General grouping mechanism

The generally accepted mechanism for nucleophilic aromatic substitution m nitro substituted aryl halides illustrated for the reaction of p fluoromtrobenzene with sodium methoxide is outlined m Figure 23 3 It is a two step addition-elimination mechanism, m which addition of the nucleophile to the aryl halide is followed by elimination of the halide leaving group Figure 23 4 shows the structure of the key intermediate The mech anism is consistent with the following experimental observations... 

In general, physico-mechanical properties of polymers depend on the molecular weight. However, the physico-mechanical properties of PSs decreased in the presence of cationic catalysis, but increased in the case of the binding of functional groups to the aromatic ring in spite of the destruction of PS. Therefore, new properties such as adhesion and photosensitive capability increase... 

The ionization of (E)-diazo methyl ethers is catalyzed by the general acid mechanism, as shown by Broxton and Stray (1980, 1982) using acetic acid and six other aliphatic and aromatic carboxylic acids. The observation of general acid catalysis is evidence that proton transfer occurs in the rate-determining part of the reaction (Scheme 6-5). The Bronsted a value is 0.32, which indicates that in the transition state the proton is still closer to the carboxylic acid than to the oxygen atom of the methanol to be formed. If the benzene ring of the diazo ether (Ar in Scheme 6-5) contains a carboxy group in the 2-position, intramolecular acid catalysis is observed (Broxton and McLeish, 1983). 

Radicals can be obtained from reduction of molecules followed either by protonation or departure of a nucleophile as illustrated in Schemes 6 [10] and 7 [11], respectively. In the first example, a generally accepted mechanism involves a reduction of a double bond activated by an electron-withdrawing group to a radical anion followed by protonation and cyclization of the resulting radical. The addition of a second electron and proton completes the process. 

For more than forty years the Bartlett butterfly TS was the generally accepted mechanism for peracid epoxidation and numerous experimental studies supported this transition structure" " . During these formative years theoretical calculations did not play a major role due to limitations of available methods that could adequately treat the peroxide functional group. Theoretical contributions in 1978 were at the Hartree-Fock (HF) level since... 

Correlation of the observed rates with the concentrations of the substrate species (10) indicates that the metal ion does not catalyze the hydrolysis of the monoanionic form of salicyl phosphate. Combination of the monoanionic form of the substrate with the vanadyl ion would result in an unreactive complex having a neutral carboxyl group. Shift of the proton to the phosphate group could not take place in accordance with the requirements of the general reaction mechanism illustrated in Figure 3. Thus the vanadyl ion would be expected to catalyze the hydrolysis of only the di- and trinegative forms of the substrate. 

Generally, the mechanism proposed for structure direction and self-assembly in the synthesis of Si-ZSM-5 involves the formation of an ordered, hydrophobic hydration sphere around the TPA cation [11]. The assembly process of MCM-41 is controlled by electrostatic interaction between silicate species and charged surfactant head groups [12]. 

Catalytic decarboxylation processes occur in aliphatic keto acids in which the keto group is in an a-position to one carboxyl group and in a P-relationship to another. Thus, the normal decarboxylation of a p-keto acid is facilitated by metal coordination to the a-keto acid moiety. The most-studied example is oxaloacetic acid and it has been shown that its decarboxylation is catalyzed by many metals following the general order Ca2+ < Mn2+ < Co2+ < Zn2+ < Ni2+ < Cu2+ < Fe3+ < Al3"1".66 67 The overall rate constants can be correlated with the stability constants of 1 1 complexes of oxalic acid rather than oxaloacetic acid, as the uncoordinated carboxylate anion is essential for the decarboxylation. The generally accepted mechanism is shown in Scheme 15. Catalysis can be increased by the introduction of x-bonding ligands, which not only increase the... 

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