Monomolecular intermediate

Indazole -> benzimidazole photoisomerization involves a singlet state and has been determined to be monomolecular and monophotonic. The UV spectrum of an intermediate with a lifetime of the order of seconds was recorded. Irradiation of the indazole (526) resulted in a 96% yield of the 1-methylbenzimidazole (528) probably via the intermediate (527)... 

Until now, none of these reactions has ever lead to stable monomolecular stannylenes. Nevertheless, trapping reactions have confirmed the presence of intermediate Sn(II) species 7,80,81). 

Adducts of the type Me2Si(NCMe3)2Sn H2N—R are unstable and cleaved to Me2Si(N(CMe3)H)2 and the intermediate SnN—R 176,177>. It is not yet clear whether this scission is a monomolecular process, as might be inferred from the structure... 

MCM-22, with a larger pore volume than ZSM-5, revealed behavior intermediate between what was observed for large- and medium-pore zeolites (126). Unverricht et al. (141) also examined MCM-22 at 353 and 393 K, it was found to produce mainly cracked products and dimethylhexanes and to deactivate rapidly. MCM-36 gained considerable interest that is evidenced by the patent literature (171-174). MCM-36 is a pillared zeolite based on the structure of MCM-22. Ideally, it should contain mesopores between layers of MCM-22 crystallites. This structure was found to be much more active and stable than MCM-22 (175). Alkane cracking experiments with zeolites having various pore dimensions evidenced the preference of monomolecular over sterically more demanding bimolecular pathways, such as hydride transfer, in small- and medium-pore zeolites (146). 

The enzyme monomolecular layer prepared as depicted in Section 5.3.3 may serve as a template for the step-by-step building of a multimonomolecular layered system.26 As shown in Figure 5.28, the key intermediate building... 

In the examples studied, neither the dihydride intermediates nor the alkyl intermediates have been observed and therefore it seems reasonable to assume that addition of H2 is also the rate-determining step. bimolecular reaction and the other ones are monomolecular rearrangement reactions one cannot say in absolute terms that oxidative addition is rate-determining. >... 

Industrial metal-zeolite catalysts undergo a bifunctional, monomolecular mechanism [1-5, 7]. Carbenium ions are the critical reaction intermediates to complete chain reactions. In the zeolite channels, carbenium ions likely exist as an absorbed alkoxyl species, rather than as free-moving charged ions [8], Figure 14.2 illustrates the accepted reaction mechanism, using hexanes as an example. 

With increasing reaction severity, the concentrations of the individual isomers approach their equilibrium values. The monomolecular route is the most effective for achieving high yields of PX, which is typically the most desirable for petrochemical applications. The schematic above shows the stepwise interconversion of OX to MX and MX to PX, which is consistent with a 1,2-methyl shift route. However, the results of kinetics studies provide some indications in favor of a reaction step that directly converts OX to PX [62]. It is not clear what the form of the reaction intermediate for this transformation is. Some in situ time-resolved spectroscopic methods have been used to look at how modification of zeoMtes like MFl affects the monomolecular mechanism by constraining the diffusion of MX [63]. 

A second method for interconverting xylene isomers occurs by way of a bimo-lecular mechanism. This can be catalyzed by materials such as Y-zeolite. The reaction proceeds through a bridged, diphenylmethane-like intermediate, so that the space requirements are more demanding than the monomolecular route (Figure 14.8). 

The interaction of alkyl halides with mercaptans or alkaline mercaptides prodnces thioalkyl derivatives. This is a typical nncleophilic substitution reaction, and one cannot tell by the nature of products whether or not it proceeds through the ion-radical stage. However, the version of the reaction between 5-bromo-5-nitro-l,3-dioxan and sodium ethylmercaptide can be explained only by the intermediate stage involving electron transfer. As found (Zorin et al. 1983), this reaction in DMSO leads to diethyldisulfide (yield 95%), sodium bromide (quantitative yield), and 5,5 -bis(5-nitro-l,3-dioxanyl) (yield 90%). UV irradiation markedly accelerates this reaction, whereas benzene nitro derivatives decelerate it. The result obtained shows that the process begins with the formation of ethylthiyl radicals and anion-radical of the substrate. Ethylthiyl radicals dimerize (diethyldisulfide is obtained), and anion-radicals of the substrate decompose monomolecularly to give 5-nitro-l,3-dioxa-5-cyclohexyl radicals. The latter radicals recombine and form the final dioxanyl (Scheme 4.4). 

In an extensive review on abiotic catalysis, Huang (2000) noted that the reactivity of hydrolyzable organic contaminants arises from the presence of electron-deficient (electrophilic) sites within the molecules. Figures 14.2 and 14.3 show the patterns of reactivity in two cases of nucleophihc substitution and monomolecular nucleophilic substitution. The Sj 2 mechanism (nucleophilic substitution) involves attack of the electrophilic sites by OH" or H O, generation of a higher coordination nnmber intermediate, subsequent elimination of the leaving group, and the formation of an hydrolysis product (Fig. 14.2). 

