Ammonium phthalamate

Phthalimide is formed when ammonium phthalamate is heated. 

On the other hand, the reduction of phthalic anhydride to phthalide is simpler. The key step in this synthesis is the electrochemical reduction of ammonium phthalamates... 

Phthalic anhydride (60 g. or 0.41 mole) is added gradually with stirring to 90 ml. of warm aqueous ammonia solution (sp. gr. 0.88). The anhydride dissolves with evolution of heat, and ammonium phthalamate separates from the solution. Cooling precipitates 70 g. (94%) of the ammonium salt. 

A solution of 30 g. (0.165 mole) of ammonium phthalamate in 30 ml. of cold water is treated with 20 ml. of concentrated hydrochloric acid. The solid formed is collected by filtration, and the white paste obtained is washed with cold water until free of chloride. The yield is 22 g. (81%). The phthalamic acid melts at 149 with loss of water, resolidifies at about 155 , and remelts at 231 . This represents conversion of phthalamic acid to phthalimide (m.p. 231 ). 

Phthalic anhydride reacts with excess ammonia to give the ammonium salt of a compound known as phthalamic acid. 

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