Mechanism of displacement reactions

Figure 11-11. Displacement reactions of the type AX+BY = AY+BX. a) lost mechanism, schematic, b) Equipotential surfaces and the evolution of phase boundary instability, lost mechanism, c) Wagner mechanism of displacement reactions, schematic.

Libby s proposals were essentially non-inechanistic. The formation of ethyl bromide by the recombination of an ethyl radical and a Br-atom was, in the strictest sense of the term, a no-mechanism reaction. Perhaps the difficulty with the early models was that they attempted to explain everything. The approach most prevalent today is to consider each recoil species separately by investigating the nature of the labeled products and the effects of various physical and chemical parameters on the yields of these products. As we shall see later, rationalization of product distribution in terms of mechanism has had considerable success. Application of known mechanisms of displacement reactions, insertion reactions, free radical reactions, etc. to recoil reactions has been quite useful. 

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 effects of adsorbed inhibitors on the individual electrode reactions of corrosion may be determined from the effects on the anodic and cathodic polarisation curves of the corroding metaP . A displacement of the polarisation curve without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the mechanism of the reaction. An increase in the Tafel slope of the polarisation curve due to the inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. However, the determination of the Tafel slope will often require the metal to be polarised under conditions of current density and potential which are far removed from those of normal corrosion. This may result in differences in the adsorption and mechanistic effects of inhibitors at polarised metals compared to naturally corroding metals . Thus the interpretation of the effects of inhibitors at the corrosion potential from applied current-potential polarisation curves, as usually measured, may not be conclusive. This difficulty can be overcome in part by the use of rapid polarisation methods . A better procedure is the determination of true polarisation curves near the corrosion potential by simultaneous measurements of applied current, corrosion rate (equivalent to the true anodic current) and potential. However, this method is rather laborious and has been little used. 

The importance of displacement reactions on carbonyl compounds in chemistry and biochemistry has resulted in numerous mechanistic studies. In solution, there is general acceptance of the following mechanism for addition of anionic nucleophiles which features a tetrahedral intermediate, 1, and is designated (1). However, recent experimental (2 10) and theoretical (11-17)... 

The mechanism of the reaction involves the initial formation of a protonated alkyl dibromophosphite by a nucleophilic displacement on phosphorus ... 

The first step in oxygen transfer is ligand substitution at an oxorhenium(V) center, Eq. (14). The final step (see Scheme 2, step 2) very likely is also ligand substitution. We have therefore examined the kinetics and mechanism of several reactions in which one monodentate ligand displaces another, represented in general as follows ... 

A free-radical mechanism has been suggested for the nitrosation of 1,2-phenylenediamine (22) by peroxynitrite PN/CO2. 1,2,3-Benzotriazole (26) was formed as a result of an intramolecular nucleophilic displacement on the diazo hydroxide (25) by the neighbouring amine group. The authors suggest that the mechanism involves an initial H-atom abstraction or one-electron oxidation from (22) by CO3 , followed by the reaction of the product (23) with NO. The inhibitory effects of azide support a free-radical mechanism of the reaction. 

A discussion of nucleophilic reactions in carbonyl systems, and in particular in derivatives of carboxylic acids, is included in the present text because of the importance of these reactions and their analogy with well-known processes in solution. While the actual mechanism of these reactions is more adequately described in many cases as an addition-elimination, we shall restrict our comments to systems which formally behave as displacement reactions. 

Catalysis of the hydrolysis of acetic anhydride by acetate ion cannot be explained in this way, since nucleophilic displacement simply generates another molecule of acetic anhydride. The mechanism of this reaction is presumed to be general base catalysis, usually written as... 

When the 3-(l-chloroalkyl)benzo[6]thiophene-l,1-dioxides (338 R = H or Me) are treated with piperidine, they readily form the enamines (339 R = H or Me).717b The mechanism of these reactions involves an Ss2 nucleophilic displacement followed by a rearrangement. 

The mechanism of the reaction of 1-butanol with hydrogen bromide proceeds by displacement of water by bromide ion from the protonated form of the alcohol (the alkyloxonium ion). 

In the first case [1] Y can be either a hydroxy, alkoxy or nitro group. The first two groups are important but variable constituents in coals and the last is probably minor or non-existent. The second active class of species are the alkyl-pyridines [2]. The final case [3] includes substituents on the benzyl carbon where X can be an ether or carbonyl functional group. The general mechanism of this reaction is most probably the base catalyzed iodination of the benzyl carbon with subsequent displacement of the iodide by the pyridine to form the pyridinium salt. In all three modes of activation, the single aromatic ring can be replaced with polycyclic rings. 

The silane substrate would then displace H2 to give back the starting silane complex for further alcoholysis, and this was determined to be the rate-limiting step. These are all known reactions, and this mechanism and rate-determining step were recently supported by theoretical calculations that showed the heterolysis to be a highly concerted process, i.e., transformation of the a-silane complex to the H2 complex could even take place in a single step, thus circumventing the transient hydride complex (124). It is noteworthy that the mechanism of this reaction involves two different a complexes M(r 2 -Si-H) and M(r 2 -H2). 

The synthesis of the important quinolone antibiotic 6.33 is shown. The key stages are the Gould-Jacobson quinolone synthesis to give 6.32, and the displacement reaction to afford 6.33. What are the mechanisms of these reactions ... 

Figure 2.34 shows the mechanism of this reaction. A key intermediate is the alkylated phosphine oxide A, with which the carboxylate ion reacts to displace the leaving group 0=PPh3. Figure 2.34 also shows that this carboxylate ion results from the deprotonation of the carboxylic acid used by the intermediate carbamate anion B. Nucleophiles that can be deproto-nated by B analogously, i.e., quantitatively, are also alkylated under Mitsunobu-like conditions (see Figure 2.36). In contrast, nucleophiles that are too weakly acidic cannot undergo Mitsunobu alkylation. Thus, for example, there are Mitsunobu etherifications of phenols, but not of alcohols.

Pt electrodeposition produces a decreases of the overpotential of 02 electroreduction and therefore a displacement of the maximum efficiency of H202 photoproduction toward potentials more anodic (see Fig. 3). However the mechanisms of the reaction seem to be the same as for naked Ti02 (Tafalla and Salvador, 1987). 

Primary and secondary alcohols are readily converted to mesylate or tosylate esters by reaction with the corresponding sulfonyl chloride. The mesylate and tosylate esters derived from tertiary alcohols are too reactive and cannot be isolated. (Although we will not go into the mechanism of these reactions in detail at this point, the reactions involve the attack of the oxygen [the nucleophile] of the alcohol at the sulfur [the electrophile], ultimately displacing chloride [the leaving group].) Pyridine is often used as a solvent for these preparations in order to react with the HC1 that is produced as a by-product. An example of the preparation of a methanesulfonate (mesylate) ester is shown in the following equation ... 

Figure 2.29 shows the mechanism of this reaction. A key intermediate is the alkylated phosphine oxide A, into which the carboxylate ion enters in a backside attack and displaces the leaving group 0=PPh3. 

Elimination of hydrogen halides, particularly in the presence of base, is also a common reachon (equation 54). The actual mechanism of these reactions could involve nucleophihc displacement of halide by the metal carbonyl halide that is formed in situ from the hydride (equations 55 and 56). 

The mechanism of displacement of chlorine and bromine by fluoride from the side chain of these systems is of interest. It has been suggested that an Sn2 type of displacement of fluorine from 3-trifluoromethylquinoline occurs in reactions with sodium eth-oxide [141] (Figure 9.55), and a similar process could account for the displacements of chloride or bromide by fluoride from 9.54A that were indicated in Figure 9.54. 

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