Pseudo-aminate mechanism

FIGURE 12.7 Base catalyzed hydrolysis of this class of pentaaminecobalt(lll) complex is believed to proceed via a pseudo-aminate mechanism via deprotonation of the blue hydrogen atoms. 

Substitution of several metal-carbonyl complexes Cr(CO)6 and Mn(CO)5 (amine) show a small dependence on the nature and concentration of the entering hgand. Under pseudo-first-order conditions, the rate laws for these substitutions have two terms, as shown for Cr(CO)6 (as for some substitution reactions with 16e complexes, see equation 5). The second-order term was always much smaller than the first-order term. A mechanism that ascribes the second-order term to dissociative interchange (U) has been suggested for the Mo(CO)5Am system (Am = amine) and involves a solvent-encased substrate and a species occupying a favorable site for exchange. Thus, the body of evidence for the simple metal carbonyls indicates that CO dissociation and is the mechanism of ligand substitution reactions. 

An investigation on the nature of the reaction between CO2 and blends of DEA and MDEA in aqueous solution is presented in this paper. The concentration of the total amine mixture was varied from one to two molars with temperatures varying from 293K to 313 K. The kinetic parameters associated with this reaction were evaluated using the zwitterion mechanism. Based on the pseudo first-order regime, the apparent rate constant was calculated from the overall pseudo first order rate constant. The overall absolute error in the calculation of the overall pseudo first order rate constant was less than 7% which upholds the validity of the chosen reaction rate model. 

X=Me, H, or Cl, and R = Me or Et, carried out under pseudo-first-order conditions (excess amine. A), gave pseudo-first-order rate constants koXis which exhibit a complex dependence on the concentration of amine since the second-order coefficient A a= o1)s/[A] is a function of [A]. Two types of behaviour were found (i) plots of kA vs. [A] are linear or (ii) plots of kA vs, [A] are curvilinear although 1/A a vs. 1/[A] is linear, to account for which a stepwise mechanism is proposed (Scheme 10). The... 

Using an excess of amine, pseudo-first-order rate coefficients were recorded, but not all plots against amine concentration were linear. The observed behaviours are interpreted in terms of a tetrahedral zwitterionic intermediate, the formation of which is rate-determining for the phenyl substrate. The relative rates of the steps of the mechanism are more finely balanced for the methyl case. The effect of changing from C=S to C=0 in the reactant is also examined. 

Pseudo-first-order rate constants (kou) have been measured for aminolysis of dibenzo-[l,2]oxathiin-6-oxide (166) (Scheme 29). The plot of fcobs vs amine concentration exhibits a downward curvature which levels off as the amine concentration increases. Such a downward curvature is proposed as definitive evidence for a stepwise mechanism, which is supported by the microscopic rate constants determined. ... 

In an excess of iron(n), however, the forward reaction does not go to completion. Under these conditions, the rate of approach to equilibrium is still of the pseudo-first-order type and is consistent with a reversible process. In the case of the bromo-complexes, the mechanism postulated involves a five-co-ordinate intermediate of platinum(ii), e.g. (amine ligands omitted)... 

A search of the literature on the reactions of primary and secondary amines with esters, amides, and imides reveals that generally SB-catalyzed aminolysis term in the rate law appears only when the rate law contains an additional GB-catalyzed term. It is also quite conunon that the rate law contains only a GB-catalyzed term. But it is almost rare or nonexistent that a rate law does not contain a GB-catalyzed term but contains an SB-catalyzed term. These general observations can be easily understood in terms of a stepwise mechanism similar to Scheme 2.32 with HNu = RNH2. However, a recent report on the kinetic studies of the cleavage of A-phthaloylglysine (46) in the buffers of hydrazine shows that pseudo-first-order rate constants (k bs) for the cleavage of 46 follow Equation 2.40 ... 

Pseudo-first-order rate constants (k bs) for the nucleophilic cleavage of phthalim-ide in the presence of buffer solutions of pyrrolidine follow Eqnation 7.52." These observed data may be explained by a brief reaction mechanism as shown by Scheme 7.8, in which SH and S represent respective nonionized and ionized phthalimide and Am represents a secondary amine in base form. 

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