Ch 3 · Acyl chlorides · AS & A Level Chemistry 9701

Topic 3 of 3 in Carboxylic acids and derivatives — AS & A Level Chemistry 9701 — 57 lessons.

Introduction

Acyl chlorides, RCOCl, are the most reactive common carboxylic acid derivative, reacting readily with water, alcohols, phenols, ammonia and amines. A single mechanism, addition-elimination, explains every one of these reactions, and comparing an acyl chloride's ease of hydrolysis with that of an alkyl chloride and a halogenoarene shows just how unusually reactive its carbonyl carbon is.

Key ideas — Producing acyl chlorides from carboxylic acids

A carboxylic acid is converted into its acyl chloride by reaction with phosphorus(V) chloride, phosphorus(III) chloride, or sulfur dichloride oxide (thionyl chloride): CH_3COOH + PCl_5 arrow CH_3COCl + POCl_3 + HCl Thionyl chloride is often preferred, since its by-products, sulfur dioxide and hydrogen chloride, are both gases, leaving the acyl chloride easy to isolate.

🧑‍🏫 Teacher's explanation

Think of an acyl chloride as a 'hyper-reactive' version of a carboxylic acid. In a carboxylic acid, the -OH group is a relatively stable leaving group. However, if we replace that -OH with a chlorine atom, we create a much more 'uncomfortable' molecule. The chlorine atom is highly electronegative, pulling electron density away from the carbonyl carbon and making it extremely hungry for electrons.

📖 Definitions

🔍 Worked example — Easy

Calculate the molar mass of ethanoyl chloride, CH_3COCl. (H=1, C=12, O=16, Cl=35.5)

🔍 Worked example — Standard

Write the balanced equation for the reaction between propanoyl chloride and water.

🔍 Worked example — Exam-style

Explain why SOCl_2 is preferred over PCl_5 for the synthesis of acyl chlorides.

⚠️ Common errors

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🎯 Exam question

Key ideas — Hydrolysis of acyl chlorides

Acyl chlorides react violently with cold water, with no catalyst needed, producing misty fumes of hydrogen chloride and re-forming the parent carboxylic acid: CH_3COCl + H_2O arrow CH_3COOH + HCl

🧑‍🏫 Teacher's explanation

Imagine you are trying to open a door that is held shut by a very weak latch. An acyl chloride is like a door with a highly unstable, 'stressed' latch (the C-Cl bond). Because the chlorine atom is very electronegative, it pulls electron density away from the carbon, making that carbon extremely 'hungry' for electrons. This makes the carbon highly electrophilic.

📖 Definitions

🔍 Worked example — Easy

Write a balanced equation for the hydrolysis of ethanoyl chloride, ceCH3COCl, in water.

🔍 Worked example — Standard

Calculate the mass of ceHCl gas produced when 5.00 g of ethanoyl chloride (M_r = 78.5) reacts completely with excess water.

🔍 Worked example — Exam-style

Explain why acyl chlorides undergo hydrolysis much more rapidly than alkyl chlorides (e.g., ceCH3CH2Cl) under the same conditions.

⚠️ Common errors

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🎯 Exam question

(a) Write a balanced equation for the reaction between ethanoyl chloride and water. [1] (b) Explain why acyl chlorides are more reactive towards nucleophilic attack than chloroalkanes. [2] [3 marks]

Key ideas — Reactions with alcohols, phenols, ammonia and amines

Acyl chlorides react with each of these nucleophiles at room temperature, releasing hydrogen chloride, which is taken up by any excess nucleophile present: CH_3COCl + C_2H_5OH arrow CH_3COOC_2H_5 + HCl (ester) CH_3COCl + C_6H_5OH arrow CH_3COOC_6H_5 + HCl (phenyl ester) CH_3COCl + 2NH_3 arrow CH_3CONH_2 + NH_4Cl (primary amide) CH_3COCl + 2CH_3NH_2 arrow CH_3CONHCH_3 + CH_3NH_3Cl (N-substituted amide)

🧑‍🏫 Teacher's explanation

Think of an acyl chloride as a very 'unhappy' molecule. The chlorine atom is extremely electronegative and pulls electron density away from the carbonyl carbon, making that carbon incredibly positive and 'hungry' for electrons. This makes the carbon a perfect target for nucleophiles.

