What is the structure of an acid anhydride?

(RCO)₂O, for example (CH₃CO)₂O.

A reaction in which an acyl group, RCO–, is added to a molecule.

What does an acyl chloride form with ammonia?

An amide, and HCl (as NH₄Cl with excess ammonia).

What does an acyl chloride form with a primary amine?

An N-substituted amide and HCl.

What type of mechanism do acyl chlorides show with nucleophiles?

Nucleophilic addition–elimination.

Give two industrial advantages of ethanoic anhydride over ethanoyl chloride.

It is cheaper, and it is less corrosive and reacts less vigorously with water (no HCl produced).

Write the equation for making aspirin.

C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH

How is the purity of a solid checked?

Its melting point: a pure solid melts sharply at the known value; an impure one melts lower and over a range.

Why is recrystallisation cooled slowly?

So that pure crystals form while soluble impurities stay in solution.

Acid anhydrides, acyl chlorides and acylationAQA A-Level Chemistry: Flashcards

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What is the structure of an acyl chloride?

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What is the structure of an acyl chloride?: RCOCl, for example CH₃COCl.
What is the structure of an acid anhydride?: (RCO)₂O, for example (CH₃CO)₂O.
What is acylation?: A reaction in which an acyl group, RCO–, is added to a molecule.
What does ethanoyl chloride form with water?: Ethanoic acid and HCl.
What does an acyl chloride form with an alcohol?: An ester and HCl.
What does an acyl chloride form with ammonia?: An amide, and HCl (as NH₄Cl with excess ammonia).
What does an acyl chloride form with a primary amine?: An N-substituted amide and HCl.
What type of mechanism do acyl chlorides show with nucleophiles?: Nucleophilic addition–elimination.
What is the by-product when an acid anhydride acylates an alcohol?: A carboxylic acid.
Give two industrial advantages of ethanoic anhydride over ethanoyl chloride.: It is cheaper, and it is less corrosive and reacts less vigorously with water (no HCl produced).
Write the equation for making aspirin.: C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH
How is the purity of a solid checked?: Its melting point: a pure solid melts sharply at the known value; an impure one melts lower and over a range.
Why is recrystallisation cooled slowly?: So that pure crystals form while soluble impurities stay in solution.

Exam questions on Acid anhydrides, acyl chlorides and acylation

A technician adds ethanoyl chloride, CH₃COCl, in separate experiments to water, to ethanol, to concentrated ammonia and to methylamine, CH₃NH₂. Each reaction is vigorous and produces misty fumes.
Write the equation for the reaction of ethanoyl chloride with ethanol and name the organic product.2 marks
Aspirin is made industrially by reacting 2-hydroxybenzoic acid with ethanoic anhydride, (CH₃CO)₂O, rather than with ethanoyl chloride. Both reagents can acetylate the OH group on the benzene ring.
Give two industrial advantages of using ethanoic anhydride rather than ethanoyl chloride to make aspirin.2 marks
A chemist reacts propanoyl chloride, CH₃CH₂COCl, separately with ammonia and with ethylamine, CH₃CH₂NH₂. In each case an amide is formed.
Write the equation for the reaction of propanoyl chloride with ethylamine, name the organic product and name the type of mechanism.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).