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 chlorides). It covers the use of phosphorus(V) chloride and phosphorus(III) chloride as well as sulfur 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 chloride.
Acyl chlorides are very reactive, and can be used to make a wide range of other things. That's why they are important.
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.
\[CH_3COOH + PCl_5 \rightarrow CH_3COCl + POCl_3 + HCl\]
The acyl chloride can be separated by fractional distillation.
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:
\[CH_3COOH + PCl_3 \rightarrow 3CH_3COCl + H_3PO3 \]
Again, the ethanoyl chloride can be separated by fractional distillation.
Replacing the -OH group using thionyl chloride
Sulfur 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 sulfur dichloride oxide reacts with carboxylic acids to produce an acyl chloride, and sulfur dioxide and hydrogen chloride gases are given off. For example:
\[CH_3COOH + SOCl_2 \rightarrow CH_3COCl + SO_2 + HCl\]
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.
Jim Clark (Chemguide.co.uk)