Rates and Mechanisms of Protolysis of Methylammonium Ion in Aqueous Solution Studied by Proton Magnetic Resonance

Abstract

The application of the nuclear magnetic resonance technique to the study of fast protolysis reactions of methylammonium ion in water is described. It is shown that reaction rates can be measured quantitatively for half‐times in the range, 0.002 to 0.2 sec. Reaction rates were measured as a function of the methylammonium ion concentration and of the hydrogen ion concentration. Detailed information on the reaction mechanism was obtained by correlating the rate constants obtained from the CH₃, the NH₃⁺ and the H₂O lines of the NMR spectrum. In the pH range employed (between 3 and 5), the exchange rate is very nearly proportional to [CH₃NH₃⁺][CH₃NH₂], the second‐order rate constant being 6×10⁸ sec^—1M^—1 at 19°. Two separate reactions contribute to this rate constant: One of them, which accounts for about 40% of the total rate, is a direct proton transfer from CH₃NH₃⁺ to CH₃NH₂. The other involves also at least one molecule of water, CH₃NH₂ receiving a proton from the solvation shell of CH₃NH₃⁺ while simultaneously, or soon thereafter, a proton is transferred from CH₃NH₃⁺ to the solvation shell.