A substitution mechanism

According to this suggestion, the electrophilic attack is at the para position (or the ortho, which leads to the same produet) and the meta orientation of the amino group arises indirectly. This mechanism is called the a-substitution mechanism. 

SiC>2N2 were clearly assigned. Since NH substitution for O in zeolite Y occurs preferably near A1 [70-72], a substitution mechanism may be surmised which involves Lewis acidic framework aluminum which reacts with the Lewis base NH3. 

The large and positive Avalues and, particularly the large and positive AV values obtained (21) for kon and represent signatures for a substitution mechanism dominated by ligand dissociation, for the ferri-heme complexes, i.e.,... 

In their mechanism, presented as a series of proton equilibria in Scheme 10, the reaction is controlled by three steps (a) ionization of the zinc-bound water, which destabilizes the binary complex to an extent that substrate binding cannot occur (p/ 3, Scheme 10) (b) a stabilizing effect of alcoholate ion formation in the ternary complex. The pH dependence of this step is the result of ionization of the alcohol.1449 (c) The dissociation of the alcohol from the ternary complex. This is similar in rate to the dissociation of aldehydes, which might be expected for a substitution mechanism, both neutral species forming structurally similar ternary complexes. 

As noted above, ethanol will react with the silica surface via a substitution mechanism during the extraction of the amine template at elevated temperatures. Our... 

The structure of the solid solutions is very complex. Investigations have shown that partial (and only partial) occupancies of up to five interstitial atomic positions occur. These atomic positions are situated between the icosahedra of the stmcture. In a few cases a substitutional mechanism for the solid solubility has been demonstrated, similar to the silicon... 

Aluminum. Previous Al NMR studies have demonstrated four possible local environments for Al in SAPO materials (3,4). These environments are illustrated in Figure 3, and may be classified as either phosphorous rich (i.e., ALPO -like) with a chemical shift ranging from 30 to 40 ppm, or silicon rich (i.e., zeolite-like) with a chemical shift greater than 48 ppm. Both types of environments are characteristic of a substitution mechanism involving silicon substitution for phosphorus. A fifth possibility for an Al environment involves two Si and two P second nearest neighbors. However, no such environment has yet been identified by NMR, either because the Al chemical shift is similar to that for the silicon- or phosporous-rich environments, or because materials with an appropriate level of Si to give rise to... 

In a specific instance a substitution mechanism should take into account the individual geometry of the starting compound. It is convenient to discuss the experimental results on the 77-C5H5 substitution and transfer reactions under three headings (1) substitution occurring in compounds leading to increased ionic character of the metal-ligand 77-bond (2) substitution of monocyclopentadienyl compounds (3) substitution in the ferrocene-like sandwich complexes. 

Side group reactions are common during pyrolysis and they may take place before chain scission. The presence of water and carbon dioxide as main pyrolysis products in numerous pyrolytic processes can be explained by this type of reaction. The reaction can have either an elimination mechanism or, as indicated in Section 2.5 for the decarboxylation of aromatic acids, it can have a substitution mechanism. Two other examples of side group reactions were given previously in Section 2.2, namely the water elimination during the pyrolysis of cellulose and ethanol elimination during the pyrolysis of ethyl cellulose. The elimination of water from the side chain of a peptide (as shown in Section 2.5) also falls in this type of reaction. Side eliminations are common for many linear polymers. However, because these reactions generate smaller molecules but do not affect the chain of the polymeric materials, they are usually continued with chain scission reactions. 

Impurities introduced into the crystal lattice can strongly affect the defect equilibria [9, 15-19]. This influence can be illustrated by the example of MO oxide matrix doped with Me2O3, assuming that predominant defects in pure MO are vacancies in the metal and oxygen sublattices (Vm, V ) in accordance with Equation (3.30). The dissolution of Me2O3 is also assumed to occur via a substitution mechanism, that is, Me atoms incorporate into the M-sublattice ... 

This argument was explored by Reynard et al. (1999), using values of E and Vg obtained from the experimental partitioning data of Fujimaki (1986). Reynard et al. (1999) used Equation (1) to predict equilibrium REE-apatite partition coefficients at surface temperature and pressure, assuming that the crystal chemistry of bone apatite is broadly similar to that of HAP, and that crystal-melt partition coefficients can be used to estimate crystal-water partitioning. Reynard et al. (1999) then compared the predicted partition coefficients with measured adsorption coefficients for the REE between seawater and HAP derived by Koeppenkastrop and DeCarlo (1992), and concluded that incorporation of REE into bone via a substitution mechanism produces bell shaped REE patterns with relatively little fractionation between La and Lu. Incorporation of REE into bone via an adsorption mechanism, on the other hand, produces significant fractionation between La and Lu (La/Lu = 5). Based on REE patterns found in fossil fish teeth, they concluded that REE uptake in fossil bone was dominated by adsorption mechanisms, but that subsequent recrystallization may superimpose a degree of substitution-related fractionation over the initial, adsorption related REE pattern. It is important to note, however, that these predictions are based on crystal chemistry of hydroxyapatite and fluorapatite, and not dahllite and francolite. Variations in E and Vo will affect relative adsorption and/or partition coefficients, and may alter the predicted partition coefficient ratios (e.g., La/Lu and La/Sm). 

Towards the end of Chapter 10 we introduced a classic example of a nucleophilic substitution reaction, namely the hydrolysis of a halogenoalkane by a warm aqueous solution of sodium hydroxide. As a result of the polarization of the carbon-halogen bond, the carbon atom is an electron-deficient centre susceptible to attack by a nucleophile such as the hydroxide ion (OH ). Primary halogenoalkanes are thought to undergo a substitution mechanism that involves a single reaction step. This one-stage reaction involves the simultaneous attack of the nucleophile and departure of the halide ion. We will use as an example the reaction between bromomethane and sodium hydroxide solution ... 

The reaction mechanism is not known. Seyferth et al. [11b] suggested a substitution mechanism of the Si-R groups by nucleophilic attack of Ae ammonia. Bums and Chandra [3] offered a radical mechanism in which the ammonia dissociates to H2N- and H radicals. None of these mechanisms can explain the reduction of carbon in the case where the alkyl groups are bonded to backbone nitrogen atoms. 

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