Lesson 25 of 34

An acyl chloride reacts readily with water, alcohols, phenols and amines

Its polar carbonyl and leaving chloride make it more reactive than an alkyl or aryl chloride.

A-Level 9476 (2026-2027)

I can describe acyl-chloride hydrolysis.Syllabus
Syllabus 9476, 11.8(d). Describe acyl-chloride hydrolysis.
I can describe acyl-chloride condensation reactions.Syllabus
Syllabus 9476, 11.8(e). Describe acyl-chloride condensation reactions.
I can compare acyl, alkyl and aryl chloride hydrolysis.Syllabus
Syllabus 9476, 11.8(f). Compare acyl, alkyl and aryl chloride hydrolysis.
I can prepare an ester from an acyl chloride.Syllabus
Syllabus 9476, 11.8(g). Prepare an ester from an acyl chloride.

An acyl chloride reacts readily with water, alcohols, phenols and amines

Its polar carbonyl and leaving chloride make it more reactive than an alkyl or aryl chloride.

Reactions at the acyl carbon
Reagent	Organic product	Representative equation
Water	Carboxylic acid; rapid hydrolysis under ordinary conditions.	CH₃COCl + H₂O → CH₃COOH + HCl.
Alcohol	Ester.	CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl.
Phenol or phenoxide	Phenyl ester; base can increase nucleophilicity and remove acid.	C₆H₅COCl + C₆H₅OH → C₆H₅COOC₆H₅ + HCl.
Primary amine	N-substituted amide; excess amine or another base removes HCl.	R'COCl + 2RNH₂ → R'CONHR + RNH₃⁺Cl^-.

Phenyl benzoate is C₆H₅COOC₆H₅. It forms from benzoyl chloride and phenol or phenoxide. The phenyl group after the ester oxygen comes from phenol, and the benzoyl part comes from the acyl chloride. Phenol does not form esters easily with a carboxylic acid, so the acyl chloride route is needed.

In an acyl chloride, the carbonyl carbon is strongly δ⁺, so a nucleophile attacks it. The nucleophile adds, then chloride leaves. So an acyl chloride hydrolyses much more easily than an alkyl chloride, which needs substitution at a saturated carbon and usually heating with an aqueous nucleophile. An aryl chloride such as chlorobenzene is even harder to hydrolyse. Delocalisation strengthens its C-Cl bond and the ring blocks the usual substitution routes.

Worked exampleDistinguish three chlorides by water and hydrolysis

Compare ethanoyl chloride, chloroethane and chlorobenzene with water under ordinary conditions, then with heated aqueous NaOH.

Try it first. Then show the worked stepsHide the worked steps

Ethanoyl chloride hydrolyses readily with water, releasing HCl and forming ethanoic acid.
Chloroethane does not show the same immediate vigorous hydrolysis but reacts on heating with aqueous hydroxide to give ethanol and chloride.
Chlorobenzene resists the ordinary heated aqueous-hydroxide substitution conditions used for halogenoalkanes.

Answer

Ease of hydrolysis: acyl chloride > comparable alkyl chloride > aryl chloride under the stated conditions. This comparison concerns the bonding environment, not simply the presence of chlorine.

Check your understanding

Why does ethanoyl chloride react with cold water at once, while chloroethane needs heating with aqueous alkali?

Its C-Cl bond is weaker than any other bond, so it breaks on contact with water.
Chloroethane is insoluble in water, so the two compounds do not meet each other.
Its carbonyl carbon is strongly delta+, so water attacks it easily before Cl^- leaves.

Show the answer

Its carbonyl carbon is strongly delta+, so water attacks it easily before Cl^- leaves.

In an acyl chloride, both O and Cl pull electron density from the carbonyl carbon. A nucleophile adds to it, then chloride leaves. Chloroethane needs substitution at a saturated carbon, which is slower.