**4. Conclusions**

The presented research confirmed the great reactivity of methimazole in the 1,2-dichloroethane solution under mild conditions, leading to its spontaneous *S*-bis alkylation to 1,2-bis[(1-methyl-1*H*imidazole-2-yl)thio]ethane in the form of dihydrochloride tetrahydrate (**2b**). Therefore, the use of 1,2-dichloroetane should be avoided when working with methimazole. Dihydrochloride tetrahydrate 2b, anhydrous **2a**, and dihydrate **2c** crystal forms were prepared and their structures were confirmed by the single crystal X-ray diffraction analysis. Dehydration process of the dihydrate **2c**, studied using complementary solid-state techniques, led to the formation of the anhydrous product **2a**, which was identical to the one obtained by crystallization. No intermediate monohydrate or hemihydrate phases were detected.

**Author Contributions:** Conceptualization, L.Š. and M.D.; investigation, L.Š., D.M.-C., D.F.; data curation, L.Š., ˇ D.M.-C., and M.D.; writing—original draft preparation L.Š. and M.D.; writing-review and editing L.Š., D.M.- ˇ C., ˇ D.F. and M.D.; supervision M.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors appreciate Lara Safti´c Martinovi´c (U. of Rijeka, Department for Biotechnology) for recording HR-MS spectra and deep discussion, Ivica Ðilovi´c (U. of Zagreb, Faculty of Science, Department of Chemistry) for single crystal data collection of 2a and Ana Cikoš (Institute Ru ˇ der Boškovi´c, Center for NMR, ¯ Zagreb) for recording NMR spectra and deep discussion.

**Conflicts of Interest:** The authors declare no conflict of interest.
