A mechanism for Omethyloxime formation from benzaldehydes in aqueous solution is proposed that involves three sequential kinetically significant steps; namely (1) formation of an unstable zwitterionic tetrahedral intermediate, TA(2) protonation of T* in a diffusion-controlled reaction, and (3) hydronium ion catalyzed dehydration of a neutral tetrahedral intermediate that is in rapid equilibrium with the protonated species formed in step 2. The following evidence is presented in support of this mechanism. (1) The pH-rate profiles at zero buffer concentration for the reaction of methoxyamine free base with three substituted benzaldehydes show two negative breaks in the pH region below neutrality. ( 2) A change in ratedetermining step that cannot be accounted for by a transition from rate-determining carbinolamine formation to dehydration is observed in the presence of increasing concentrations of general acid catalysts with pmethoxybenzaldehyde at pH 2-3.(3) Absolute rates for the hydronium ion catalyzed proton transfer process, calculated from observed rate constants and estimated equilibrium constants for formation of TA are 7 X 109-1010 M~1 sec-1, in the range expected for a diffusion-controlled proton transfer. Absolute rates for the uncatalyzed conversion of T* to a neutral tetrahedral intermediate are 106-107 sec-1, consistent with a solvent-mediated intramolecular proton transfer. (4) Structure-reactivity correlations for three substituted benzaldehydes are consistent with the proposed mechanism; in particular, a logarithmic plot of the observed rate constants for the hydronium ion catalyzed proton transfer step against the equilibrium constants for formation of the neutral tetrahedral intermediates has a slope of 0.9 in satisfactory agreement with the expected value of 1.0 for a diffusion-controlled reaction of TA (5) The Br{fnsted plot for general acid catalysis of the reaction of p-methoxybenzaldehyde undergoes a transition from slope 0 to -1.0 at pK ~8.6, which is in good agreement with the estimated pK of ~9 for the hydroxyl group of the protonated tetrahedral intermediate. In addition to the mechanism involving a stepwise proton transfer, a second, concurrent mechanism for catalysis by the hydronium ion, which is ascribed to a proton transfer that is more-or-less "concerted" with carbon-nitrogen bond formation, is observed at high acidity.' for the addition step (extrapolated where necessary to zero methoxyammonium ion concentration), are plotted against acidity in Figure 2. The correction for the contribution of the dehydration step at these low pH values never exceeded 5% of the observed rate constants.A similar small break in the pH-rate profile for p-nitrobenzaldehyde occurs at pH ~4.0, such that the rate con-
The multiplicity patterns of olefin oxidation, catalyzed by a Pt wire controlled to maintain a preset average temperature, are traced by varying the reactant concentrations and are mapped in the concentration plane. The similar bifurcation maps of ethylene and propylene oxidations exhibit regions of tristability and isolated branches in the directions of olefin and oxygen concentrations.Isobutylene oxidation exhibits mushroom-shaped bistability. Several features indicate the occurrence of symmetry breaking to form a partially ignited wire. Comparison of these results with the characteristic bifurcation maps of inhomogeneous solutions, drawn in part 1 of this work, shows that these complex patterns are induced by coupling of reactant inhibition with strong exothermicity. This work also develops an efficient methodology for fast tracing of all the multiplicity features of reacting systems. Bifurcation diagrams are traced automatically with a microcomputer-governed experimental system, and the continuity of features in the second and third dimensions is studied off-line to decide on further experimentation.In the first part of this work (Sheintuch, 1989), we demonstrated that surprisingly complex multiplicity patterns can be induced by relatively simple kinetics in systems that attain stable inhomogeneous solutions. We reviewed previous observations of bistability in reactions catalyzed by an "isothermal" wire, i.e., controlled to maintain a preset (average) temperature, and showed that the patterns with counterclockwise hysteresis or with isolated branches can be accounted for by stationary thermal fronts in systems with simple kinetics or with reactant inhibition. The analysis was also extended to systems that show tristability in order to account for the observations reported in this part.This work presents a comparative study of multiplicity patterns and bifurcation maps in olefin oxidation on a Pt wire, using a thermochemic method. Regions of uniqueness, bistability, and tristability are mapped in the oxygen versus olefin concentration plane at several temperatures.
The decomposition of diethyl ether has been investigated with considerable accuracy at pressures up to 260 atmospheres at 426°C. The results confirm the previous approximate values found by Steacie and Solomon. The rate of the reaction is still increasing at the highest pressures investigated. The results may be qualitatively explained by the Rice-Herzfeld mechanism, and support the idea that the reaction is not a simple unimolecular change.
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