Ch 1 · Alcohols · AS & A Level Chemistry 9701
Topic 1 of 2 in Hydroxy compounds — AS & A Level Chemistry 9701 — 48 lessons.
Introduction
At AS level, esters were made by warming an alcohol with a carboxylic acid and a trace of concentrated sulfuric acid, a slow, reversible reaction. A2 introduces the second, far more direct route: reacting the alcohol with an acyl chloride. Comparing the two methods tests whether a candidate understands why one is fast and irreversible while the other is slow and reaches equilibrium.
Key ideas — Reaction of an alcohol with an acyl chloride
An alcohol reacts with an acyl chloride at room temperature, with no catalyst and no heating, to form an ester and hydrogen chloride gas. The reaction is exemplified by ethanol reacting with ethanoyl chloride to form ethyl ethanoate: CH_3CH_2OH + CH_3COCl arrow CH_3COOCH_2CH_3 + HCl The lone pair on the alcohol's oxygen atom attacks the electrophilic carbonyl carbon of the acyl chloride; the chlorine atom is displaced as chloride ion, which then removes a proton to leave as HCl gas. The reaction is effectively instantaneous and goes to completion, unlike esterification with a carboxylic acid.
🧑🏫 Teacher's explanation
Imagine you are trying to force two people to shake hands. Using a carboxylic acid is like trying to shake hands with someone who is very shy and stubborn; they resist, so you need to add a catalyst and heat it up to force the interaction. Using an acyl chloride is like having a high-energy, aggressive matchmaker who forces the two people together instantly. Because the chlorine atom is so 'electron-hungry' (electronegative), it pulls electron density away from the carbonyl carbon, making that carbon extremely 'hungry' for electrons itself. This makes the carbon a much stronger target for the alcohol's oxygen.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass of the ester formed when methanol (CH_3 extOH) reacts with ethanoyl chloride (CH_3 extCOCl).
🔍 Worked example — Standard
A student reacts 0.10 mol of propan-1-ol with excess ethanoyl chloride. Calculate the volume of HCl gas produced at RTP (assume V_m = 24 dm^3mol^-1).
🔍 Worked example — Exam-style
Identify the name of the ester formed when butan-1-ol reacts with ethanoyl chloride and state the observation made during the reaction.
⚠️ Common errors
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🎯 Exam question
Key ideas — Observations
Adding an alcohol to an acyl chloride produces immediate misty or steamy white fumes of hydrogen chloride gas at room temperature, without any need for heating or a catalyst. The mixture becomes warm, since the reaction is exothermic, and an oily layer of the ester product, often with a sweet, fruity smell, separates from the aqueous residue.
🧑🏫 Teacher's explanation
Think of an acyl chloride as a highly 'aggressive' version of a carboxylic acid. While a carboxylic acid is like a polite person asking for a reaction, an acyl chloride is like someone bursting through a door. Because the chlorine atom is very electronegative, it pulls electron density away from the carbonyl carbon, making that carbon extremely 'hungry' for electrons (highly electrophilic).
📖 Definitions
🔍 Worked example — Easy
Identify the observations when ethanol is added to ethanoyl chloride.
🔍 Worked example — Standard
Write the balanced chemical equation for the reaction between methanol and propanoyl chloride.
🔍 Worked example — Exam-style
Calculate the mass of ethyl ethanoate produced when 0.10 mol of ethanoyl chloride reacts completely with excess ethanol. (Molar mass: CH_3COCl = 92.5, C_4H_8O_2 = 88.1)
⚠️ Common errors
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🎯 Exam question
Key ideas — Why esterification with an acyl chloride occurs more readily
Both routes involve a nucleophilic alcohol oxygen attacking an electrophilic carbonyl carbon, but the two leaving groups differ enormously in ability. In an acyl chloride, chlorine is far more electronegative than the carbon it is bonded to, so it withdraws electron density strongly by induction, making the carbonyl carbon more delta^+ and therefore more attractive to a nucleophile than in a carboxylic acid. Chloride ion, Cl^-, is also a very stable, weakly basic ion, so it is an excellent leaving group that departs readily once the nucleophile has attacked. In a carboxylic acid, the leaving group would have to be the hydroxide ion, OH^-, a much stronger base and far poorer leaving group; this is why carboxylic acid esterification instead proceeds through acid-catalysed protonation of the carbonyl oxygen, needs heating, and remains a reversible equilibrium reaction rather than going to completion.
🧑🏫 Teacher's explanation
Imagine you are trying to push a heavy door open. In a carboxylic acid, the door is heavy and stuck (the OH^- group), so you need a helper (an acid catalyst) and a lot of energy (heat) to nudge it. In an acyl chloride, the door is already unlatched and light (the Cl^- group), so even a tiny nudge sends it flying open immediately.
📖 Definitions
🔍 Worked example — Easy
Predict the products and state symbols for the reaction between ethanol and ethanoyl chloride: CH_3CH_2OH + CH_3COCl arrow ?
🔍 Worked example — Standard
Explain why the reaction between ethanol and ethanoyl chloride produces HCl gas rather than H_2O and why this reaction is faster than using ethanoic acid.
🔍 Worked example — Exam-style
Calculate the mass of ethyl ethanoate produced when 0.10 moles of ethanoyl chloride reacts completely with excess ethanol. (Molar mass of ethyl ethanoate C_4H_8O_2 = 88)
⚠️ Common errors
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🎯 Exam question
Key ideas — Naming esters from acyl chloride reactions
Esters formed from an acyl chloride are named exactly as esters formed from a carboxylic acid: the alkyl group from the alcohol is named first, ending in "-yl", followed by the acid-derived part, ending in "-oate". Propan-1-ol reacting with ethanoyl chloride gives propyl ethanoate, CH_3COOCH_2CH_2CH_3; methanol reacting with propanoyl chloride gives methyl propanoate, CH_3CH_2COOCH_3.
🧑🏫 Teacher's explanation
Think of making an ester like building a LEGO model from two distinct kits. One kit provides the 'acid' part (the carbonyl group) and the other provides the 'alcohol' part. When you react an acyl chloride with an alcohol, the acyl chloride is much more 'aggressive' or reactive than a standard carboxylic acid. It's like using superglue instead of regular school glue; it works much faster and more efficiently.
📖 Definitions
🔍 Worked example — Easy
Name the ester formed when ethanol reacts with ethanoyl chloride.
🔍 Worked example — Standard
Write the structural formula for the ester produced from methanol and propanoyl chloride.
🔍 Worked example — Exam-style
Calculate the molar mass of ethyl ethanoate, C_4H_8O_2.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. Ethanol is added dropwise to propanoyl chloride at room temperature. (a) State two observations that would be made. [2] (b) Write an equation for the reaction, including state symbols. [2] (c) Name the ester formed. [1] (d) Explain why this reaction occurs far more readily than the reaction of ethanol with propanoic acid. [2]
🔍 Worked example — Easy
Identify the observations when methanol is added to ethanoyl chloride.
🔍 Worked example — Standard
Write the balanced equation for the reaction between ethanol and propanoyl chloride.
🔍 Worked example — Exam-style
Explain why the reaction of an alcohol with an acyl chloride is faster than with a carboxylic acid.
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