The traditional methods utilize sulfur or phosphorous halides to convert the acid to the acid chloride.
ACID CHLORIDES
Traditional
textbook preparations of acid chlorides from carboxylic acids include
The
traditional methods utilize sulfur or phosphorous halides to convert the acid
to the acid chloride. Of these methods, thionyl chloride [often with a
catalytic amount of dimethyl formamide (DMF)] is the most useful since the
by-products of the reaction are gases (SO2, HCl) which can be easily
purged from the reaction mixture with a stream of nitrogen. The acid chloride
product can then be purified on a small scale by bulb-to-bulb distillation or
crystallization. Because an excess of thionyl chloride is usually used, there
must be a purification step to remove the excess reagent.
Another
superior reagent for the preparation of acid chlorides is oxallyl chlo-ride in
methylene chloride. Addition of a carboxylic acid leads to the smooth evolution
of gas (CO2, CO, HCl) which can be used as a crude monitor of the
reaction progress. The acid chloride is very easily purified since oxallyl
chloride boils at 62◦
C and is easily evaporated from the product. In many instances, the crude
product is sufficiently pure to be used directly.
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