33.3 Acyl chlorides

Carboxylic acids are converted to acyl chlorides by PCl₃ with heat, PCl₅ or SOCl₂. The hydroxyl group is replaced by chlorine at the acyl carbon.

Choose the reagent named in the question and remember that each route has different inorganic by-products. The product is a reactive derivative ready for further acyl substitution.

Ethanoic acid + SOCl₂ → ethanoyl chloride; the reagent is convenient because gaseous by-products can leave the mixture.

Do not confuse an acyl chloride RCOCl with an alkyl chloride RCl; the carbonyl group changes its reactivity and naming.

Acyl chlorides undergo nucleophilic acyl substitution. Water gives a carboxylic acid, an alcohol gives an ester, ammonia gives a primary amide, and a primary amine gives a substituted amide; HCl is formed.

The nucleophile attacks the carbonyl carbon, then chloride leaves. Match the nucleophile to the product family before writing the equation.

CH₃COCl + H₂O → CH₃CO₂H + HCl; CH₃COCl + NH₃ → CH₃CONH₂ + HCl.

Do not write an alkene addition product or omit HCl when balancing the overall reaction.

A nucleophile adds to the electrophilic carbonyl carbon of an acyl chloride, forming a tetrahedral intermediate. The intermediate then eliminates chloride and reforms the C=O bond.

This mechanism explains why several nucleophiles give different products while the acyl carbon remains the reaction centre.

In ethanolysis, ethanol attacks CH₃COCl, the tetrahedral intermediate collapses, and ethyl ethanoate forms with HCl as the by-product.

The mechanism is not direct displacement without an intermediate, and the carbonyl bond is temporarily changed rather than permanently lost.

Acyl chlorides hydrolyse rapidly because the carbonyl carbon is strongly electrophilic and chloride is a good leaving group. Alkyl chlorides need suitable substitution conditions, while aryl chlorides resist because the C–Cl bond has partial double-bond character on an sp² carbon.

Compare the carbon attached to chlorine and the stabilisation of the possible intermediate before deciding reactivity.

Ethanoyl chloride reacts vigorously with water at room temperature; chloroethane reacts much more slowly under ordinary aqueous conditions; chlorobenzene is least susceptible.

All three contain chlorine, but hydrolysis rate is governed by the local structure, not the element alone.