Solid-state transformations and crystal structure analysis of α- and β-o-acetamidobenzamide

Abstract

o-Acetamidobenzamide (I) has been found to crystallize in at least two distinct forms, both of which undergo complex thermal solid-state transformations which result in the formation of anhydrous 2-methylquinazol-4-one (II). We discuss the first transformation in this series which is a solid-state polymorphic transformation of (Iα) to (Iβ), occurring at 150 °C with the heat of transformation of 0.7 kcal mol^–1. Single crystal X-ray structure determinations of (Iα and β) revealed that the transformation involves loss of an intramolecular N–H ⋯ O hydrogen bond from (Iα) and formation of a new intermolecular N–H ⋯ O hydrogen bond in the structure of (Iβ). Two other intermolecular hydrogen bonds are present in both (Iα and β). The significance of these hydrogen-bond interactions to the mechanisms of disordering in solid-state transformations is discussed. Crystal data for the two polymorphic forms of (I) are: (Iα), P2₁/n, a= 4.952(2)Å, b= 14.496(2), c= 12.471(3), β= 93.99(3)°; (Iβ), P2₁/c, a= 7.807(2)Å, b= 9.039(3), c= 12.706(3), β= 101.12(2)°.