In the case of monomolecular nucleophilic substitution (S l) the reaction proceeds with the loss of the leaving group to generate a lower coordination number intermediate, followed by generation of the hydrolysis product by nucleophilic addition, as shown in Fig. 14.3. 

The conditions favoring cracking by the monomolecular path are high temperature and low olefin concentrations, i.e. low paraffin partial pressure and/or low conversion. The proposed reaction intermediate is formed by protonation of the paraffin feed by a Brdnsted acid site of the catalyst. We may compare this with similar paraffin protonation by CH5 in chemical ionizations occurring in an ion cyclotron resonance mass spectrometer [10], The C0H15 ion produced collapses to the same products as we have observed with zeolites HZ as the proton source (Fig.1). This is surprising, since the... 

In developing oxidation processes a major source of free radical formation was found to be degenerate chain branching. Among the products derived from the branching were intermediate peroxides ROOH. Formation of radicals from the hydroperoxides proceeded not only by monomolecular breakdown of hydroperoxides ... 

The mechanisms of stepwise monomolecular thermal decomposition of 1,5- and 2,5-disubstituted tetrazoles feature nitrogen evolution by rate-limiting breakdown of intermediate azidoazomethines and azodiazo compounds, respectively 67 the activation parameters have been reported. 

At RT it decomposes at a rate characteristic of monomolecular reactions. Decompn is catalyzed in the dry state, but not in aq solns, by an intermediate prod or by the thiocyamc acid formed ... 

Conversions of only surface compounds are considered. It is suggested that the gas-phase composition remains unchanged. If, in this case, the mechanism is linear with respect to intermediates, the conversion mechanism for these intermediates is monomolecular. 

Modern methods of vibrational analysis have shown themselves to be unexpectedly powerful tools to study two-dimensional monomolecular films at gas/liquid interfaces. In particular, current work with external reflection-absorbance infrared spectroscopy has been able to derive detailed conformational and orientational information concerning the nature of the monolayer film. The LE-LC first order phase transition as seen by IR involves a conformational gauche-trans isomerization of the hydrocarbon chains a second transition in the acyl chains is seen at low molecular areas that may be related to a solid-solid type hydrocarbon phase change. Orientations and tilt angles of the hydrocarbon chains are able to be calculated from the polarized external reflectance spectra. These calculations find that the lipid acyl chains are relatively unoriented (or possibly randomly oriented) at low-to-intermediate surface pressures, while the orientation at high surface pressures is similar to that of the solid (gel phase) bulk lipid. 

As many organic compounds may transform simultaneously through mono molecular (intramolecular) and bimolecular (intermolecular) processes, it is easy to understand that the shape and size of the space available near the active sites often determine the selectivity of their transformation. Indeed the transition state of a bimolecular reaction is always bulkier than that of a monomolecular reaction, hence the first type of reaction will be much more sensitive to steric constraints than the second one. This explains the key role played by the pore structure of zeolites on the selectivity of many reactions. A typical example is the selective isomerization of xylenes over HMFI the intermediates leading to disproportionation, the main secondary reaction over non-spatioselective catalysts, cannot be accommodated at its channel intersections (32). Furthermore, if a reaction can occur through mono and bimolecular mechanisms, the significance of the bimolecular path will decrease with the size of the space available near the active sites (41). 

Consider a monomolecular reaction in Example 3.18, and determine the condition for minimum entropy production when the rate of entropy production is expressed in terms of the concentration. In this open reaction system, the chemical potentials of reactant R and the product B are maintained at a fixed value by an inflow of reactant R and an outflow of product B. The concentration of intermediate X is maintained at a nonequilibrium value, while the temperature is kept constant by exchanging the heat of reaction with the environment. Determine the condition for minimum entropy production. 

This statement on the relationship between the stationary rate of the stepwise process and the thermodynamic force that initiates it can be easily generahzed for the case of an arbitrary combination of monomolecular transformations of intermediates. The simplest and most visual way to do that is to use the perfect analogy between equation (1.31) of the rate of elementary chemical reaction Vy and the Ohm s law for electric current ly between two points, i and j, of an electric circuit with electric potentials Ui and Uj, respectively ... 

In a stationary process, the direction of changes in actual values of pj and Yi for intermediates Yj is always strongly determined. For example, in the course of consecutive monomolecular transformations, the values of Pi (and, correspondingly, of Yi) always decrease along the pathway from the initial reactant to the final product (Figures 1.4 and 1.5). 

Figure 1.5 The world of mountains of actual chemical potentials of transition states and lakes of stationary chemical potentials of intermediates in the systems with a plurality of the allowed monomolecular transformation of initial reactant R into final product P. The surface levels of the stationary lakes are gradually decreasing from the reactant to the product.

This specific feature of the stationary state of chemical systems that undergo their evolution via an arbitrary combination of only monomolec ular (or reduced to monomolecular) transformations, as well as transforma tions that are linear in respect to the intermediates, is of practical importance to simplify the analysis of complex stepwise chemical processes with the use of methods of nonequilibrium thermodynamics. 

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