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Learn7405Paper 2 · Paper 2: Organic chemistryCarboxylic acids and derivativesAcyl chlorides and acylation
Carboxylic acids and derivatives · Subtopic 2 of 2

Acyl Chlorides and Acylation

Acyl chlorides (RCOCl) are made from carboxylic acids with SOCl₂, PCl₅ or PCl₃. The carbonyl carbon is very δ+ (bonded to O and Cl), so they react vigorously at room temperature with nucleophiles by nucleophilic addition–elimination, giving off steamy HCl fumes: water → carboxylic acid; alcohol (or phenol) → ester; ammonia → primary amide; primary amine → N-substituted amide. Acid anhydrides ((RCO)₂O) react the same way but more slowly, releasing RCOOH instead of HCl; ethanoic anhydride is preferred industrially for aspirin (cheaper, safer, less corrosive). Ease of hydrolysis: acyl chloride ≫ alkyl chloride ≫ aryl chloride.

Cambridge 9701 Paper 4Edexcel YCH11 WCH14AQA 7405 Paper 2≈ 45 minRevision 1 · updated 2026-11-01
Section 1 of 14

Learning objectives

  • Prepare acyl chlorides from carboxylic acids.
  • Describe reactions with water, alcohols, phenols, ammonia and amines.
  • Draw the addition–elimination mechanism.
  • Compare acyl chlorides with acid anhydrides.
Pause and think

Phenol reacts only slowly with ethanoic acid to make an ester. How else could you make phenyl ethanoate?

Section 2 of 14

Core idea

Start hereThe fastest way to make an ester

Acid plus alcohol makes an ester slowly and never completely. Swap the acid for an acyl chloride and the reaction is over in seconds.

The big idea

The carbonyl carbon in RCOCl is bonded to both O and Cl, so it is strongly δ+. Nucleophiles add to it, then Cl⁻ is eliminated: addition–elimination.

1. Mechanism

Ethanoyl chloride + water
  1. H2O:→Cδ+\mathrm{H_2O{:}\rightarrow C^{\delta+}}
    Lone pair on O attacks the carbonyl carbon; C=O π electrons move onto O.
  2. CH3C(O−)(Cl)(OH2+)\mathrm{CH_3C(O^-)(Cl)(OH_2^+)}
    Tetrahedral intermediate.
  3. CH3COOH+HCl\mathrm{CH_3COOH+HCl}
    C=O reforms, Cl⁻ leaves, H⁺ is lost.
One reagent, four products

Replace Cl with the nucleophile.

Worked exampleEthanamide yield

3.93 g CH₃COCl (MrM_r 78.5) with excess NH₃. Maximum mass of ethanamide (MrM_r 59.0)?

Goal
Mass.
Strategy
1 : 1 ratio.
  1. n=3.9378.5=0.0500n=\frac{3.93}{78.5}=0.0500
    Moles of acyl chloride.
  2. m=0.0500×59.0=2.95 gm=0.0500\times59.0=2.95\ \text{g}
    Mass of amide.
Answer2.95 g.
✓ CheckSecond NH₃ only removes HCl ✓.

Summary

  • SOCl₂ makes RCOCl.
  • Addition–elimination.
  • Acid, ester, amide, N-substituted amide.
  • Anhydrides: slower, safer.
Key words
Acyl chloride
Compound containing –COCl.
Acylation
Adding an RCO– group to a molecule.
HistoryAspirin, 1897. Felix Hoffmann at Bayer acetylated salicylic acid with ethanoic anhydride to make aspirin, gentler on the stomach than salicylic acid itself.
Fun factSteamy fumes. Open a bottle of ethanoyl chloride in a humid lab and it fumes: it reacts with water vapour to make HCl mist.
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