📖 Definitions

🔍 Worked example — Easy

Calculate the mass of ethyl ethanoate produced when 0.10 mol of ethanoyl chloride reacts completely with excess ethanol. CH_3 extCOCl + C_2 extH_5 extOH arrow CH_3 extCOOC_2H_5 + HCl. (M_r: C_4H_8O_2 = 88)

🔍 Worked example — Standard

A student reacts 0.50 mol of ethanoyl chloride with 0.50 mol of ammonia. Write the balanced equation for the formation of the amide and the salt produced. CH_3COCl + NH_3 arrow dots

🔍 Worked example — Exam-style

Explain why acyl chlorides undergo hydrolysis much faster than alkyl chlorides. (Focus on the carbonyl carbon)

⚠️ Common errors

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🎯 Exam question

(a) Predict the products of the reaction between ethanoyl chloride and ethanol. (b) Draw the mechanism for the reaction between ethanoyl chloride and ammonia, showing all lone pairs and formal charges during the intermediate stage. [4 marks]

Key ideas — The addition-elimination mechanism

Every reaction above proceeds by the same two-stage mechanism. First, the nucleophile's lone pair attacks the delta^+ carbonyl carbon; simultaneously, the electron pair of the mathrmC=O π bond moves onto the oxygen atom, forming a tetrahedral intermediate that carries a negative charge on oxygen and still bears the chlorine atom (the addition stage). In the second stage, the lone pair on the oxygen atom re-forms the mathrmC=O double bond as the chlorine atom leaves with the bonding pair of electrons, giving chloride ion (the elimination stage). The chloride ion then removes a proton from the nitrogen or oxygen nucleophile that has just bonded, releasing hydrogen chloride or its salt.

🧑‍🏫 Teacher's explanation

Think of an acyl chloride as a high-stakes negotiation. The carbonyl carbon is extremely 'hungry' for electrons because it is being pulled in two directions by the oxygen and the chlorine. This makes the carbon a very strong electrophile (an electron-lover). Imagine a revolving door: first, someone pushes into the door (the nucleophile attacks), causing the door to spin and change shape (the tetrahedral intermediate). Then, someone must exit the door for the rotation to continue (the chloride leaves).

📖 Definitions

🔍 Worked example — Easy

Identify the nucleophile and the electrophile in the reaction between ethanoyl chloride and water: CH_3COCl + H_2O arrow CH_3COOH + HCl.

🔍 Worked example — Standard

Draw the tetrahedral intermediate formed when ethanol reacts with ethanoyl chloride.

🔍 Worked example — Exam-style

Calculate the mass of ethanoyl chloride (C_2H_3ClO) required to react completely with 0.50 moles of ammonia (NH_3) to produce ethanamide and HCl.

⚠️ Common errors

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🎯 Exam question

(a) Describe the mechanism by which X reacts with ammonia (NH_3) to form ethanamide (CH_3CONH_2). (4 marks) (b) Explain why X reacts more rapidly with ammonia than CH_3COOH does. (2 marks) [6 marks]

Key ideas — Relative ease of hydrolysis: acyl chlorides, alkyl chlorides and halogenoarenes

Acyl chlorides hydrolyse instantly in cold water. This is because the carbonyl carbon is made strongly electrophilic by the electron-withdrawing mathrmC=O group and the electronegative chlorine, and chloride is an excellent leaving group, so addition-elimination proceeds without any catalyst. Alkyl chlorides, such as chloroethane, hydrolyse far more slowly, needing to be heated under reflux with aqueous sodium hydroxide, since their sp^3 carbon is much less electrophilic and reacts by nucleophilic substitution rather than addition-elimination. Halogenoarenes, such as chlorobenzene, essentially do not hydrolyse under these conditions at all: a lone pair on the halogen atom is delocalised into the aromatic ring, giving the carbon-halogen bond partial double-bond character that shortens and strengthens it, while the ring's increased electron density repels the approaching nucleophile; only very forcing conditions (high temperature and pressure with concentrated sodium hydroxide) bring about any hydrolysis.

Ch 3 · Acyl chlorides · AS & A Level Chemistry 9701

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