Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy. Differential Rates of N-H Proton-exchange Reactions of Hydrazinecarbothioamide and 4-Methylhydrazinecarbothioamide in Dimethyl Sulfoxide

The proton-coupled nitrogen-15 NMR spectra of hydrazinecarbothioamide and 4-methylhydrazinecarbothioamide have been taken at the natural-abundance level in neutral, basic and acidic solutions at 25°C. The N-H proton-exchange reactions of the hydrazino-NH_2 groups in both compounds were found to be very rapid in the presence of acid, but quite slow in the presence of base. The hydrazino-NH protons of hydrazinecarbothioamide exhange six times and 200 times faster than the amide protons in the presence of either base or acid, respectively. Similarly, acid- and base-catalyzed N-H proton exchanges of the hydrazino-NH group of 4-methylhydrazinecarbothioamide were found to be two to three orders of magnitude faster than those of N-methylamido protons. These results can be rationalized by consideration of the effect of the lone pair on the hydrazino-NH_2 group on the reactivity of the adjacent -NH- group.