Carboxylic Acid Catalyzed Hydrogen-Deuterium Exchange Reaction of 2-Methylpyridine in Deuterium Oxide

Hydrogen-deuterium exchange reaction of 2-methylpyridine has been studied by using several carboxylic acids as a catalyst in deuterium oxide. The reaction was carried out in a sealed pyrex tube at 120°C, and the contents of the product were determined by mass spectrometry. The exchange rate catalyzed by acids (DCl, RCO2H : R=CH3, ClCH2, Cl2CH, Cl3C, CF3, Ar) is larger than that by bases (NaOD, NaOAc). At lower acid concentrations, strong acids (DCl, RCO2H : R=CF3, Cl2CH, ClCH2) are more effective than acetic acid, while at higher acid concentrations vice versa. For the acetic acid catalyzed exchange, the rate increases with increasing the initial concentrations of the substrate and acid. For the reaction by the strong acids, the rate increases up to a maximum and then decreases with increasing the acid concentration, higher conversion being observed at higher concentration of the substrare. A possible reaction mechanism is proposed and discussed.