Crystal structure, thermal behaviour, protonation and mass spectroscopic studies of racemic 4-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole hydrochlorides.

The crystal structure, thermal behaviour, mass spectrum and protonation of 4-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole (medetomidine) hydrochloride have been investigated. The title compound crystallizes in both hydrated and anhydrous forms, and their structures have been determined by three-dimensional X-ray structure analysis. The crystals of the anhydrous form are monoclinic and those of the hydrated form (containing one hydrate water molecule) are triclinic with unit-cell dimensions: a = 23.861(9), b = 7.721(4), c = 22.037(9) A, beta = 140.20(4) degrees, Z = 8, and space group C2/c, and a = 7.841(4), b = 8.380(3), c = 12.743(6) A, alpha = 93.66(3), beta = 102.90(3), gamma = 116.85(3) degrees, Z = 2, and space group P1, respectively. Thermal decomposition of the title compound has been interpreted from the TG, DTG and DSC curves with the help of mass spectrometry. Medetomidine hydrochloride monohydrate decomposes in four stages. The first is dehydration at 45-100 degrees C, the second is evaporation of HCl and medetomidine base at 200-320 degrees C, and the third and fourth are decomposition at 340-570 degrees C. The protonation constant is 7.04 in aqueous 0.1 M NaClO4 (25 degrees C).