Introduction
In this post, I will describe the preparation of phthalimide from the reaction of phthalic anhydride with urea. The reaction is the following:
The mechanism of the reaction is fairly simple and starts with the nucleophilic attack of the urea nitrogen on one of the carbonyl groups of phthalic anhydride. This forms a tetrahedral intermediate that, by collapsing, opens the original anhydride function expelling a carboxylate ion. This can in turn act as a nucleophile toward the carbonyl carbon of urea that ends up forming an amide on one side of the molecule and a carbamate-like moiety on the other side. The reaction is completed by a ring-closing reaction in which the amide nitrogen intramolecularly attacks the other side of the molecule generating an intermediate that collapses forming the desired phthalimide and carbamic acid. The carbamic acid can, in turn, react with another molecule of phthalic anhydride possibly following a similar mechanism terminating with the production of carbon dioxide and water from the decarboxylation of a carbonic acid-like adduct.
The reaction can carried out, without the use of any solvent, by melting the two solids together. Both reagents melt at the relatively low temperatures of 131.6°C for phthalic anhydride and 133°C for urea.
Experimental part
1g of urea (16.7mmol, 1eq.) and 5g of phthalic anhydride (33.8mmol, 2eq.) are mixed and ground together with mortar and pestle. The powder mixture is transferred to a 100mL reaction flask and heated using either a heat gun or an oil bath. The solid mass starts to melt and some white smoke is observed. Heating is continued until the mass suddenly solidifies and puffs up. The reaction flask is left to cool down and 12.5mL of water are added to break up the solid and dissolve eventually unreacted urea. The solid product is recovered by vacuum filtration and recrystallized from about 100mL of hot ethanol. After drying, a total of 3.32g of recrystallized product in the form of white needles have been recovered representing a final yield of nearly 68%.
