Non-steady state intermediates: a re-examination of the kinetics of hydrolysis of N-methylisobutylidene and N-isopropylethylidene under acidic conditions

Direct measurement by stopped flow and UV–vis spectroscopy of the decay of two protonated imines, N-methylisobutylidene and N-isopropylethylidene, below a pH 3 resulted in a biphasic kinetic profile involving a rapid decay of the iminium ion to a carbinolamine intermediate. Previously published hydrolysis studies on N-methylisobutylidene did not report observation of non-first order kinetics. Spectroscopic evidence of the carbinolamine intermediate of N-methylisobutylidene was observed by 1H NMR. The rate constants for the pH independent step, k1, were similar for both compounds, 0.94s−1 for N-methylisobutylidene versus 0.87s−1 for N-isopropylethylidene. Following k1, the rate constants, k−1 and k±Ka(OH) differed by a factor of approximately 3 with the carbinolamine intermediate of N-isopropylethylidene being more stable over the carbinolamine of N-methylisobutylidene. An in-depth examination of the rate constants is presented.