Formation mechanisms reaction pathway

Like tert butyloxonium ion tert butyl cation is an intermediate along the reaction pathway It is however a relatively unstable species and its formation by dissociation of the alkyloxonium ion is endothermic Step 2 is the slowest step m the mechanism and has the highest activation energy Figure 4 8 shows a potential energy diagram for this step... 

A number of reaction pathways have been proposed for the Fischer indolization reaction. The mechanism proposed by Robinson and Robinson in 1918, which was extended by Allen and Wilson in 1943 and interpreted in light of modem electronic theory by Carlin and Fischer in 1948 is now generally accepted. The mechanism consists of three stages (I) hydrazone-ene-hydrazine equilibrium (II) formation of the new C-C bond via a [3,3]-sigmatropic rearrangement (III) generation of the indole nucleus by loss of... 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

Monoalkylthallium(III) compounds can be prepared easily and rapidly by treatment of olefins with thallium(III) salts, i.e., oxythallation (66). In marked contrast to the analogous oxymercuration reaction (66), however, where treatment of olefins with mercury(II) salts results in formation of stable organomercurials, the monoalkylthallium(III) derivatives obtained from oxythallation are in the vast majority of cases spontaneously unstable, and cannot be isolated under the reaction conditions employed. Oxythallation adducts have been isolated on a number of occasions (61, 71,104,128), but the predominant reaction pathway which has been observed in oxythallation reactions is initial formation of an alkylthallium(III) derivative and subsequent rapid decomposition of this intermediate to give products derived by oxidation of the organic substrate and simultaneous reduction of the thallium from thallium(III) to thallium(I). The ease and rapidity with which these reactions occur have stimulated interest not only in the preparation and properties of monoalkylthallium(III) derivatives, but in the mechanism and stereochemistry of oxythallation, and in the development of specific synthetic organic transformations based on oxidation of unsaturated systems by thallium(III) salts. 

While the data provide clear evidence for the formation of incomplete oxidation products, and help to identify the nature of the stable adsorbate(s) formed upon interaction with the respective Ci molecules, the molecular-scale information on the actual reaction mechanism and the main reaction intermediates is very indirect. Also, the reaction step(s) at which branching into the different reaction pathways occurs (e.g., direct versus indirect pathway, or complete oxidation versus incomplete oxidation) cannot be identified directly from these data. Nevertheless, by combining these and the many previous experimental data, as well as theoretical results, conclusions on the molecular-scale mechanism are possible, and are substantiated by a solid data base. 

For both methanol oxidation and formic acid oxidation, a dual-pathway mechanism has been proposed (for methanol oxidation, see Lamy et al. [1983] Jarvi and Stuve [1998] Cuesta [2006] Housmans et al. [2006] Iwasita [2003] for formic acid oxidation, see Parsons and VanderNoot [1988] Sun et al. [1988] Willsau and Heitbaum [1986] Miki et al. [2002] Samjeske and Osawa [2005] Chen et al. [2006a, b, c] Samjeske et al. [2005, 2006] Miki et al. [2004], Chang et al. [1989]), in which one reaction pathway proceeds via formation and subsequent oxidation of COad (P, indirect pathway ), while the other leads, via one or more reaction intermediates RI, directly to CO2 ( direct pathway ) (Fig. 13.8a). 

In the original proposal of the dual-pathway mechanism (for formic acid oxidation, see [Capon and Parsons, 1973a, b, c] for methanol oxidation, see [Parsons and VanderNoot, 1988 Jarvi and Stuve, 1998 Leung and Weaver, 1990 Lopes et al., 1991 Herrero et al., 1994, 1995]), both pathways lead to CO2 as the final product, as illustrated in the reaction scheme depicted in Fig. 13.8a [Jarvi and Smve, 1998]. In this mechanism, desorption of incomplete oxidation products was not included. The existence of a direct reaction pathway for methanol oxidation, following the dual-pathway mechanism, was justified by the observation of a methanol oxidation current at potentials where COad oxidation is not yet active [Sriramulu et al., 1998, 1999 Herrero et al., 1994, 1995]. The validity of this interpretation was questioned, however, by Vielstich and Xia (1995), who claimed that CO2 formation is observed only with the onset of COad oxidation and that the faradaic current measured at lower potentials is due to the formation of the incomplete oxidation products formaldehyde and formic acid. The latter findings were later confirmed by Wang et al. [2001], Korzeniewski and Childers [1998], and Jusys et al. [2001, 2003]. In more... 

