Isotopic exchange reactions of amines. 2. Amino-amide proton exchange in the potassium methylamide-methylamine system

Abstract

The kinetics of the methylamine-potassium methylamide amino proton exchange reaction were studied by /sup 1/H NMR as a function of temperature and catalyst concentration. Rates were determined by analysis of the line shapes of both the amino and methyl peaks. The exchange kinetics cannot be described by a single bimolecular rate constant based on solvent and stoichiometric amide concentration. By using published data for the dissociation constant of potassium methylamide in methylamine, we calculated bimolecular rate constants for two cataytically active species in solution: free methylamide ions and potassium methylamide ion pairs. These are at 25/sup 0/C as follows: k/sub 2/(free ion) = 1.42 x 10/sup 9/ L.mol/sup -1/.s/sup -1/, k/sub 2/(ion pair) = 1.04 x 10/sup 8/ L.mol/sup -1/.s/sup -1/, with activation parameters ..delta..H double dagger = 12.8 and 10.3 kJ.mol/sup -1/ and ..delta..S double dagger = -27 and -57 J.mol/sup -1/.K/sup -1/, respectively. The NMR spectrum of anhydrous methylamine was recorded as a function of temperature over the range -82 to +75/sup 0/C. Values of 46.7 and 7.0/sub 4/ Hz, respectively, were found for the /sup 14/N-H and H-N-C-H spin coupling constants in methylamine. In addition the spin lifetimes of the methylamine /sup 14/N yielded an activation energymore » for quadrupole relaxation, a parameter characterizing rotational diffusion in the liquid phase, of 8.7 kJ.mol/sup -1/. 8 figures, 3 tables.« less