High temperature pathways

At high temperatures the peroxy-radical becomes increasingly unstable with respect to redissociation to reactants and an alternative reaction 

In the following we summarize the experimental observations and the apparently contradictory conclusions which arise if the results are interpreted with either of the models outlined above. An alternative model is presented which invokes involvement of two electronic states of differing symmetry for the peroxy radical. The model is compatible with all the available experimental data on ethyl -f O2. 

Detailed discussions of some of the experimental data can be found elsewhere [108]. In this subsection major results are summarized to provide a basis for the subsequent mechanistic proposals. 

It is also relevant to look at the reverse reaction of the products of (—49b). Baldwin et al. [114] have measured the rate of formation of the oxirane  

They proposed that the initial stage involves addition of HO2 to form the hydroperoxy radical which represents the rate determining step. They obtained an activation energy of TOkJmoF for reaction (51) which they ascribed to the addition step of HO2 to ethene. 

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After radical initiation of an alkane fuel, chain branching is immediately accessible via new high-temperature pathways with O2 ... 

Reaction (Rl) is an exothermic, low-temperature pathway, whereas reaction (R2) is an endothermic, high-temperature pathway. The reaction rates are obtained from a model of the T-jump/FTIR experiment [15], which takes into account the heat transfer of the filament and sample. 

Table 4.7 shows representative examples of the high-temperature pathway affording cyclopentenols, and Table 4.8 depicts the low-temperature pathway which generates p,y-unsaturated ketones from intermediate 2. 

Butane-Naphtha Catalytic Liquid-Phase Oxidation. Direct Hquid-phase oxidation ofbutane and/or naphtha [8030-30-6] was once the most favored worldwide route to acetic acid because of the low cost of these hydrocarbons. Butane [106-97-8] in the presence of metallic ions, eg, cobalt, chromium, or manganese, undergoes simple air oxidation in acetic acid solvent (48). The peroxidic intermediates are decomposed by high temperature, by mechanical agitation, and by action of the metallic catalysts, to form acetic acid and a comparatively small suite of other compounds (49). Ethyl acetate and butanone are produced, and the process can be altered to provide larger quantities of these valuable materials. Ethanol is thought to be an important intermediate (50) acetone forms through a minor pathway from isobutane present in the hydrocarbon feed. Formic acid, propionic acid, and minor quantities of butyric acid are also formed. 

In other cases, it may be impossible to describe the kinetics properly using a single reaction path. A variety of pathways may contribute to the reaction kinetics. One or more paths may be dominant at low temperature, whereas other paths may be dominant at high temperatures. This results in a temperature-dependent reaction mechanism. In such situa-... 

Alternative algorithms employ global optimization methods such as simulated annealing that can explore the set of all possible reaction pathways [35]. In the MaxFlux method it is helpful to vary the value of [3 (temperamre) that appears in the differential cost function from an initially low [3 (high temperature), where the effective surface is smooth, to a high [3 (the reaction temperature of interest), where the reaction surface is more rugged. 

Pathway temperatures must be strictly controlled (especially in single-phase systems) to create a balance between low-temperature oxide dissolution and high-temperature mass transfer limitations. 

In the presence of a suitably disposed /i-hydrogen—as in alkyl-substituted thiirane oxides such as 16c—an alternative, more facile pathway for thermal fragmentation is available . In such cases the thiirene oxides are thermally rearranged to the allylic sulfenic acid, 37, similarly to the thermolysis of larger cyclic and acyclic sulfoxides (see equation 9). In sharp contrast to this type of thiirane oxide, mono- and cis-disubstituted ones have no available hydrogen for abstraction and afford on thermolysis only olefins and sulfur monoxide . However, rapid thermolysis of thiirane oxides of type 16c at high temperatures (200-340 °C), rather than at room temperature or lower, afforded mixtures of cis- and trans-olefins with the concomitant extrusion of sulfur monoxide . The rationale proposed for all these observations is that thiirane oxides may thermally... 

Given their extraordinary reactivity, one might assume that o-QMs offer plentiful applications as electrophiles in synthetic chemistry. However, unlike their more stable /tora-quinone methide (p-QM) cousin, the potential of o-QMs remains largely untapped. The reason resides with the propensity of these species to participate in undesired addition of the closest available nucleophile, which can be solvent or the o-QM itself. Methods for o-QM generation have therefore required a combination of low concentrations and high temperatures to mitigate and reverse undesired pathways and enable the redistribution into thermodynamically preferred and desired products. Hence, the principal uses for o-QMs have been as electrophilic heterodienes either in intramolecular cycloaddition reactions with nucleophilic alkenes under thermodynamic control or in intermolecular reactions under thermodynamic control where a large excess of a reactive nucleophile thwarts unwanted side reactions by its sheer vast presence. 

C. Hsu, H. Nguyen, D. Yeung, D. Brookes, G. Koe, T. Bewley, and R. Pearlman, Surface denaturation of solid-void interface—a possible pathway by which opalescent particulates form during the storage of lyophilized tissue-type plasminogen activator at high temperatures, Pharm. Res., 12, 69 (1995). 

The reaction pathway for the gas-phase methylation of m-cresol, as inferred from catalytic data here reported, can be summarized as shown in Scheme 1. Methanol and m-cresol react through two parallel reactions, yielding either 3-MA or DMPs. The relative contribution of the two reactions is a function of the physico-chemical features of the catalysts, and of the reaction temperature as well, C-methylation being kinetically favored at high temperature. Consecutive reactions occur on 3-MA, which acts as a methylating agent yielding DMPs, DMAs and polyalkylates (with co-production of m-cresol in all cases) by reaction with m-cresol, 3-MA and DMPs, respectively. Consecutive reactions may also occur on DMPs to yield polyalkylates. 

It will be clear from the results so far presented that both C5 and C dehydrocyclization products can be formed, with aromatization proceeding (one would expect) by further dehydrogenation of the initially formed C6 ring-closure species. There is another pathway for the production of aromatics based upon cyclization of a linear triene (133), but this is of relatively small importance, and is only significant at all at quite high temperatures and low hydrogen partial pressures. 

A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

AH = 2.9 kj mol-1 at 300 K and 1 atm, there is no low-energy pathway for the transformation, so the process is difficult to carry out. However, synthetic diamonds are produced on a large scale at high temperature and pressure (3000 K and 125kbar). The conversion of graphite to diamonds is catalyzed by several metals (i.e., chromium, iron, and platinum) that are in the liquid state. It is believed that... 

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