Decomposition preferential

The (thermal) decomposition of thiazol-2-yldiazonium salts in a variety of solvents at 0 C in presence of alkali generates thiazol-2-yl radicals (413). The same radicals result from the photolysis in the same solvents of 2-iodothiazole (414). Their electrophilic character is shown by their ability to attack preferentially positions of high rr-electron density of aromatic substrates in which they are generated (Fig. 1-21). The major... 

Because of the possibility of focusing laser beams, tlrin films can be produced at precisely defined locations. Using a microscope train of lenses to focus a laser beam makes possible tire production of microregions suitable for application in computer chip production. The photolytic process produces islands of product nuclei, which act as preferential nucleation sites for further deposition, and tlrus to some unevenness in tire product film. This is because the subsuate is relatively cool, and therefore tire surface mobility of the deposited atoms is low. In pyrolytic decomposition, the region over which deposition occurs depends on the drermal conductivity of the substrate, being wider the lower the thermal conductivity. For example, the surface area of a deposit of silicon on silicon is nanower dran the deposition of silicon on silica, or on a surface-oxidized silicon sample, using the same beam geomeU y. 

SIMS is inherently damaging to the sample since ion bombardment removes some material from the surface. However, other forms of damage may also occur. These include surface roughening, knock-on effects, preferential sputtering, decomposition, and implantation of source ions [49]. 

Jacobs et al. [59,925,926] (Fig. 17). While this scheme conveniently summarizes many features of the observed behaviour, a number of variations or modifications of the mechanisms indicated have been proposed. Maycock and Pai Vemeker [924,933] emphasize the possible role of point defects and suggest, on the evidence of conductivity measurements, that the initial step may be the transfer of either a proton or an electron. Boldyrev et al. [46] suggest that proton conduction permits rapid migration of HC104 within the reactant and this undergoes preferential decomposition in distorted regions. More recently, the ease of proton transfer and the mobilities of other species in or on AP crystals have been investigated by a.c. [360] and d.c. [934] conductivity measurements. Owen et al. [934] could detect no surface proton conductivity and concluded that electron transfer was the initial step in decomposition. At the present time, these inconsistencies remain unresolved. 

Kadlec and Rosmusova [1153] believe that both Ni and Co oxalates initially yield product oxide and that the proportion of metal increases with a. Since nickel oxalate decomposes at temperatures 60 K lower than those for CoC204, even a small proportion of Ni2+ markedly increases the rate of decomposition of cobalt oxalate. The effect was attributed to the catalytic properties of the preferentially formed Ni metal. The a—time curves were generally sigmoid and showed only slight deviations in shape with changes in the Ni Co ratio. In the decomposition of a mechanical... 

More functionalized 5,6-dihydro-2H-pyran-derivatives 71 and 72 have been prepared [26] by cycloaddition of 1 -methoxy-3-trialkylsilyloxy-1,3-butadienes 69 with t-butylglyoxylate (70) (Scheme 5.6). Whereas thermal reactions did not occur in good yields because of the decomposition of the cycloadducts, application of pressure (10 kbar) allowed milder conditions to be used, which markedly improved the reaction yields. The use of high pressure also gives preferentially en Jo-adduct allowing a stereocontrolled synthesis of a variety of substituted 5,6-dihydro-2H-pyran-derivatives, which are difficult to prepare by other procedures. 

Figure 14 shows the relative rates of various reactions for the decomposition of ArCOaH in acetic acid at 25 °C. Thermal homolytic decompostion is negligible under these conditions. The relative rates of reaction of ArCOaH with Co, Br" and Mn are 3900 4.7 1 (ref. 9), which is not what one would expect from the decreasing order of reduction potentials Br" > Mn > Co What this means in practice is that in a mixture containing roughly equal amounts of Co Mn and Br" together with ArCOaH more than 99% of the latter will preferentially react with the Co Similarly, replacement of 5% of Mn with Co resulted in a nine-fold increase in rate (ref. 9). 

Recent work on [CpFe(CO)2]2 was intended to test whether once again a complex molecule could be found to have a high yield and also to test a possible preferential formation of metal carbonyls over metal sandwich compounds. In this compound, thermal decomposition of the starting compound gives rise predominantly to ferrocene (28, 68). The data (50) given in Table VIII show that indeed the carbonyl is preferentially formed... 

The hemidecarboxylation of sodium phthaiate on reaction with mercuric acetate in boiling water [Eq. (82), X = H] (90) was the first reported thermal decarboxylation. The reaction has been observed for a number of arenes with two adjacent carboxylate groups (1-4,91) and has been named the Pesci reaction (91). Studies of 3-substituted sodium phthalates or of preformed mercuric 3-substituted phthalates have shown that the sterically hindered carboxyl group (the 2-carboxyl) is preferentially eliminated whether X is electron-donating or electron-withdrawing [Eq. (82), X = Me (91), Cl, N02 (91,93), Br (93), or C02H (94)]. A similar conclusion was drawn from the decomposition of mercuric 1,2-naph-thalenedicarboxylate and 3,4-phenanthrenedicarboxylate (91). 

The reactions that are more favored thermodynamically tend to be also favored kineti-cally. Semiconductor electrodes can be stabilized by using this effect. For this purpose, redox couples in the electrolyte are established with the redox potential more negative than the oxidative decomposition potential, or more positive than reductive decomposition potential in such a manner that the electrolyte redox reaction occurs preferentially compared to the electrode decomposition reaction. 

In addition to this reaction, many other reactions of hydroperoxide decay occur in solution and they will be discussed later. This is the reason why the unimolecular decomposition of hydroperoxides was studied preferentially in the gas phase. The rate constants of the unimolecular decomposition of some hydroperoxides in the gas phase and in solution are presented in Table 4.11. The decay of 1,1-dimethylethyl hydroperoxide in solution occurs more rapidly. This demonstrates the interaction of ROOH with the solvent. 

The decomposition of hydroperoxides occurs preferentially in the surface layer of water and hydrocarbon. The larger the surface per unit volume of hydrocarbon the faster the decomposition of hydroperoxide. Therefore, the increase in an aqueous phase accelerates hydrocarbon oxidation. The optimal RH H20 ratio was found to be nearly 1 1 (v/v) [19], if the calculation of the reaction rate per unit volume of the whole mixture is done. The introduction of surfactants that creates the smaller drops of hydrocarbons increases the surface and, therefore, accelerates the oxidation. 

Effective double stereodifferentiation is possible in intramolecular C-H insertion.199 For example, catalytic decomposition of enantiopure (lY,2Y)-diazoacetate 74 by Rh2(4A-MEOX)4 directed the reaction toward the preferential formation of y-lactone (lY)-75, whereas the corresponding reaction catalyzed by Rh2(4i -MEOX)4 prefers initially forming y-lactone (lY)-76 (Equation (66)). Similarly, treatment of (lY,2i )-diazoacetate 77 with Rh2(5A-MEPY)4 or Rh2(4i -MPPIM)4 gave (lY)-78 or (lY)-79, respectively (Equation (67)).199... 

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