Tert-butylhydroquinone: structural elucidation of a new polymorphic form

Thermal behavior and polymorphism of tert -butylhydroquinone (TBHQ), an antioxidant widespread used in industry and as a food additive, were investigated by differential scanning calorimetry, infrared spectroscopy, and hot stage microscopy. Depending on heating conditions, TBHQ presents sublimation at isothermal conditions or melting followed by evaporation under dynamic heating. From evaporation or sublimation, the same commercial polymorphic form was obtained. However, DSC experiments indicated that another polymorph could be obtained from cooling the melted. In the end, two polymorphs were identified depending on the thermal treatment of the sample: a commercial form and another form obtained from melt-crystallization. DSC curves showed the same melting point for the commercial form and the sample crystallized after evaporation or sublimation, but a very different melting point was found for the solid from melt-crystallization, indicating the new form. Infrared spectra presented different profiles and significant band shifts for the solid from the melted, when compared with those of the commercial form, indicating the formation of a new polymorph. Well-defined crystals were obtained from the commercial material by evaporation/sublimation and their structures were elucidated by single-crystal X-ray diffraction. Elucidation confirmed the same phase for the crystal from the evaporated/deposited and the commercial material, that crystallized in the monoclinic space group I2/a with two molecules per asymmetric unit.