Ch 2 · Esters · AS & A Level Chemistry 9701
Topic 2 of 3 in Carboxylic acids and derivatives — AS & A Level Chemistry 9701 — 46 lessons.
Introduction
There are now two routes to an ester: the slow, reversible AS route from a carboxylic acid and alcohol, and the fast, irreversible A2 route from an acyl chloride and alcohol. This lesson compares the two routes directly and extends the acyl chloride route to a substrate an alcohol-based route cannot easily reach: phenol.
Key ideas — Ester formation from an alcohol and an acyl chloride
An alcohol reacts with an acyl chloride at room temperature to give an ester and hydrogen chloride, with misty white fumes seen immediately: CH_3CH_2OH + CH_3COCl arrow CH_3COOCH_2CH_3 + HCl
🧑🏫 Teacher's explanation
Imagine you are trying to join two Lego bricks together. A carboxylic acid is like a brick with a slightly sticky surface; it needs a 'catalyst' or a lot of heat to make the connection stick. An acyl chloride, however, is like a brick with a powerful magnet attached. It is much more eager to react and will snap together with an alcohol almost instantly, even at room temperature.
📖 Definitions
🔍 Worked example — Standard
An organic liquid is formed by reacting methanol (CH_3OH) with propanoyl chloride (CH_3CH_2COCl). Write the balanced equation for this reaction.
🔍 Worked example — Exam-style
Compare the reaction of CH_3COOH with CH_3OH versus CH_3COCl with CH_3OH in terms of conditions and byproducts.
⚠️ Common errors
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🎯 Exam question
Key ideas — Comparing the two routes to an ester
The acyl chloride route needs no catalyst, occurs at room temperature, and goes essentially to completion in seconds, since chloride ion is a stable, unreactive leaving group with no tendency to react back with the ester. The carboxylic acid route instead needs a concentrated sulfuric acid catalyst and heating under reflux, and even then only reaches an equilibrium, since water, the leaving group in this case, can react back with the ester to re-form the carboxylic acid and alcohol: CH_3COOH + CH_3CH_2OH leftharpoons CH_3COOCH_2CH_3 + H_2O For a high-yield laboratory preparation of a single ester, the acyl chloride route is therefore preferred, despite the extra step needed to make the acyl chloride itself from the carboxylic acid.
🧑🏫 Teacher's explanation
Imagine you are trying to force two people to shake hands. The carboxylic acid route is like a polite, slow negotiation where both parties keep changing their minds; they might shake hands (form an ester), but they might also decide to walk away (revert to acid and alcohol) because the process is reversible. This is an equilibrium reaction.
📖 Definitions
🔍 Worked example — Standard
A student reacts ethanoyl chloride, ceCH_3COCl, with methanol, ceCH_3OH, to produce methyl ethanoate and hydrogen chloride. Write the balanced equation.
🔍 Worked example — Exam-style
Explain why the reaction ceCH_3COOH + C_2H_5OH leftharpoons CH_3COOC_2H_5 + H_2O is reversible, whereas ceCH_3COCl + C_2H_5OH arrow CH_3COOC_2H_5 + HCl is not.
⚠️ Common errors
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🎯 Exam question
Key ideas — Why acyl chlorides are more reactive towards nucleophiles
Chlorine is more electronegative than the oxygen of a hydroxyl group is able to make the carbon behave, and its strong electron-withdrawing inductive effect leaves the carbonyl carbon of an acyl chloride more delta^+, and so more attractive to a nucleophile, than the carbonyl carbon of a carboxylic acid. Once the nucleophile has attacked, chloride ion is expelled easily, since it is a very stable ion and a weak base; hydroxide ion, the equivalent leaving group from a carboxylic acid, is a much stronger base and a poor leaving group, which is why the carboxylic acid route needs acid catalysis (protonating the carbonyl oxygen first) to proceed at a useful rate at all.
🧑🏫 Teacher's explanation
Imagine a crowded room where everyone is looking at one person (the carbonyl carbon). In a carboxylic acid, the oxygen atom is like a polite friend; it pulls some attention away from the person, but not much. In an acyl chloride, the chlorine atom is like a loud, demanding person who pulls all the attention toward itself. Because chlorine is much more electronegative than oxygen, it pulls electron density away from the carbon atom, making that carbon extremely 'hungry' for electrons. This makes the carbon highly positive (delta^+) and very attractive to nucleophiles.
📖 Definitions
🔍 Worked example — Easy
Predict the products of the reaction between ethanoyl chloride and methanol: CH_3COCl + CH_3OH arrow ?
🔍 Worked example — Standard
Calculate the mass of methyl ethanoate produced when 0.50 mol of ethanoyl chloride reacts completely with 0.50 mol of methanol. (M_r of CH_3COOCH_3 = 74)
🔍 Worked example — Exam-style
Explain why the reaction between a carboxylic acid and an alcohol requires an acid catalyst, whereas the reaction with an acyl chloride does not. (3 marks)
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🎯 Exam question
Key ideas — Preparing a phenyl ester
Phenol reacts only very slowly, if at all, with a carboxylic acid to form an ester, because the lone pair on phenol's oxygen atom is delocalised into the aromatic ring, making it a much weaker nucleophile than the oxygen lone pair of an aliphatic alcohol. Phenol does, however, react with the far more electrophilic acyl chloride to give a phenyl ester and hydrogen chloride: C_6H_5OH + CH_3COCl arrow CH_3COOC_6H_5 + HCl Adding a small amount of aqueous sodium hydroxide first converts phenol into the more strongly nucleophilic phenoxide ion, C_6H_5O^-, which reacts with the acyl chloride even more readily.
🧑🏫 Teacher's explanation
Imagine you are trying to throw a ball (the lone pair of electrons) into a moving target (the carbonyl carbon). In a normal alcohol, the electrons are sitting loosely on the oxygen, ready to be thrown. But in phenol, the oxygen is part of a 'club' (the benzene ring). The electrons are being shared and pulled into the ring to help stabilize it, making them much less available to attack anything else. This makes phenol a very 'weak' nucleophile.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass of the phenyl ester produced in the reaction: ceC_6H_5OH + ceCH_3COCl arrow ceCH_3COOC_6H_5 + ceHCl. (Use C=12, H=1, O=16, Cl=35.5)
🔍 Worked example — Standard
A student reacts 0.05 mol of phenol with 0.05 mol of ethanoyl chloride. Calculate the theoretical yield of phenyl ethanoate in grams. (Molar mass of ester = 136 g/mol)
🔍 Worked example — Exam-style
Explain, in terms of electron density, why phenol reacts more slowly with carboxylic acids than ethanol does.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. A student wants to prepare phenyl ethanoate. Ethanoic acid and phenol are heated together with concentrated sulfuric acid catalyst, but almost no ester forms even after prolonged reflux. (a) Explain, in terms of nucleophilicity, why this method fails. [2] (b) Suggest a reagent that would successfully form phenyl ethanoate from phenol, and state the conditions. [2] (c) Explain why this alternative reagent reacts successfully where ethanoic acid does not. [2]
🔍 Worked example — Easy
Calculate the molar mass of ethyl ethanoate, C_4H_8O_2. (H=1, C=12, O=16)
🔍 Worked example — Standard
A student reacts 0.50 mol of ethanoyl chloride with 0.40 mol of methanol to produce methyl ethanoate and HCl. Determine the number of moles of methyl ethanoate produced, assuming the reaction goes to completion.
🔍 Worked example — Exam-style
Explain why the reaction between ethanoic acid and ethanol requires a concentrated H_2SO_4 catalyst, whereas the reaction between ethanoyl chloride and ethanol does not.
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