Kinetics and mechanism of the hydrolysis of N-isobutylidenemethylamine in aqueous solution

Measurements on the kinetics of hydrolysis of N-is3butylidenemethylamine from pH 0.3 to 8.2 have been combined with earlier kinetic messurements on the formation of this imine between pH 7.6 and 11.5 to give a fairly complete rate-pH profile at 35". The rate constant for hydrolysis increases from pH 0.3, where decomposition of the zwitterion i-PrCH(O-)NH2Me+ is rate controlling, up to around pH 4.5, where it begins to level off as hydration of the iminium ion via attack of water becomes rate controlling. The rate constant then falls around pH 7 as the equilibrium protonation of the imine begins to be significantly incomplete. Above about pH 8 the rate again levels off as attack of hydroxide ions on the iminium ion becomes rate controlling. The pKa value for the iminium ion obtained from the pH profile agrees well with the value 6.88 =t 0.08 determined by an indicator method on a stopped-flow spectrophotometer. Several rate and equilibrium constants for the individual steps of the reaction were determined directly, and others could be estimated rather reliably. It is estimated that below about pH 4.5 the principal form of the reactant is probably the conjugate acid of the carbinolamine, i-PrCH(OH)NH2Me+, rather than the iminium ion. study of the kinetics of the hydrolysis of N-isoA butylidenemethylamine was of interest to us for reasons given in the previous paper, in which a kinetic study of the formation of this imine from isobutyraldehyde and methylamine in aqueous solutions between pH 7.6 and 11.6 was described. In solutions that are so basic that neither the amine nor imine is protonated to an appreciable extent, isobutyraldehyde is transformed largely to the imine by excess methylamine at concentrations above about 0.02 M . 3 However, since the amine is considerably more basic than the imine, in acidic aqueous solutions no directly measurable fraction of the aldehyde can be transformed to imine or iminium ion at equilibrium. In solutions acidic enough that both the imine and amine are essentially completely protonated, the reaction whose kinetics we have studied has the stoichiometry shown in eq 1. The pKa of the iminium ion i-PrCH=NHMe+ + H 2 0 + i-PrCHO + MeNHaf (1) has been estimated4 to be around 7 or less. To aid in the interpretation of the kinetic results at various pH's, we have determined a more reliable value directly. We have also studied the kinetics of hydrolysis between pH 0.3 and 8.2 and drawn conclusions about the mechanism of establishment of the aldehyde-imine (or iminium ion) equilibrium over the entire pH range.