An Overall Reaction Scheme

The results presented in the previous sections show that the anodic reactions on a silicon electrode may proceed via different paths depending on the conditions and that those in HF solutions and those in KOH solutions are rather different. They also show that the mechanistic models proposed for the reactions in HF and KOH solutions from the many studies in the literature are largely separated. However, in both HF and KOH solutions, the silicon/electrolyte interface is fundamentally similar differing only in the concentrations of hydroxyl and fluoride ions. Thus, a reaction scheme must be coherent with respect to the experimental observations in both FIF and KOH solutions. For comparison. Table 5.8 summarizes the characteristic features of the reactions occurring on silicon in FIF and KOH, in terms of nature of the reaction, rate, effective dissolution valence, photoeffect, and uniformity of the surface. 

Despite the many differences in HF and KOH solutions as shown in Table 5.8, the overall reactions are similar in two important aspects the silicon surface is dynamically terminated by hydrogen and breaking of the silicon-silicon back bond is facilitated by the adsorption of electronegative ligands such as F or OH . More specifically this means (1) the initial surface is hydrogen terminated (2) the Si-Si back bond requires that the hydrogen termination is first replaced by F or OH and (3) the silicon atoms on the newly exposed layer are terminated by hydrogen so that the surface after the dissolution of one silicon layer is identical to that before the dissolution. 

A key aspect of the reaction scheme is the valence state of the silicon and hydrogen atoms at different stages of the various reaction paths. The nonpolarized nature of the Si-H bond indicates that the hydrogen atom bonded to the silicon atoms on the 

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An estimate of the pK value for benzaldehyde radical-anion has also been obtained from fast cyclic voltammetry experiments over a range of pH values [14], Interpretation the results obtained in this case requires first deduction of an overall reaction scheme followed by numerical solution of the corresponding set of differential equations allowing simulation of the cyclic voltammogram. Reaction constants are then adjusted to give good simulations over a range of experimental conditions. The pKj can then be extracted from these reaction constants. 

Each of the series-parallel reaction schemes presented in the previous sections for the Fe-assisted oxidations of phenol, o-DHB, p-DHB, and 1,4-BQ, were established considering the chemical species detected in the bulk of the water solution. Given that o-DHB, p-DHB, and 1,4-BQ are intermediates in the Fe-assisted PC oxidation of phenol, an overall reaction scheme for the Fe-assisted PC oxidation of phenol can be formulated. This overall reaction network incorporates all the detected intermediaries. 

An alternative, and equally useful form of Hess s Law says in effect that if simple reaction equations are added (or subtracted) to give an overall reaction scheme, then so may the corresponding AH values be added (or subtracted) to give an overall value of enthalpy change. Two final examples will be given to demonstrate this alternative approach. 

The high-temperature reaction between acetone and H2 can be represented by an overall reaction scheme which contains two main parallel processes (36), as shown in the following scheme. 

The adsorbed reactant may react by itself, with other adsorbed reactants, or with reactants from the gas phase. For formulation of overall rate equations, the individual partial steps are considered as elementary reactions of an overall reaction scheme. It is difficult - frequently impossible - to derive from the rates of... 

Morvillo and Turco also looked at the oxidative addition of methyl and ethyl halides to nickel(I) complexes, [NiX(PEt3)3], which is about ten times faster than the first step of the same process for nickel(O) systems, [Ni(PEt3)4] (X = Br or I). As some nickel(I) is formed in the second case, an overall reaction scheme can be constructed which includes both oxidation states, (120)-(124). Reactions (122) and (123) are slow and (124) is very slow. 

The effect of both the R and X group of RX on the overall reaction rate constant for generation of (TPP)Rh(R) from TPP)Rh and RX have been measured by electrochemical methods . The dependence of the reaction rate constant on X follows the trend I > Br > F Cl while the dependence of R follows the trend CHa > C2H5 C3H7 C4H9 C5H11. Based on this and other data, an overall reaction scheme was proposed " in which the (TPP)Rh radical attacked the carbon in the position a to the halide with consequent loss of either X- or X. This proposed mechanism differs, in part, from reactions involving [(P)Rh]2 and RX where formation of (P)Rh(R) and (P)RhX are observed. ... 

The overall reaction scheme of the luminol chemiluminescence in an aqueous medium is shown in Figure 1. The luminol oxidation leads to the formation of an aminophthalate ion in an excited state, which then emits light on return to the ground state. The quantum yield of the reaction is low ( 0.01) compared with bioluminescence reactions and the emission spectrum shows a maximum1 at 425 nm. 

Figure 2. A diagrammatic representation of those segments of an overall reaction mechanism for barbed-end actin polymerization in the absence and presence of profilin and/or thymosin- 4. For clarity the overall scheme is indicated. Experimental data were obtained for individual segments (indicated in black type), and those parts of the scheme that were not evaluated in a particular experiment are shown in gray.

