33.3 Acyl Chlorides

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Main Content

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1. What Is an Acyl Chloride?

An acyl chloride (also called an acid chloride) is an organic compound that contains the acyl chloride functional group: a carbonyl group (C=O) bonded directly to a chlorine atom.

The general structure looks like this:

    O
    ‖
R — C — Cl

The "R" represents any carbon chain or group attached to the rest of the molecule.

A simple and common example is ethanoyl chloride, CH₃COCl. Here, R = CH₃ (a methyl group).

Acyl chlorides are named by taking the parent carboxylic acid, removing the "-ic acid" ending, and replacing it with "-oyl chloride." So ethanoic acid → ethanoyl chloride.

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Method 1: Reaction with Phosphorus Trichloride (PCl₃) — requires heating

• Reagent: PCl₃
• Condition: Heat
• General equation:

$$3\text{RCOOH} + \text{PCl}_3 \xrightarrow{\Delta} 3\text{RCOCl} + \text{H}_3\text{PO}_3$$

• Example using ethanoic acid:

$$3\text{CH}_3\text{COOH} + \text{PCl}_3 \xrightarrow{\Delta} 3\text{CH}_3\text{COCl} + \text{H}_3\text{PO}_3$$

• The other product is phosphonic acid (H₃PO₃).

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Method 2: Reaction with Phosphorus Pentachloride (PCl₅) — room temperature

• Reagent: PCl₅
• Condition: Room temperature (no heating needed)
• General equation:

$$\text{RCOOH} + \text{PCl}_5 \rightarrow \text{RCOCl} + \text{POCl}_3 + \text{HCl}$$

• Example using ethanoic acid:

$$\text{CH}_3\text{COOH} + \text{PCl}_5 \rightarrow \text{CH}_3\text{COCl} + \text{POCl}_3 + \text{HCl}$$

• The other products are phosphoryl chloride (POCl₃) and hydrogen chloride (HCl) gas.
• You will notice misty white fumes of HCl being released — this is a useful observation in practicals.