Hydrogen-Deuterium Exchange Reaction of 4-Methylpyridine Catalyzed by Several Fatty Acids in Deuterium Oxide

Hydrogen-deuterium exchange of 4-methylpyridine (1b) has been studied by using fatty acid RCO2H (R=C1C10) as a catalyst in D2O at 120°C. The reaction mixture is homogeneous in the case of R=C1C3, and heterogeneous with R=C4C10. The conversion (or contents) -time curves show typical stepwise reactions (d0→d1→d2→d3). The rate increases with the increase in the concentration of the acids. The conversion decreases and increases, respectively, in the cases of R=C5C10 and R=C1C3 with increasing the concentration of the substrates. The mode of the valeric acid catalyzed reaction (R=C4) is an intermediate between the two groups above. The reactivity is in the order of 2-methylpyridine (1a) (1b) with R=C7C10. From the above observations, a possible mechanism involving protonation and rate-determining attack of free base or carboxylate ion is proposed and discussed by assuming an ion pair model.