Thermal studies of isoxazolylnaphthoquinones by simultaneous DTA-TG-DTG

The tautomeric binary system consisting of N-(3,4-dimethyl-5-isoxazolyl)-4-amino-1,2-naphthoquinone (1),m.p. 156C, and 2-hydroxy-N-(3,4-dimethyl-5-isoxazolyl)-1,4-naphthoquinon-4-imine (2),m.p. 218C, was studied by DTA-TG-DTG and IR spectrometry. Crystals of 1, left in contact with their mother liquor during 4 months, exhibited thermal behaviour similar to that of physical mixtures of 1 and 2, which reveal the presence of a eutectic at about 146C.The effects of light, heat and humidity on the stabilities of 1 and 2 were also studied. Heat (up to 110C) did not induce any change observable by DTA-TG-DTG in either 1 or 2. Moisture and daylight affected only 1. Upon melting, interconversion of the isomers was not observed.The authors thank the Consejo National de Investigaciones Científicas y Técnicas de la RepÚblica Argentina (CONICET) and the Consejo de Investigaciones Cientificas y Tecnológicas de la provincia de Córdoba (CONICOR) for financial support. They also thank Eng. Martín Portela (C.I.M.M., INTI, Córdoba) for recording the DTA-TG-DTG curves and for helpful discussions.