All subjects · Chemistry · WCH14 (Unit 4)

Acyl chlorides, esters and polyesters

Acyl chlorides are very reactive derivatives of carboxylic acids. They make esters and amides quickly and completely. Esters can be hydrolysed back, and diacids with diols make polyesters.

Naming (spec 15.13)

Reactions of acyl chlorides (spec 15.14)

All at room temperature, fast, giving steamy fumes of HCl:

Hydrolysis of esters (spec 15.15)

Polyesters (spec 15.16)

A diol and a dicarboxylic acid join by condensation polymerisation, losing a water molecule at each ester link. Terylene (PET) is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol; its repeat unit is -OC-C6H4-COO-CH2CH2-O-. Polyesters are used for clothing fibres and drinks bottles, and can be hydrolysed, which makes them more biodegradable than addition polymers.

Worked example

Write equations for ethanoyl chloride reacting with ethanol and with concentrated ammonia. Name the organic products.

  1. CH3COCl + C2H5OH -> CH3COOC2H5 + HCl
  2. CH3COCl + 2NH3 -> CH3CONH2 + NH4Cl

Answer: Ethyl ethanoate; ethanamide

Key idea: Acyl chlorides + water/alcohol/ammonia/amine give acid/ester/amide/N-substituted amide plus HCl, fast and irreversibly. Esters: acid hydrolysis is reversible; alkaline hydrolysis gives the salt and is complete. Diacid + diol gives a polyester by condensation.

Check yourself

  1. What are the products when ethanoyl chloride reacts with water?
    • Ethanol and hydrogen chloride
    • Ethanal and chlorine
    • Ethanoic acid and hydrogen chloride
    • Ethanoic acid and chlorine
    Show answer

    Ethanoic acid and hydrogen chloride. -Cl is replaced by -OH; HCl is released as steamy fumes.

  2. Why is ethanoyl chloride often used instead of ethanoic acid to make esters?
    • The reaction is fast and goes to completion
    • It needs a concentrated sulfuric acid catalyst
    • It produces water as the only by-product
    • It is a weaker reagent
    Show answer

    The reaction is fast and goes to completion. Acyl chlorides react rapidly at room temperature and the reaction is not reversible.

  3. What forms when ethanoyl chloride reacts with concentrated ammonia?
    • Ethylamine
    • Ethanoic acid
    • Ethanamide
    • Ammonium ethanoate only
    Show answer

    Ethanamide. -Cl is replaced by -NH2, giving CH3CONH2.

  4. What is the organic product of ethanoyl chloride with methylamine?
    • Ethylmethylamine
    • N-methylethanamide
    • Methyl ethanoate
    • Ethanamide
    Show answer

    N-methylethanamide. The amine N bonds to the acyl carbon: CH3CONHCH3.

  5. Methyl propanoate is heated under reflux with sodium hydroxide solution. What are the products?
    • Propanoic acid and methanol
    • Sodium methanoate and propan-1-ol
    • Propan-1-ol and methanoic acid
    • Sodium propanoate and methanol
    Show answer

    Sodium propanoate and methanol. Alkaline hydrolysis gives the carboxylate salt and the alcohol.

  6. Why does acid hydrolysis of an ester not go to completion?
    • The acid is used up
    • The reaction is reversible
    • The ester is insoluble
    • Water is a product
    Show answer

    The reaction is reversible. Ester + water ⇌ acid + alcohol reaches equilibrium.

  7. Which pair of monomers forms Terylene?
    • Ethene and propene
    • Hexane-1,6-diamine and hexanedioic acid
    • Ethanoic acid and ethanol
    • Benzene-1,4-dicarboxylic acid and ethane-1,2-diol
    Show answer

    Benzene-1,4-dicarboxylic acid and ethane-1,2-diol. A dicarboxylic acid and a diol form ester links.

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Last updated 7 November 2026. Spotted a mistake? Tell us.