Remark. The reaction scheme (1.4) corresponds to an overall reaction... 

A set of stoichiometric numbers of the stages producing an overall reaction equation is called, after Horiuti, a reaction route. Thus, in scheme (35) there are two reaction routes, N(l) and iV(2) route Na> cannot be obtained from N(i) through multiplication by a number. Therefore, routes N(1) and N(2) are essentially different, although their respective overall equations are identical. 

Carbonyl groups can be transformed into amines via formation of their hydrazones followed by transfer hydrogenation. Thus the benzoyl formate shown in Scheme 4.35 was converted into the phenylhydrazone under microwave irradiation in ethylene glycol as solvent. Subsequent reduction using ammonium formate and 10% Pd/C as a catalyst provides the amine in an overall reaction time of 10 min and a total yield of 83%.14... 

The term substitution in an unrestricted sense is rather too broad to be useful in classification of radical reactions, since most of them result in replacement of one group by another. We have already seen typical examples of bond homolysis, in which a molecule dissociates to yield two radicals which combine with each other or with another molecule. We are primarily concerned in this section with those elementary reaction steps in which a radical attacks directly an atom of another molecule (Equation 9.64), displacing from the site of attack another group, and with the overall reaction schemes in which these elementary reactions occur. 

In 2000, Dubac s group reported the microwave-assisted Friedel-Crafts acylation of slightly activated and deactivated arenes under solvent-free conditions with FeCl3 as catalyst. Here, for the acylation of toluene a 90% product yield is obtained after 5 min of irradiation and an overall reaction time of 30 min in the presence of only 5 mol% of FeCl3. A sequential MW irradiation at 300 W afforded the acylation of fluorobenzene with 2 -chlorobenzoyl chloride, with a surprisingly high yield of 92% of 2-chloro-4 -fluorobenzophenone (Scheme 6.11). 

Catalyst inhibition is traditionally associated with biocatalytic processes, but can also apply to homogeneous and heterogeneous catalysis. Competitive inhibition is analogous to competitive adsorption in gas/solid heterogeneous catalysis, where two molecules from the gas phase compete for the same active site on the catalyst surface. A competitive inhibitor is any chemical species I which can bind to the same site as the substrate, or to another site on the enzyme (an allosteric site). The overall reaction scheme is then given by Eqs. (2.58)-(2.60), where El indicates an enzyme-inhibitor complex. 

The mechanism of the decomposition reaction of 5-methoxy- 1,2,3,4-thiatriazole to dinitrogen sulfide and methoxy-nitrile was studied by the DFT method at the CCSD(T)//MP2/6-31+G level of theory <2003JOC6049>. The calculations indicated that this is a concerted retro-[2+3]-dipolar cycloaddition process with an activation energy of 28.9 kcal mol 1 and a reaction energy of 1.9 kcal mol. This unimolecular decomposition is favored due to the entropy gain (25.8 eu) involved in the overall reaction (Scheme 1 and Table 2). 

The reduction of the anaesthetic halothane at pH 13 was studied on a variety of electrode materials. The overall reaction has been shown to involve two electrons with the production of a bromide ion. The first electron transfer is the slowest electrochemical step. An overall reaction mechanism consistent with these observations is shown in Scheme 18. The first electron transfer involves two electrons and results in the production of a bromide ion. Of these, the first electron transfer is irreversible and the slowest246,247. 

The underlying assumption in this reaction scheme is that all chemical species behave the same as a model pollutant or as an intermediate. To exemplify, o-DHB is an intermediate in the Fe-assisted oxidation of phenol and when used as a model reactant, o-DHB forms p-DHB. It is h)q)othesized therefore that o-DHB as an intermediate in phenol Fe-assisted PC conversion forms p-DHB. Hence, the same assumptions for the development of the overall reaction scheme in the unpromoted PC reaction are considered for the Fe-assisted PC system. 

Transfer of a hydride Ion to an Isoalkyl cation Is often the rate controlling step In the overall reaction scheme (16). Isoalkyl cations Including C5 and higher ones react forming the various Isoparaffin molecules found In alkylate, e.g., IMP+ s react to form TMP s. 

In 1970 Kennedy and Gillham (9) proposed a comprehensive scheme for the study of competitive reactions in cationk polymerization of isobutylene. Their scheme was based on reacting an initiator-coinitiator system with a nonpolymerizing olefin. In particular, they examined the t-BuQ/MejAl system with 2,4,4-trimethyl-l-paitene, the model compound for isobutylene, using methyl chloride at — 78°C. The overall reaction scheme is shown in Fig. 1. 

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