A detailed examination of the kinetics of dimethylaminolysis of N3P3C16 by Krishnamurthy and co-workers has revealed that there is a gradual and subtle mechanistic change that occurs as the degree of replacement of chlorines increases (92). While the first chlorine replacement follows an Sn2 pathway involving the formation of a neutral five-coordinate intermediate [Fig. 8(A)], at the second stage the mechanism can be induced to follow a concerted path [Fig. 8(B)] by using acetonitrile as the solvent. The polar transition state of the concerted path reaction pathway is stabilized in acetonitrile. This postulate has sup-... 

The oxidation of a-tocopherol (1) to dimers29,50 and trimers15,51 has been reported already in the early days of vitamin E chemistry, including standard procedures for near-quantitative preparation of these compounds. The formation generally proceeds via orf/zo-quinone methide 3 as the key intermediate. The dimerization of 3 into spiro dimer 9 is one of the most frequently occurring reactions in tocopherol chemistry, being almost ubiquitous as side reaction as soon as the o-QM 3 occurs as reaction intermediate. Early accounts proposed numerous incorrect structures,52 which found entry into review articles and thus survived in the literature until today.22 Also several different proposals as to the formation mechanisms of these compounds existed. Only recently, a consistent model of their formation pathways and interconversions as well as a complete NMR assignment of the different diastereomers was achieved.28... 

In this chapter we have seen that enzymatic catalysis is initiated by the reversible interactions of a substrate molecule with the active site of the enzyme to form a non-covalent binary complex. The chemical transformation of the substrate to the product molecule occurs within the context of the enzyme active site subsequent to initial complex formation. We saw that the enormous rate enhancements for enzyme-catalyzed reactions are the result of specific mechanisms that enzymes use to achieve large reductions in the energy of activation associated with attainment of the reaction transition state structure. Stabilization of the reaction transition state in the context of the enzymatic reaction is the key contributor to both enzymatic rate enhancement and substrate specificity. We described several chemical strategies by which enzymes achieve this transition state stabilization. We also saw in this chapter that enzyme reactions are most commonly studied by following the kinetics of these reactions under steady state conditions. We defined three kinetic constants—kai KM, and kcJKM—that can be used to define the efficiency of enzymatic catalysis, and each reports on different portions of the enzymatic reaction pathway. Perturbations... 

Unlike other enzymes that we have discussed, the completion of a catalytic cycle of primer extension does not result in release of the product (TP(n+1)) and recovery of the free enzyme. Instead, the product remains bound to the enzyme, in the form of a new template-primer complex, and this acts as a new substrate for continued primer extension. Catalysis continues in this way until the entire template sequence has been complemented. The overall rate of reaction is limited by the chemical steps composing cat these include the chemical step of phosphodiester bond formation and requisite conformational changes in the enzyme structure. Hence there are several potential mechanisms for inhibiting the reaction of HIV RT. Competitive inhibitors could be prepared that would block binding of either the dNTPs or the TP. Alternatively, noncompetitive compounds could be prepared that function to block the chemistry of bond formation, that block the required enzyme conformational transition(s) of turnover, or that alter the reaction pathway in a manner that alters the rate-limiting step of turnover. 

The reaction mechanism of SCR of NOx with decane on acid and iron-exchanged MFI-type zeolite was also investigated by operando FTIR spectroscopy and a special reactor cell enabling the use of water-containing gas mixtures. Brosius et al. found that water has a dramatic influence on the reaction pathway, while the formation of organic nitro and nitrite compounds does not proceed via chemisorbed states of NO, as observed in SCR with dry gases [174],... 

The autocatalytic reaction mechanism apparent at low temperatures is expected to apply to catalytic hydrogen oxidation at high pressures. In addition, the above study is the first to use STM to observe the formation of dynamic surface patterns at the mesoscopic level, which had previously been observed by other imaging techniques in surface reactions with nonlinear kinetics [57]. This study illustrates the ability of in situ STM to visualize reaction intermediates and to reveal the reaction pathway with atomic resolution. 

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