Relative reduction rates in different

In Figure 9 plots of the logarithms of the relative reduction rates (k ei, relative to 4-Cl) of ten mono-substituted nitrobenzenes versus their values for two different systems illustrate the usefulness of initial rates for inferring information on reaction mechanism(s). 

Figure 14. Comparison of the relative reduction rates (krei, the rates are relative to 4-Cl) of the mono-substituted nitrobenzenes listed in Table 4 in three different systems (a) in aqueous solutions containing hydrogen sulfide and natural organic matter data from (11) (b) in aqueous solutions containing cysteine and a water soluble iron porphyrin data from (64) (c) in aquifer columns under ferrogenic conditions data from (25) The relative rates, expressed as log k gi, are plotted against the normalized one-electron reduction potentials, [El, (ArN02)/0.059 V], of the compounds.

Several additional studies were carried out to obtain information about the precise behavior of the various components in the model system. The interplay between the manganese porphyrin and the rhodium cofactor was found to be crucial for an efficient catalytic performance of the whole assembly and, hence, their properties were studied in detail at different pH values in vesicle bilayers composed of various types of amphiphiles, viz. cationic (DODAC), anionic (DHP), and zwitterionic (DPPC) [30]. At pH values where the reduced rhodium species is expected to be present as Rh only, the rate of the reduction of 13 by formate increased in the series DPPC < DHP < DODAC, which is in line with an expected higher concentration of formate ions at the surface of the cationic vesicles. The reduction rates of 12 incorporated in the vesicle bilayers catalyzed by 13-formate increased in the same order, because formation of the Rh-formate complex is the rate-determining step in this reduction. When the rates of epoxidation of styrene were studied at pH 7, however, the relative rates were found to be reversed DODAC DPPC < DHP. Apparently, for epoxidation to occur, an efficient supply of protons to the vesicle surface is essential, probably for the step in which the Mn -02 complex breaks down into the active epoxidizing Mn =0 species and water. Using a-pinene as the substrate in the DHP-based system, a turnover number of 360 was observed, which is comparable to the turnover numbers observed for cytochrome P450 itself. 

In our second example we look at the reduction of chlorinated ethenes at a nickel electrode and at the surfaces of two zero-valent metals [Fe(0), Zn(0)]. To gain insight into the rate-limiting process(es) in these cases, we consider how the relative overall reduction rates (relative to PCE) of PCE, TCE, and the three DCE isomers (see Fig. 14.15 for structures) vary as a function of two common descriptors used in QSARs, the one-electron reduction potential (EJ Fig- 14.17a) and the bond dissociation energy (DR X Fig. 14.176). In all these systems, the reduction rates were found to be significantly slower than diffusion of the compounds to the respective surfaces. Therefore, the large differences in the relative reactivities of the compounds between the systems reflect differences in the actual reaction at the metal surface. 

Arrhenius parameters for nickel carbide hydrogenation 162) is close to both lines on Fig. 3. Compensation behavior for reactions on the carbide phase must include an additional feature in the postulated equilibria, to explain the removal of excess deposited carbon, if the active surface is not to be poisoned completely. The relative reduction in the effective active area of the catalyst accounts for the lower rates of reaction on nickel carbide, and the difference in the compensation line from that of the metal (Fig. 3) is identified as a consequence of the poisoning-regeneration process. After any change in reaction conditions, a period of reestablishment of surface equilibria was required before a new constant reaction rate was attained (22). 

Further studies367 on the hydrolysis of (107) have shown that a reduction in the chloride ion concentration leads to a marked increase in rate. The approximate relative reactivities of the different mercury(Il) species in solution are Hg" (l) = HgCl+ (l) = HgCl2 (1)> HgCV" (0.1) HgCl42 (CO.001). The rate constant for reaction via Hg11 is ca. 106-fold larger than for hydrolysis in the presence of HjO+ alone. Addition of HC1 leads to a reduction in rate, rather than an increase, presumably due to the formation of less reactive chloro complexes. Hydrolysis of (107)... 

The different biological properties of NO and HNO can be partially explained by the high reduction potential for NO and the slow rate of deprotonation of HNO. However, HNO is a mild reductant (163, 164), and biomolecules such as ferricyt c (170) and SOD (83, 84) are reduced, at least formally, by HNO donors, resulting in formation of free NO. The relevance of these and other reactions that have been observed with purified biomolecules to the complex, heterogeneous environments of cells and tissue can be determined by elucidation of the chemical biology of HNO. This process includes identification of potential reactions, mechanistic determinations, and systematic comparisons of relative reaction rates, particularly for modification of biological targets in relation to consumption pathways. 

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