Acyl Chlorides and Acid Anhydrides

Acyl chlorides and acid anhydrides are reactive derivatives of carboxylic acids. They are used extensively in organic synthesis because they react much more readily with nucleophiles than carboxylic acids do. Understanding their reactions and mechanisms is essential for Edexcel A-Level Chemistry.

---

Acyl Chlorides

Structure and Naming

An acyl chloride has the functional group –COCl. The general structure is RCOCl. They are named by replacing the -oic acid ending of the parent carboxylic acid with -oyl chloride:

• Ethanoic acid → Ethanoyl chloride (CH₃COCl)
• Propanoic acid → Propanoyl chloride (CH₃CH₂COCl)
• Benzoic acid → Benzoyl chloride (C₆H₅COCl)

Formation of Acyl Chlorides

Acyl chlorides are prepared from carboxylic acids using reagents that replace the –OH group with –Cl:

Using thionyl chloride (SOCl₂): CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl

This is the preferred method because both by-products (SO₂ and HCl) are gases that leave the reaction mixture, making purification straightforward.

Using phosphorus pentachloride (PCl₅): CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl

The white fumes of HCl gas produced serve as a test for the presence of a –OH group (both in carboxylic acids and alcohols).

---