PROTON EXCHANGE OF N-METHYLACETAMIDE IN CONCENTRATED AQUEOUS ELECTROLYTE SOLUTIONS .I. ACID CATALYSIS

Acid-catalyzed amide hydrogen exchange kinetics of N-methylacetamide (NMA) have been measured in a variety of concentrated electrolyte solutions. Second-order rate constants for the exchange process (k), determined by nmr line-shape analyses, have been shown to be markedly dependent on the nature of the supporting cation, and quite insensitive (except for SCN-) to the species of supporting anion present. Furthermore, it has been shown that log k is a linear function of supporting electrolyte concentration. A linear relationship has also been observed between log k and the reciprocal of the crystal radius of the supporting cation, suggesting that cationic charge density is an important factor in determining the exchange rate. The effects of added electrolyte on the rate constant do not seem to correlate with any of the usual solution electrolyte properties, but can be accounted for formally by postulating salt-induced changes in the activity of the transition-state complex. A mechanistic model is proposed involving a cation-dependent alteration in the equilibrium between the various tautomeric forms of protonated NMA, not all of which are active in the acid-catalyzed proton-exchange reaction. 7Li+ nmr spin-lattice relaxation measurements performed in lithium halide mixed amide-water solutions revealed no anion dependence, within experimental error, of the spin-lattice relaxation time (T1), confirming further that the interaction between the lithium ion and the amide dipole is not affected by the associated anion. © 1968, American Chemical Society. All rights reserved.