Hydrogen deuterium exchange reaction of 2-methylpyridine has been studied by using several fatty acids as catalysts. The reaction was carried out in a sealed pyrex tube at 120°C, and the contents of the products were determined by mass spectrometry. Reaction of 2--methylpyridine with monodeuteroacetic acid (1 : 1, mol/mol) arrived at a equibrium (d
0_??_d
1_??_d
2_??_d
3) in 2 hr (d
0 41%, d
1 42%, d
2 15%, d
3 2%). No exchange was observed for the reaction of pyridine with monodeuteroacetic acid. The conversion-time curves of typical series reactions (d
0 → d
1→ d
2 → d
3) were obtained for the fatty acid catalyzed exchange in deuterium oxide. The effect of the fatty acid RCO
2H (substrate fatty acid : D
2O=1 : 0.86 : 276, mol/mol/mol) on the conversion was in the order of R; C
1-C
3<C
4-C
10, where the reaction mixtures were homogeneous in the case of C
1-C
3 and were heterogeneous in the case of C
4-C
10. The effects of the initial concentration of the substrates and the catalysts (RCO
2H) on the total conversion were studied by using some fatty acids (R; C
2, C
4 and C
9) in deuterium oxide (for 2 hr). The total conversion of the substrate increases with increasing the concentration of the acids. The total conversion decreases in the case of R=C
9, but, increases in the case of R=C
2 with increasing the concentration of the substrate. In the case of reactions with low concentrations of the substrate, the reactivity was in the order of C
9>C
4>C
2, while with high concentrations, the reactivity was in the order of C
4>C
2C
9 and C
9>C
4>C
2 with high and low concentrations of the acids, respectively. A possible reaction mechanism was proposed and discussed.
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