Preparation and Properties of Acyl Chlorides

PREPARATION AND PROPERTIES OF ACYL CHLORIDES (Acylation of Hydroxy and Amino Compounds) 

Introduction. Of all acyl halides, the chlorides are the most commonly used. They are usually prepared by the action of phosphorus trichloride, phosphorus pentachloride, or thionyl chloride on the 

The method to be used is determined by the practicability of the separation of the chloride from the by-products of the reaction, and the excess of the reagent used. Thus, in the laboratory, phosphorus trichloride is used for the lower fatty acids because their chlorides are more volatile than phosphorous acid. The acid, together with 1.5 times the amount of phosphorus trichloride, as indicated in equation (1), is refluxed, and then the acid chloride is distilled from the viscous phosphorous acid and fractionated. Phosphorus pentachloride is used with aromatic acids as shown in equation (2). The dry, finely pulverized potassium or sodium salt of the acid is mixed with a little more than the calculated amount of the pentachloride and heated until the reaction is finished. The resulting oxychloride (POCI3) is removed by distillation. If the boiling points are too close, then the mixture of aryl acid chloride and phosphorus oxychloride is added to finely chopped ice. The oxychloride decomposes instantly, while the aryl acid chloride does not react appreciably at this temperature. The chloride is either separated or extracted with ether, dried, and then fractionated. 

The use of thionyl chloride has the great advantage of forming gaseous products, sulfur dioxide and hydrogen chloride, as shown in equation (3), thus rendering the purification simpler. In addition to this, it permits the preparation of small amounts for the 

The object of this experiment is to illustrate the preparation of an acyl halide and its application to the acylation of amines. 

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