Symmetrical and Unsymmetrical o-Phthalyl Chlorides

Symmetrical o-Pkthalyl Chloride A mixture of 148 g. (1 mole) of phthalic anhydride (Note 1) and 220 g. (1.06 moles) of phosphorus pentachloride (Note 2) is placed in a 500-cc. Claisen flask. The flask is equipped with a reflux condenser, the upper end of which is provided with a calcium chloride tube, and the side-arm of the flask is closed with a cork. The flask is inclined slightly so that any phosphorus oxychloride which collects in the stoppered side-arm will run back into the flask. After heating in an oil bath at 150° for twelve hours, the air condenser and the stopper in the end of the side-arm are removed, and the flask is connected to a water-cooled condenser. The temperature is then raised gradually to 250°, during which 

A good grade of sublimed phthalic anhydride should be used (m. p. 128-129°). If this cannot be obtained the ordinary phthalic anhydride can be purified by sublimation. 

The phosphorus pentachloride should be freed of any phosphorus trichloride or oxychloride present. This may be done by placing the pentachloride in a flask connected to an ice-cooled receiver and heating on a water bath. The pressure in the apparatus is reduced as much as possible by means of a water pump. A calcium chloride tube should be inserted between the pump and the receiver. 

Pure symmetrical o-phthalyl chloride cannot be obtained, even by recrystallization from carbon tetrachloride. A product melting at 16° may be obtained by distilling the unsymmetrical phthalyl chloride at atmospheric pressure. 

The extraction requires from eight to ten hours. The residue should be practically pure phthalic anhydride (m. p. 126°). 

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