Substitution mechanisms types

In this mechanism, a complexation of the electrophile with the 7t-electron system of the aromatic ring is the first step. This species, called the 7t-complex, m or ms not be involved directly in the substitution mechanism. 7t-Complex formation is, in general, rapidly reversible, and in many cases the equilibrium constant is small. The 7t-complex is a donor-acceptor type complex, with the n electrons of the aromatic ring donating electron density to the electrophile. No position selectivity is associated with the 7t-complex. 

The second major type of nucleophilic substitution mechanism is the S 1 mechanism. This mechanism proceeds via two steps. The first step (the slow step) involves the breakdown of the alkyl halide into an alkyl carbocation and a leaving group anion. The second step (the fast step) involves the formation of a bond between the nucleophile and the alkyl carbocation. 

Although a substantial number of reactions are described in the text, they belong to a relatively modest number of mechanistic types. The preparation of alkyl halides from alcohols and HX, the cleavage of ethers, and the preparation of amines from alkyl halides and ammonia (and many other reactions) all, for example, occur by a nucleophilic substitution mechanism. The following is a brief review of the main mechanistic pathways discussed in the text. 

The orfAo-Claisen rearrangement, a no-mechanism type of reaction, is included here since it is, effectively, an alkylation reaction though not an electrophilic substitution. Two reports have recently appeared describing the rearrangement of 2-allyloxypyridines in tertiary amine solvents or in the absence of a solvent. Rearrangement... 

Ideally, the measurement of obs as a function of Xe concentration would have provided strong evidence for the type of mechanism, since for a dissociative mechanism, obs l/[Xe], whereas an associative mechanism would have no dependence on [Xe], However, it is not possible to vary the concentration of Xe in liquid Xe, and so pre-exponential factors A obtained from Arrhenius plots were used to differentiate the two mechanisms. The value of logA was found to lie within the expected range for a unimolecular dissociation reaction, and it was concluded that this reaction occurs by a dissociative substitution mechanism in liquid Xe. The Arrhenius plot therefore gave an estimate of the W—Xe BDE, AHw—Xe = 35.1 0.8 kJ mol . ... 

A variety of other y-type phases with high Li+ conductivity are derived from the Li3X04 phases with X = P, As, or V. The substitution mechanisms are of the type X (Si, Ge, Ti) - - Li, and lead to the creation of interstitial Li+ ions which are responsible for the high ionic conductivity. The highest conductivity at room temperature, 4 x 10 S cm , is found in the series Li3+j (Gej Vi j )04. Neutron diffraction has been nsed to locate the interstitial lithium ions, to determine their site occnpancy, and correlate the high ionic conductivity with the connectivity of the interstitial sites ... 

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... 

This picture is also consistent with the fact that, in general, to becomes shorter on increasing the content of tertiary amine co-units in the copolymer [118]. The observation that Rcmax decreases in poly(MBA-co-DAPA) and poly(MBA-co-DEPA) on increasing the content of tertiary amine co-units and that the minimum activity is observed in poly(MBA-co-DMEA) and poly(MBA-co-DEEA) in correspondence with the highest content of tertiary amine co-imits [118] can easily be explained. In fact, due to the mechanism proposed in Scheme 29, as soon as the traces of oxygen are consumed, the residual amine co-units, in excess with respect to MBA units, continue the conversion of the substituted benzyl-type polymeric radicals into the alkylamino radicals, which are known to display lower reinitiation constants for acrylic monomers. 

Although the kinetic rate law is helpful in determining the mechanism of a reaction, it does not always provide sufficient information. In cases of ambiguity, other evidence must be used to find the mechanism. This chapter will describe a number of examples in which the rate law and other experimental evidence have been used to find the mechanism of a reaction. Our goal is to provide two related types of information (1) the type of information that is used to determine mechanisms, and (2) a selection of specific reactions for which the mechanisms seem to be fairly completely determined. The first is the more important, because it enables a chemist to examine data for other reactions critically and to evaluate the proposed mechanisms. The second is also helpful, because it provides part of the collection of knowledge that is required for designing new syntheses. Each of the substitution mechanisms is described with its... 

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. 

Use CFAE calculations to compare dissociative-type and associativc-typc substitution mechanisms for the [Fe(CN)5(H20)]3 complex. 

Martens and Jacobs further elaborated the types of isomorphic substitution according to various substitution mechanisms.[17] Figure 2.10 shows different types of substitutions. The isomorphic substitution mechanism (SM) can be classified as i) SM I - substitution of Al atoms. SM la, SM lb, and SM Ic refer to monovalent, divalent and trivalent element substitutions of Al atoms, respectively, thus resulting in an M—O—P bond ii) SM II substitution of P atoms. SM Ila and SM II/ refer to tetravalent and pentavalent element substitutions, respectively, thus resulting in an M—O—Al bond iii) SM III substitution of pairs of adjacent Al and P atoms. 

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