The Absorption Spectra and Ionic Dissociation of Thiouracil Derivatives with Reference to Their Anti-Thyroid Activity

Summary The ultra-violet absorption spectra of 2-thiouracil, 2-thio-4-methyluracil, 1:4-dimethyl-2-methylmercapto-6-oxypyrimidine and 2-methylmercapto-4-methyl-6-oxypyrimidine have been determined at varying pH values. Electrometric titration curves have been plotted for 2-thio-4-methyluracil, 2-methylmercapto-4-methyl-6-oxypyrimidine and 1:4-dimethyl-2-methylmercapto-6-oxypyrimidine, and pKa values for these compounds in aqueous ethyl alcohol have been recorded. The close connection between change in pH and change in ultraviolet light absorption for the compounds studied shows that absorption changes are due primarily to ionisation and subsequent ionic rearrangement. On the basis of the iodine oxidation theory, the probable ionic nature of the iodine absorption mechanism suggests that thiouracil derivatives which are appreciably ionised at pH 7ṁ3 are most likely to exhibit anti-thyroid activity.