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Bimolecular displacement complexes

Figure C shows an extreme case of the dependence of a substitution reaction rate on the nature of the incoming group. This happens to be the hydrolysis of the trisacetylacetonate complex of silicon (IV), cationic species, which Kirchner studied first—the rate of racemization or rate of dissociation. We studied the base-catalyzed rate of dissociation and showed that a large number of anions and nucleophilic groups, in general, would catalyze in the dissociation process. We found that the reaction rates were actually for a second-order process, so these units are liters per mole per second. But the reaction rate did vary over an enormous range—in this case, about a factor of 109—and this is typical of the sort of variation in rates of reaction (that you can get) for processes that seem to be Sn2 bimolecular displacement processes. Figure C shows an extreme case of the dependence of a substitution reaction rate on the nature of the incoming group. This happens to be the hydrolysis of the trisacetylacetonate complex of silicon (IV), cationic species, which Kirchner studied first—the rate of racemization or rate of dissociation. We studied the base-catalyzed rate of dissociation and showed that a large number of anions and nucleophilic groups, in general, would catalyze in the dissociation process. We found that the reaction rates were actually for a second-order process, so these units are liters per mole per second. But the reaction rate did vary over an enormous range—in this case, about a factor of 109—and this is typical of the sort of variation in rates of reaction (that you can get) for processes that seem to be Sn2 bimolecular displacement processes.
Figure 2. Bimolecular displacement mechanism for substitution reactions of square planar complexes. ka is the rate constant for the solvent path and ky is the rate constant for the direct reagent path. Figure 2. Bimolecular displacement mechanism for substitution reactions of square planar complexes. ka is the rate constant for the solvent path and ky is the rate constant for the direct reagent path.
Fig. 5-5. Schematic one-dimensional relative enthalpy diagram for the exothermic bimolecular displacement reaction HO + CH3—Br —> HO—CH3 + Br in the gas phase and at various degrees of hydration of the hydroxide ion [485]. Ordinate standard molar enthalpies of (a) the reactants, (b, d) loose ion-molecule clusters held together by ion-dipole and ion-induced dipole forces, (c) the activated complex, and (e) the products. Abscissa not defined, expresses only the sequence of (a). .. (e) as they occur in the chemical reaction. The barrier heights ascribed to the activated complex at intermediate degrees of hydration were chosen to be qualitatively consistent with the experimental rate measurements cf. Table 5-3 [485]. Possible hydration of the neutral reactant and product molecules, CH3—Br and HO—CH3, is ignored. The barrier height ascribed to the activated complex in aqueous solution corresponds to the measured Arrhenius activation energy. A somewhat different picture of this Sn2 reaction in the gas phase, which calls into question the simultaneous solvent-transfer from HO to Br , is given in reference [487]. Fig. 5-5. Schematic one-dimensional relative enthalpy diagram for the exothermic bimolecular displacement reaction HO + CH3—Br —> HO—CH3 + Br in the gas phase and at various degrees of hydration of the hydroxide ion [485]. Ordinate standard molar enthalpies of (a) the reactants, (b, d) loose ion-molecule clusters held together by ion-dipole and ion-induced dipole forces, (c) the activated complex, and (e) the products. Abscissa not defined, expresses only the sequence of (a). .. (e) as they occur in the chemical reaction. The barrier heights ascribed to the activated complex at intermediate degrees of hydration were chosen to be qualitatively consistent with the experimental rate measurements cf. Table 5-3 [485]. Possible hydration of the neutral reactant and product molecules, CH3—Br and HO—CH3, is ignored. The barrier height ascribed to the activated complex in aqueous solution corresponds to the measured Arrhenius activation energy. A somewhat different picture of this Sn2 reaction in the gas phase, which calls into question the simultaneous solvent-transfer from HO to Br , is given in reference [487].
Hydrogen transfer has been induced with macrocyclic receptors bearing 1,4-di-hydropyridyl (DHP) groups. Bound pyridinium substrates are reduced by hydrogen transfer from DHP side chains within the supramolecular species 78 the first-order intracomplex reaction is inhibited and becomes bimolecular on displacement of the bound substrate by complexable cations [5.19]. Reactions with carbonyl or sulphonium substrates have been performed with other DHP containing macrocycles, such as 79 [5.20]. [Pg.59]

One useful approach to examining the dynamics of reactive bimolecular collisions involves analysis of the unimolecular dissociation of species that correspond to a reaction intermediate. This criterion was applied to the S 2 reaction in three independent investigations which made use of different experimental techniques and conditions to study the decomposition to products of specific ion-dipole complexes, the presumed intermediates of these nucleophilic displacement reactions239-241. [Pg.236]

Attractive interactions may be viewed as incipient valence shell expansion and as the stages of bimolecular nucleophilic displacement reactions . Crystallographic data for the reaction pathways first described for cadmium complexes by Burgi reveal a correlation between the two X—Si and Si<-N distances in the linear... [Pg.140]

When both reactants were coadsorbed on Rb-X at 308 K, indications for the formation of reaction products or bimolecular complexes were not found in the IR spectra. The spectra rather suggest that toluene and methanol are independently sorbed. It should be noted, however, that after equilibration with equal partial pressures of both reactants, toluene was the main sorbed species. Note that only part of the sites can be covered by toluene molecules due to steric reasons (theoretically 2/3 of the cations are accessible) and pore filling, while methanol achieved a coverage of approximately one molecule/cation at elevated partial pressures (p = 1 mbar). Coadsorption of toluene onto a surface preequilibrated with methanol resulted in the displacement of the main fraction of the methanol molecules (80 %) from the sorption sites [24].The same coadsorbed state was reached irrespective of the sequence of adsorption of the two reactants. If toluene was adsorbed first, coadsorption of methanol did not change the coverage of toluene. [Pg.453]

There is now much kinetic data on substitution reactions of square-planar complexes all of which are explained in terms of a bimolecular (8 2) displacement mechanism. For reactions such as... [Pg.317]

The type of reaction which is probably of most importance in the enzymatic degradation of polymers is the bimolecular reaction illustrated above, in which the enzyme catalyzes the interaction of the polymer and a low molecular reagent (such as water in a hydrolysis reaction). These reactions can occur by either a single displacement or a double displacement mechanism. In the former, both substrates, A and B below, are bound to the enzyme by consecutive, reversible reactions, after which the final complex, EAB, dissociates into the products, C and D, and the free enzyme, as follows ... [Pg.6]

The displacement of water in the analogous complexes /ran.v- Tc 0(0H2)(CN)4] and /rans-[Rc 0(0H2)(CN)4] by NCS ions in an aqueous medium was monitored spectrophotomctrically [62] using a stopped-flow technique [63]. The substitution of water by thiocyanate can be represented by the bimolecular reaction ... [Pg.51]


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