · 3 → a (potassium cyanide) · 4 → d (saponification)

    • –CN → –COOH.
    • Triglycerides.
    • Adds one carbon.
    • Fat + NaOH.

  2. Drag and drop

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

    glycerol  ·  –C≡N  ·  ethanal  ·  ethanoic acid
    1. The products of saponification are soap and .
    2. The functional group of nitriles is .
    3. Ethanoic anhydride reacts with water to give .
    Show answer

    1: glycerol · 2: –C≡N · 3: ethanoic acid

    • Propane-1,2,3-triol.
    • Cyano group.
    • Two molecules of acid.

  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. PET (polyester) bottles contain which linkage?a. hydrogen bonding
    2. Esters typically smell:b. fruity
    3. Soap removes grease because its molecules have:c. a polar head and a non-polar tail
    4. Amides have high melting points because of:d. Ester
    Show answer

    1 → d (Ester) · 2 → b (fruity) · 3 → c (a polar head and a non-polar tail) · 4 → a (hydrogen bonding)

    • Polyethylene terephthalate.
    • Used in flavourings.
    • They form micelles around grease.
    • N–H...O=C hydrogen bonds.

  4. Drag and drop

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

    a nitrile  ·  ethanoic anhydride  ·  methyl ethanoate  ·  ethyl ethanoate
    1. Dehydration of an amide with P4O10 gives .
    2. The ester formed from methanol and ethanoic acid is .
    3. Aspirin is made from salicylic acid and .
    Show answer

    1: a nitrile · 2: methyl ethanoate · 3: ethanoic anhydride

    • Removes water.
    • Alcohol part first.
    • Acetylation of the phenol group.

Take the Unit 8 quiz → · Back to contents