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Questions on acylation: acyl chlorides and acid anhydrides
Acyl chlorides and acid anhydrides as acylating agents, their reactions with water, alcohols, ammonia and primary amines to give carboxylic acids, esters, amides and N-substituted amides, the observations, the nucleophilic addition–elimination mechanism, making acyl chlorides with SOCl₂, PCl₅ or PCl₃, the ease of hydrolysis of acyl, alkyl and aryl chlorides, hydrolysis and LiAlH₄ reduction of amides and why amides are weak bases, phenyl esters from phenol, and why ethanoic anhydride is used to make aspirin industrially.
19 original questions · 62 marks · the acylation: acyl chlorides and acid anhydrides notes · Carbonyls and carboxylic acids · Flashcards for this unit
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Evaluate the use of ethanoic anhydride rather than ethanoyl chloride for the industrial manufacture of aspirin from 2-hydroxybenzoic acid.
Mark scheme
Indicative content
- the acyl chloride route has the higher atom economy, because its by-product, HCl, is lighter than ethanoic acid, which on its own favours it
- ethanoic anhydride is cheaper to make in bulk, which matters more than atom economy at industrial scale
- the anhydride is less corrosive and reacts less violently, so the reaction is easier to control in a large vessel
- it is hydrolysed only slowly by water vapour in the air, so it is safer to store and handle than ethanoyl chloride
- its by-product, ethanoic acid, is far less hazardous than hydrogen chloride gas and can be recovered and sold or reused
- a justified conclusion: cost, safety and a useful by-product outweigh the lower atom economy, so the anhydride is the better choice
Levels
- Level 3 (5–6 marks) the higher atom economy of the acyl chloride route is acknowledged and outweighed by all the factors on the other side, cost, controllability, ease of storage and the safer, saleable by-product, with a justified conclusion in favour of the anhydride; the answer compares the two reagents throughout rather than describing one
- Level 2 (3–4 marks) two or three reasons for the anhydride are right and a conclusion is drawn; atom economy is left out, or the by-products are not set side by side
- Level 1 (1–2 marks) the anhydride is called safer or cheaper with no reason, or is preferred for a higher atom economy, which in fact belongs to the acyl chloride route
Outline the mechanism for the reaction of ethanoyl chloride with ethanol, and name the type of mechanism.
Mark scheme
- B1 curly arrow from a lone pair on the oxygen of ethanol to the carbonyl carbon
- B1 curly arrow from the C=O double bond to the oxygen atom
- B1 intermediate drawn with O⁻ and with a positive charge on the oxygen of the ethanol group
- B1 three arrows: from O⁻ back to reform C=O, from the C–Cl bond to Cl, and from the O–H bond to O⁺
- B1 the mechanism is nucleophilic addition–elimination
Name the compounds CH₃CH₂COCl and (CH₃CO)₂O, and name the organic products formed when ethanoyl chloride reacts with methanol and with ethylamine.
Mark scheme
- B1 CH₃CH₂COCl is propanoyl chloride
- B1 (CH₃CO)₂O is ethanoic anhydride
- B1 methanol gives methyl ethanoate, CH₃COOCH₃
- B1 ethylamine gives N-ethylethanamide, CH₃CONHCH₂CH₃; the N is required
Give equations for the reactions of ethanoyl chloride with water and with excess ammonia, an observation for the reaction with water, and the name of the organic product with ammonia.
Mark scheme
- B1 CH₃COCl + H₂O → CH₃COOH + HCl
- B1 steamy white fumes of hydrogen chloride, and a vigorous reaction; 'gas given off' alone is not credited
- B1 CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl
- B1 the organic product is ethanamide
Outline the mechanism for the reaction of ethanoyl chloride with methylamine.
Mark scheme
- B1 curly arrow from the lone pair on the nitrogen of CH₃NH₂ to the carbonyl carbon
- B1 curly arrow from the C=O double bond to the oxygen atom
- B1 intermediate drawn with O⁻ and with a positive charge on the nitrogen of the CH₃NH₂⁺ group
- B1 three arrows: from O⁻ back to reform C=O, from the C–Cl bond to Cl, and from an N–H bond to N⁺
Give an equation for the hydrolysis of ethanamide, CH₃CONH₂, when it is heated under reflux with dilute hydrochloric acid, and another for its hydrolysis with aqueous sodium hydroxide, naming the organic product of each.
Mark scheme
- B1 CH₃CONH₂ + H₂O + HCl → CH₃COOH + NH₄Cl; allow H⁺ in place of HCl with NH₄⁺ as the product
- B1 the organic product with acid is ethanoic acid, the nitrogen leaving as ammonium ions
- B1 CH₃CONH₂ + NaOH → CH₃COONa + NH₃; allow OH⁻ with CH₃COO⁻ as the product
- B1 the organic product with alkali is sodium ethanoate, and ammonia gas is given off, turning damp red litmus blue
Explain why ethanoyl chloride reacts with ethanol much faster than ethanoic acid does.
Mark scheme
- B1 the carbonyl carbon of the acyl chloride is bonded to two electronegative atoms, O and Cl
- B1 so it carries a larger δ+ charge and attracts the nucleophile more strongly
- B1 the chloride ion is a good leaving group, much better than the hydroxide ion an acid would lose
Calculate the atom economy for making ethyl ethanoate, Mr 88.0, from ethanoyl chloride, Mr 78.5, and ethanol, Mr 46.0.
