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Preparation of Acyl Chlorides (Acid Chlorides)

This page looks at ways of swapping the -OH group in the -COOH group of a carboxylic acid for a chlorine atom to make acyl chlorides (acid chlorides), and is a very slightly modified version of a page in the section on reactions of carboxylic acids.

It covers the use of phosphorus(V) chloride, phosphorus(III) chloride and sulfur dichloride oxide (thionyl chloride).

Replacing the -OH in a Carboxylic Acid by -Cl

We'll take the conversion of ethanoic acid to ethanoyl chloride as typical.

Note: If you haven'talready done so, it would probably pay you to have a quick look at the beginning of the introduction to acyl chlorides before you go on.

Replacing the -OH Group Using Phosphorus(V) chloride, PCl5

Phosphorus(V) chloride is a solid which reacts with carboxylic acids in the cold to give steamy acidic fumes of hydrogen chloride.It leaves a liquid mixture of the acyl chloride and a phosphorus compound, phosphorus trichloride oxide (phosphorus oxychloride) – POCl3.

The acyl chloride can be separated by fractional distillation.

For example:
CH_3COOH + P\text{Cl}_5 \longrightarrow CH_3CO\text{Cl} + PO\text{Cl}_3 + H\text{Cl}

Replacing the -OH Group Using Phosphorus(III) chloride, PCl3

Phosphorus(III) chloride is a liquid at room temperature. Its reaction with a carboxylic acid is less dramatic than that of phosphorus(V) chloride because there is no hydrogen chloride produced.

You end up with a mixture of the acyl chloride and phosphoric(III) acid (old names: phosphorous acid or orthophosphorous acid), H3PO3.

For example:
3CH_3COOH + P\text{Cl}_3 \longrightarrow 3CH_3CO\text{Cl} + H_3PO_3

Again, the ethanoyl chloride can be separated by fractional distillation.

Replacing the -OH Group Using Sulfur dichloride oxide (thionyl chloride)

Sulphur dichloride oxide (thionyl chloride) is a liquid at room temperature and has the formula SOCl2.

Traditionally, the formula is written as shown, despite the fact that the modern name has the chlorine before the oxygen (alphabetical order).

The sulfur dichloride oxide reacts with carboxylic acids to produce an acyl chloride, and sulfur dioxide and hydrogen chloride gases are given off.

For example:
CH_3COOH + SO\text{Cl}_2 \longrightarrow CH_3CO\text{Cl} + SO_2 + H\text{Cl}

The separation is simplified to an extent because the by-products are both gases. You would obviously still have to fractionally distil the mixture to separate the acyl chloride from any excess acid or sulfur dichloride oxide.

Questions to test your understanding

Questions on the preparation of acyl chlorides Answers