Salt forms of amides : protonation and polymorphism of carbamazepine and cytenamide

Buist, Amanda and Kennedy, Alan and Shankland, Kenneth and Shankland, Norman and Spillman, Mark J. (2013) Salt forms of amides : protonation and polymorphism of carbamazepine and cytenamide. Crystal Growth and Design, 13 (11). pp. 5121-5127. ISSN 1528-7483

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Abstract

In situ generation of HCl or HBr in alcohol leads to O-protonation of the amide group of carbamazepine. Six salt phases have been produced using this method and their crystal structures determined by single crystal diffraction. A new polymorph of carbamazepine hydrochloride is described as are two polymorphs of carbamazepine hydrobromide. All are protonated at the amide O atom to give RC(OH)NH2 cations. Prolonged exposure to air results in addition of water to the solid salt forms. Such hydration of carbamazepine hydrobromide simply gives a monohydrated phase, but similar treatment of the equivalent hydrochloride results in partial loss of HCl and the transfer of the remaining proton from the amide group to water to give [carbamazepine][H3O]0.5[Cl]0.5·H2O. A similar hydronium chloride species is the only product isolated after reaction of the carbamazepine analogue cytenamide with HCl generated in methanol.