Displacement, bimolecular

According to Habid and Malek49 and to Basolo and Pearson210, this highly negative value of AS for metal ion-catalyzed esterification is typical of bimolecular displacement... 

The rate of displacement and of inversion are thus identical within the limits of experimental error, and it thus follows that each act of bimolecular displacement must thus proceed with inversion of configuration. Having shown that SN2 reactions are attended by inversion of configuration, independent demonstration that a particular reaction occurs via the SN2 mode is often used to correlate the configuration of product and starting material in the reaction. 

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.

In the present case, alkaline solvolysis forms almost completely racemic product in contrast with that from the neutral hydrolysis which is stereospecific. It is clear that the alkaline solvolysis involves a symmetrical intermediate, a metaphosphorimidothioate. which reasonably may be expected to be planar (isoelectronic with S03). The close similarity in reactions of thiophosphoryl and phosphoryl centers is well established for bimolecular displacements on phosphorus and the resemblance apparently extends to the metaphosphate eliminations. 

Structures III and IV assist ionisation of the C-X bond, whereas structure II facilitates nucleophilic addition and consequently a bimolecular displacement of X. The various derivatives of carboxylic acids form a series with varying degrees of resonance stabilisation decreasing in the following order ... 

Steric effects provide a useful method of probing mechanisms. If a bimolecular displacement is involved, the increased steric hindrance on the ligand causes a decrease in rate, whereas steric acceleration is generally observed for a dissociation process. The data in Table 17 show that increasing the steric bulk of the ligands L decreases the reaction rate, hence the data strongly support an associative mechanism. 

The value of the steric coefficient s also suggests a bimolecular displacement reaction with a pentacoordinate intermediate. In this mechanism, the spJ hybridized silicon is rehybridized to an sp-V-like transition state. If the transition state TS 1 is the highest energy point along the reaction coordinate, there should be considerable bond formation between the incoming hydroxide anion and the... 

Bimolecular displacements on sulfur occur when sulfur is di-, tri-, or tetra-coordinated. Examples are shown in Equations 4.27-4.29.57... 

West (91), amplifying on these results, argued that since the solvolysis is bimolecular it must proceed either through a normal Sn2 bimolecular displacement or involve a rather stable pentacovalent intermediate. Both mechanisms. West believes, must involve a 5-coordinate transition state, and therefore may really be thought of as equivalent. West found that silacyclopentane was 13 times as reactive as diethylmethylsilane and 200 times as reactive as silacydohexane (which could be construed as evidence for I-strain in silacyclopentane). Since this order of reactivity is the same as that found in carbocyclic compounds, it was concluded that similar considerations of energy and entropy of reaction are encountered, a possibility that had also been advanced by Price. 

By choosing suitable forcing-conditions, it is possible to induce poly-0-acetylglycosyl halides to undergo bimolecular displacement-reactions. For example. Chapman and Laird studied the reactions of such halides with piperidine in acetone. The second-order law was obeyed, but the reactions were complicated by a concurrent, bimolecular (E-2), elimination reaction. (See Fig. 5 for changes with the D-glucosyl halides.) A selection of the re-... 

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].

The relative rates of reaction for the hydrolysis and condensation dictate the structure and properties of an alkoxide gel. These reaction rates are schematically described in Figure 8.18 [43] for the example of a silicon ethoxide. In acidic solutions, hydrolysis is achieved by a bimolecular displacement mechanism that substitutes a hydronium ion (H" ) for an alkyl [44]. Under these conditions the hydroljreis is rapid compared to the condensation of the hydrolyzed monomers and promotes the development of larger and more linear molecules, as is described in Figure 8.19. Under basic conditions, hydrolysis occurs by nucleophilic substitution of hydrojgrl ions (OH ) for alkyl groups [45]. Here the condensation is rapid relative to hydrolysis, promoting the precipitation of three-dimensional colloidal particles as shown in Figure 8.17(b) and 8.19. 

If the leaving groups were present, as in a bimolecular displacement process, then the product ratios may differ for different substrates. 

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