Acyl Chlorides

Acyl chlorides — sometimes called acid chlorides — are the most reactive class of simple carbonyl derivatives. They are so reactive that they fume in moist air (reacting with water vapour) and attack skin on contact. This high reactivity, although a nuisance in the laboratory, makes acyl chlorides incredibly useful as acylating agents: they can install a –COR group onto almost any nucleophile in one step.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.2.2: structure, preparation and reactions of acyl chlorides, including the nucleophilic addition-elimination mechanism.

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1. Structure and Naming

An acyl chloride has the general formula R–COCl. The carbonyl carbon is bonded directly to a chlorine atom instead of an OH (carboxylic acid), OR (ester) or NR₂ (amide).

Key Definition — Acyl group: The R–CO– fragment derived from a carboxylic acid by removing the OH. An acyl chloride is an acyl group bonded to Cl.

Naming rules

Drop the -ic acid ending of the parent carboxylic acid and replace it with -yl chloride.

Acid	Acyl chloride	Formula
Ethanoic acid	Ethanoyl chloride	CH₃COCl
Propanoic acid	Propanoyl chloride	CH₃CH₂COCl
Benzoic acid	Benzoyl chloride	C₆H₅COCl
Butanoic acid	Butanoyl chloride	CH₃(CH₂)₂COCl

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2. Why Acyl Chlorides Are So Reactive

Three factors combine to make the carbonyl carbon of an acyl chloride extraordinarily electrophilic:

1. Both the C=O oxygen and the Cl withdraw electron density from the carbonyl carbon.
2. Chloride is an excellent leaving group (it is a stable anion with a low-energy lone pair).
3. The C–Cl bond is polar (Cl δ–, C δ+) and relatively weak (~338 kJ mol⁻¹) compared to C=O (~745 kJ mol⁻¹).

The result is that even weakly nucleophilic species — water, alcohols, amines — react with acyl chlorides at room temperature without any catalyst. Compare this with carboxylic acid + alcohol (needs concentrated H₂SO₄ and reflux) and you can see why acyl chlorides are preferred in synthesis.

graph LR
  A[R-COCl] --> B[Strong delta+ on C]
  B --> C[Attacked by any nucleophile]
  C --> D[Nu displaces Cl-]
  D --> E[New C-Nu bond]

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3. Preparation of Acyl Chlorides

You do not need a detailed mechanism, but you should know the preparation:

$R-COOH + SOCl_2 \longrightarrow R-COCl + SO_2 + HCl$R−COOH+SOCl2​⟶R−COCl+SO2​+HCl

Carboxylic acid + thionyl chloride (SOCl₂) at room temperature or with gentle warming. The by-products SO₂ and HCl are both gases, so the acyl chloride is left behind and can be isolated by distillation.

OCR also accepts PCl₅ and PCl₃ as alternative reagents. All three generate HCl and so must be done in a fume cupboard.

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4. Reactions of Acyl Chlorides

All the reactions below follow the same nucleophilic addition-elimination mechanism (Section 5). The pattern is always:

1. Nucleophile attacks the δ+ C of C=O.
2. C=O breaks, giving a tetrahedral intermediate.
3. C–Cl breaks as Cl⁻ leaves.
4. C=O reforms; the final product contains the new Nu group in place of the original Cl.

The only thing that changes from reaction to reaction is which nucleophile you start with.

4.1 With water — violent hydrolysis

$CH_3COCl + H_2O \longrightarrow CH_3COOH + HCl$CH3​COCl+H2​O⟶CH3​COOH+HCl

• Vigorous exothermic reaction at room temperature.
• Produces misty white fumes of HCl (visible when exposed to moist air).
• No catalyst needed.

This reaction is why acyl chlorides must be stored in tightly sealed bottles and used in dry conditions — atmospheric moisture alone will hydrolyse them.

4.2 With alcohols — the fast route to esters

$CH_3COCl + C_2H_5OH \longrightarrow CH_3COOC_2H_5 + HCl$CH3​COCl+C2​H5​OH⟶CH3​COOC2​H5​+HCl

• Fast, exothermic, at room temperature.
• No catalyst, no reflux needed.
• Irreversible — unlike the H₂SO₄-catalysed esterification, this goes to completion.
• HCl gas is evolved (misty fumes with ammonia).

Exam Tip: Acyl chloride + alcohol is the preferred way to make an ester in quantitative yield. Carboxylic acid + alcohol only gives 65–70% conversion at equilibrium.

4.3 With ammonia — amide formation

$CH_3COCl + 2NH_3 \longrightarrow CH_3CONH_2 + NH_4Cl$CH3​COCl+2NH3​⟶CH3​CONH2​+NH4​Cl

• One NH₃ molecule acts as a nucleophile, attacking the carbonyl and displacing Cl⁻.
• A second NH₃ molecule neutralises the HCl produced, forming NH₄Cl.
• Product: ethanamide CH₃CONH₂ — a primary amide.
• Vigorous reaction, white smoke of NH₄Cl.

4.4 With primary amines — N-substituted amide formation

$CH_3COCl + 2CH_3NH_2 \longrightarrow CH_3CONHCH_3 + CH_3NH_3^+Cl^-$CH3​COCl+2CH3​NH2​⟶CH3​CONHCH3​+CH3​NH3+​Cl−

• One methylamine acts as nucleophile, the other neutralises HCl.
• Product: N-methylethanamide CH₃CONHCH₃ — a secondary amide.
• The N-nomenclature indicates the substituent is on nitrogen rather than on a carbon.

4.5 Summary table