Migration of methyl groups between aliphatic amines in water.

Glycine undergoes spontaneous decarboxylation in dilute aqueous solution at elevated temperatures to form methylamine. During that process, we noticed the appearance of dimethylamine and trimethylamine in smaller amounts that increased gradually with time. These observations suggested the existence of disproportionation reactions of methylamines in water, for which there appears to be no direct precedent in the literature. Every member of the methylamine series is found to yield other members of the methylamine series. When the total concentration of amine was held constant and the rate of reaction was examined as a function of changing pH using the amine itself as the buffer, the initial rate of appearance of the products was found to reach a maximum when the conjugate acid and the conjugate base were present at equivalent concentrations. Near this equivalence point, the rate of reaction varied with pH as expected for a second-order reaction between the protonated and the unprotonated species. Under similar conditions, methyl groups were also found to migrate between the nitrogen atoms of N,N-dimethyl-1,3-propanediamine in a first-order process. With dimethylamine as a common acceptor, trimethylsulfonium ion was found to be approximately 104-fold more reactive than the tetramethylammonium ion at ambient temperature.