Bimolecular displacement reaction

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... 

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].

In his 1968 review of donor-acceptor interactions, Bent suggested that certain kinds of attractive intermolecular interactions may be viewed as incipient valence shell expansions and often as the first stage of bimolecular displacement reactions [44]. In particular he drew attention to the correlation between the two F-I distances observed in the linear IJ anion (Fig. 5,a) and suggested that the curve may be presumed to show, approximately, the changes that occur in the distances between nearest neighbors in the linear exchange reaction f + ff Similarly, the curves shown in Fig. 

Jencks (1969) has pointed out that the role of the enzyme itself is particularly difficult to understand in bimolecular displacement reactions in which general acid-base catalysis would give little advantage, or is not possible. As examples of such reactions Jencks included methyl transfer from S-adenosylmethionine to nicotinic acid to form the N-meth-yl pyridinium, trigonelline (Joshi and Handler, 1960), or to methionine to form the new sulfonium, S-methylmethionine (Greene and Davis,... 

Fig. 5. Bimolecular displacement reaction of iraras- Co(en)2N02Cl]+. The electron-withdrawal capacity of N02 by vp-d bonding decreases the electron density in the opposite octahedral faces and promotes an Sn2 process.

Miller s (60) calculations call for such an inverse isotope effect (fcn < ku) in bimolecular displacement reactions, with A h/ d increasing with temperature. In contrast, they require SnI dissociations to have the usual effect (fcn > fcn) the magnitude of which should decrease with temperature. 

These effects, determined with tertiary chlorides, unquestionably refer to 8n1 reactions so also do Lewis and Boozer s effects on acetolysis and formolysis—-if we can rely on the constancy of the effect of a-deuteration (Table VIII). The /3-effects then suggest that formolysis of the secondary bromide and solvolysis in aqueous ethanol of the tosylate are also SnI—or nearly so, unless bimolecular displacement reactions are also subject to similar isotope effects. Shiner (117) had however already shown that this was not so, since deuteration in the two methyl groups of isopropyl bromide did not lead to an experimentally significant effect on the displacement reaction with ethoxide ion in ethanol. It is thus also reasonable to interpret the very small effect (AAF" = 6 cal.) cited by Lewis (74f) for acetolysis of ethyl-2d8 brosylate as evidence that acetolysis of primary sulfonate esters is borderline if not Sn2. This conclusion, already suggested by the abnormally low a-effect for unassisted acetolysis of phenylethyl tosylate [Table VIII and text of Sec. VA, 2(b) 1 is supported by a similarly... 

The apphcation of bimolecular, nucleophilic substitution (S ) reactions to sucrose sulfonates has led to a number of deoxhalogeno derivatives. Selective displacement reactions of tosyl (79,85), mesyl (86), and tripsyl (84,87) derivatives of sucrose with different nucleophiles have been reported. The order of reactivity of the sulfonate groups in sucrose toward reaction has been found to be 6 > 6 > 4 > 1. ... 

Studies of the stereochemical course of rmcleophilic substitution reactions are a powerful tool for investigation of the mechanisms of these reactions. Bimolecular direct displacement reactions by the limSj.j2 meohanism are expected to result in 100% inversion of configuration. The stereochemical outcome of the lirnSj l ionization mechanism is less predictable because it depends on whether reaction occurs via one of the ion-pair intermediates or through a completely dissociated ion. Borderline mechanisms may also show variable stereochemistry, depending upon the lifetime of the intermediates and the extent of internal return. It is important to dissect the overall stereochemical outcome into the various steps of such reactions. 

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. 

It is interesting that when EtOe, in fairly high concentration, is used as the nucleophile in preference to EtOH, the reaction of (19) becomes SN2 in type and yields only the one ether (21). Allylic rearrangements have been observed, however, in the course of displacement reactions that are proceeding by a bimolecular process. Such reactions are referred to as SN2 and are believed to proceed ... 

These problems can be somewhat overcome by a study of reactions in solution where much greater densities are possible than in the gas phase and fast bimolecular reaction are diffusion limited [1,28,29]. However, since coordinatively unsaturated metal carbonyls have shown a great affinity for coordinating solvent we felt that the appropriate place to begin a study of the spectroscopy and kinetics of these species would be in a phase where there is no solvent the gas phase. In the gas phase, the observed spectrum is expected to be that of the "naked" coordinatively unsaturated species and reactions of these species with added ligands are addition reactions rather than displacement reactions. However, since many of the saturated metal carbonyls have limited vapor pressures, the gas phase places additional constraints on the sensitivity of the transient spectroscopy apparatus. 

Bimolecular, nucleophilic-displacement reactions of sugar sulfonates have been reviewed in this Series.58,59 The value of these reactions in the preparation of deoxyhalo sugars has been emphasized by Hanessian.92... 

Gas-phase intracomplex substitution in (R)-(- -)-l-arylethanol/CHs OH2 adducts. It is well established that bimolecular Sn2 reactions generally involve predominant inversion of configuration of the reaction center. Unimolecular SnI displacements instead proceed through the intermediacy of free carbocations and, therefore, usually lead to racemates. However, many alleged SnI solvolyses do not give fully racemized products. The enantiomer in excess often, but not always, corresponds to inversion. Furthermore, the stereochemical distribution of products may be highly sensitive to the solvolytic conditions.These observations have led to the concept of competing ° or mixed SNl-SN2 mechanisms. More recently, the existence itself of SnI reactions has been put into question. ... 

In order to strengthen evidence in favour of the proposition that concerted inplane 5n2 displacement reactions can occur at vinylic carbon the kinetics of reactions of some /3-alkyl-substituted vinyliodonium salts (17) with chloride ion have been studied. Substitution and elimination reactions with formation of (21) and (22), respectively, compete following initial formation of a chloro-A, -iodane reaction intermediate (18). Both (17) and (18) undergo bimolecular substitution by chloride ion while (18) also undergoes a unimolecular (intramolecular) jS-elimination of iodoben-zene and HCl. The [21]/[22] ratios for reactions of (18a-b) increase with halide ion concentration, and there is no evidence for formation of the -isomer of (Z)-alkene (21) iodonium ion (17d) forms only the products of elimination, (22d) and (23). 

Bruice and Benkovic (1964) and Bruice (1970) have noted that conversion of a bimolecular reaction into an intramolecular reaction corresponds to a reduction in kinetic order. Comparison of reactions of varying kinetic order (Table 5) reveals that a change of 4-5kcalmole in TAS accompanies reduction in order by one. These reactions include 17 displacement reactions on phenyl esters and 4 on thiol esters with an average value of rAS /(kinetic order)... 

The abbreviation Sn2 conveys the information sub-stitution-nucleophilic-bimolecular . The reaction is essentially the displacement of one group, a leaving group, by another group, a nucleophile. It is a bimolecular reaction, since kinetic data indicate that two species are involved in the rate-determining step ... 

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.

Bimolecular, nucleophilic-displacement reactions of sulfonic esters of carbohydrates have been reviewed.77,78... 

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