ESR study of the kinetics of H/D exchange in the 5-hydroxypyrimidine series

1. The effect of temperature on the value of the pH of an aqueous solution of 5-hydroxypyrimidine and 4,6-dimethyl-5-hydroxypyrimidine in the pH range of 0.5–10 and the temperature range of 24–90°C was investigated, and equations describing the dependence of the ionization constants on the temperature were derived. 2. Based on the analysis of the kinetic mechanisms of H/D exchange in 5-hydroxypyrimidine in the pD region of 0.5–10 and at 160°, it was found that the reaction takes place according to an electrophilic mechanism with participation of the anionic and neutral forms: The cationic form also participates in the reaction in the case of 4,6-dimethyl-5-hydroxypyrimidine. 3. The exchange rate constants characterizing the contribution of the cationic, neutral, and anionic forms of 5-hydroxypyrimidine derivatives in H/D exchange of the protons in the ring were determined.