Mark scheme
- M1 total Mr of reactants = 78.5 + 46.0 = 124.5
- M1 atom economy = 88.0 ÷ 124.5 × 100
- A1 atom economy = 70.7%, to 3 significant figures
Calculate the percentage yield of aspirin when 2-hydroxybenzoic acid reacts with excess ethanoic anhydride, using the data given in the table.
Quantity Value mass of 2-hydroxybenzoic acid 2.76 g Mr of 2-hydroxybenzoic acid 138.0 mass of aspirin collected 2.52 g Mr of aspirin 180.0 Mark scheme
- M1 amount of 2-hydroxybenzoic acid = 2.76 ÷ 138.0 = 0.0200 mol, so the maximum amount of aspirin is 0.0200 mol
- M1 amount of aspirin obtained = 2.52 ÷ 180.0 = 0.0140 mol
- A1 percentage yield = 0.0140 ÷ 0.0200 × 100 = 70.0%, to 3 significant figures
Compare the reactions of ethanoyl chloride and ethanoic anhydride with ethanol.
Mark scheme
- B1 both give the same organic product, ethyl ethanoate, by nucleophilic addition–elimination
- B1 ethanoyl chloride reacts quickly at room temperature, while the anhydride reacts more slowly and needs warming
- B1 the by-product is HCl with ethanoyl chloride and ethanoic acid with the anhydride
Suggest what is seen when a few drops of ethanoyl chloride and of 1-chlorobutane are added to separate tubes of aqueous silver nitrate, and explain the difference.
Mark scheme
- B1 ethanoyl chloride gives a white precipitate of silver chloride at once
- B1 1-chlorobutane gives a white precipitate only slowly, after warming
- B1 the acyl chloride is hydrolysed rapidly, releasing Cl⁻ ions, because its carbonyl carbon is strongly δ+ and is attacked by water, while the C–Cl bond of 1-chlorobutane is broken far more slowly
Suggest the products formed when propanoic anhydride reacts with excess ammonia, and give an equation for the reaction.
Mark scheme
- B1 the organic product is propanamide, CH₃CH₂CONH₂
- B1 ammonium propanoate, CH₃CH₂COONH₄, from propanoic acid and the second ammonia molecule
- B1 (CH₃CH₂CO)₂O + 2NH₃ → CH₃CH₂CONH₂ + CH₃CH₂COONH₄; allow 1 : 1 with propanoic acid as the by-product
Describe how the melting point of a sample of aspirin shows whether it is pure, and a chemical test for unreacted 2-hydroxybenzoic acid in it.
Mark scheme
- B1 a pure sample melts sharply at the data-book value, 135 °C
- B1 an impure sample melts below that value and over a range of several degrees
- B1 iron(III) chloride solution gives a purple colour with the free phenol group of 2-hydroxybenzoic acid, and none with pure aspirin
Explain why ethanoyl chloride is hydrolysed by water at room temperature, 1-chlorobutane only slowly on warming, and chlorobenzene not at all on boiling.
Mark scheme
- B1 the carbonyl carbon of ethanoyl chloride is bonded to both O and Cl, so it is strongly δ+ and water adds to it readily
- B1 the C–Cl carbon of 1-chlorobutane is bonded to only one electronegative atom, so it is less δ+, and its C–Cl bond is broken only slowly by substitution
- B1 in chlorobenzene a chlorine lone pair overlaps with the ring's π electrons, strengthening the C–Cl bond, and the electron-rich ring repels nucleophiles
Give an equation for the reaction of phenol with benzoyl chloride, name the organic product, and give the reason phenol is first dissolved in sodium hydroxide.
Mark scheme
- B1 C₆H₅OH + C₆H₅COCl → C₆H₅COOC₆H₅ + HCl
- B1 the organic product is phenyl benzoate
- B1 the alkali turns phenol into the phenoxide ion, a far better nucleophile, because phenol's own lone pair is partly delocalised into the ring
Explain why two moles of ammonia are needed for each mole of ethanoyl chloride in the preparation of ethanamide.
Mark scheme
- B1 one mole of ammonia acts as the nucleophile and forms the amide, releasing HCl
- B1 ammonia is a base, so a second mole reacts with the HCl to form ammonium chloride
Explain why an acid anhydride reacts more slowly with nucleophiles than an acyl chloride does.
Mark scheme
- B1 the leaving group from an anhydride is a carboxylate ion, a poorer leaving group than the chloride ion
- B1 the carbonyl carbon of the anhydride is less δ+ than that of the acyl chloride, so it attracts nucleophiles less strongly
Suggest why aspirin is made from 2-hydroxybenzoic acid and ethanoic anhydride rather than by heating 2-hydroxybenzoic acid with ethanoic acid.
Mark scheme
- B1 the OH group to be acylated is a phenol group, which reacts only very slowly with a carboxylic acid
- B1 the anhydride is a more reactive acylating agent, and the reaction is not a slow equilibrium, so the yield is much higher
Suggest why SOCl₂ is often preferred to PCl₅ for converting a carboxylic acid into an acyl chloride.
Mark scheme
- B1 the by-products of SOCl₂, sulfur dioxide and hydrogen chloride, are both gases that leave the flask
- B1 so the acyl chloride needs little separating, whereas with PCl₅ it must be distilled away from liquid POCl₃
Practise acylation: acyl chlorides and acid anhydrides one question at a time