converting carboxylic acids into acyl chlorides

This page looks at ways of swapping the -OH group in the COOH group of a carboxylic acid for a chlorine atom. This
produces useful compounds called acyl chlorides (acid
It covers the use of phosphorus(V) chloride and phosphorus(III)
chloride as well as sulphur dichloride oxide (thionyl chloride).
Replacing -OH by -Cl
We are going to be looking at converting a carboxylic acid,
RCOOH, into an acyl chloride, RCOCl. Acyl chlorides are also
known as acid chlorides.
By far the most commonly used example of the conversion of a
carboxylic acid into an acyl chloride is ethanoic acid to ethanoyl
Acyl chlorides are very reactive, and can be used to make a
wide range of other things. That's why they are important.
Note: If you are want to find out more about acyl
chlorides, you can explore the acyl chlorides menu by
following this link.
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Replacing the -OH group using phosphorus(V) chloride,
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:
Replacing the -OH group using phosphorus(III) chloride,
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
You end up with a mixture of the acyl chloride and
phosphoric(III) acid (old names: phosphorous acid or
orthophosphorous acid), H3PO3.
For example:
Again, the ethanoyl chloride can be separated by fractional
Replacing the -OH group using sulphur 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 writes the chlorine before the oxygen
(alphabetical order).
The sulphur dichloride oxide reacts with carboxylic acids to
produce an acyl chloride, and sulphur dioxide and hydrogen
chloride gases are given off.
For example:
The separation is simplified to an extent because the byproducts are both gases. You would obviously still have to
fractionally distil the mixture to separate the acyl chloride from
any excess acid or sulphur dichloride oxide.
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