Kinetics of the Reduction

All cleavages of simple disulfides by mercaptans that have been studied kinetically are bimolecular ionic reactions of the SN 2 type, involving direct displacement by mercaptide ion on disulfide [20]. Because the active species in this disulfide scission process is the mercaptide ion [21] rather than the unionized mercaptan, pH is a critical factor. As a consequence, pH can determine the rate-controlling step in the reductive cleavage of cystinyl residues in keratin fibers by mercaptans. For example, in the reaction of wool fiber with dithiothreitol, Weigmann [22] has shown that the ratecontrolling step at pH 7.0 and above is diffusion of the reducing species into 

In other words, for difficult-to-wave hair or at high pH, the concentration of mercaptide ion is so high that cleavage of the disulfide bond can occur faster than mercaptide can diffuse into the fibers. As the pH is decreased to the acid side, or for easy-to-wave hair, the rate of chemical reaction decreases faster than diffusion to the point at which the chemical reaction itself becomes rate-limiting. With many mercaptans [21], further lowering of the pH to about 2 freezes or stops the reduction reaction. 

Evans et al. [25] have confirmed these conclusions of Wickett. In addition, the observation that Japanese hair is easy to perm and that fine Caucasian hair, less than 75 pm in diameter, is more difficult to perm was also confirmed. However, these scientists were unable to identify any common characteristics such as fiber diameter or cystine content that would account for this behavior. The fact that fine hair is more difficult to perm than thick hair may be due to the larger ratio of cuticle to cortex in fine hair and the fact that cortex plays a stronger role in waving than cuticle. This explanation is consistent with the experiments by Wortmann and Kure [2], demonstrating that the cuticle does inhibit the reduction reaction. In addition to pH, other important variables that influence the rate of reduction of keratin fibers by mercaptans are temperature, hair swelling, prior history of the hair, and structure of the mercaptan. 

Because the rate-controlling step in this reaction can be diffusion of the reducing agent into the fibers or the chemical reaction itself, it is important to consider the rate in terms of these two potentially rate-limiting factors. 

The pH region most commonly employed for the reduction of hair fibers by mercaptans is above neutral (generally 9 to 9.5). In the professional field, glyceryhnonothioglycolate (GMT) 

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The scheme of the interaction mechanism (Equation 88) testifies to an electro-affinity of MeFe" ions. In addition, MeFe" ions have a lower negative charge, smaller size and higher mobility compared to MeF6X(n+1> ions. The above arguments lead to the assumption that the reduction to metal form of niobium or tantalum from melts, both by electrolysis [368] and by alkali metals, most probably occurs due to interaction with MeF6 ions. The kinetics of the reduction processes are defined by flowing equilibriums between hexa-and heptacoordinated complexes. 

Shastri et have examined the kinetics of the reduction of Np(V) by U(rV). The rate of the reaction, zero order in Np(V) and first order in U(IV), shows an inverse dependence on the square of the hydrogen-ion concentration. The gross features of the mechanism are believed to be... 

The kinetics of the reduction of chlorate ion by Ir(III) have been determined by controlled-potential electrolysis to be ... 

However, the kinetics of the reduction are often simplified in that the first step (production of H02 ) is rate-determining the ensuing reactions, however, may confer radical-chain behaviour on the system. Hydrolysis of the reductant can also modify the kinetics. 

Perlinger JA, W Angst, RP Schwarzenbach (1996) Kinetics of the reduction of hexachloroethane by juglone in solutions containing hydrogen sulfide. Environ Sci Technol 30 3408-3417. 

Finally, a group from General Motors has explored the mechanistic importance of the N20 + CO reaction as an intermediate step during the reduction of NO by CO on noble metal exhaust catalysts [87,88]. Quasi-linearization of the non-linear NO + CO reaction system by identifying a critical kinetic parameter revealed that, indeed, the rate of the N20 + CO conversion as an intermediate step in the overall NO + CO conversion can be two to three orders of magnitude faster than the isolated N20 + CO reaction. This suggests that the observed suppression of N20 production at higher temperatures may be due to its fast reaction with adsorbed CO once produced, and that, contrary to the accepted wisdom, the formation of N20 and its subsequent reaction with CO can make a major contribution to the kinetics of the reduction of NO by CO in three-way catalytic converters. The validity of the theoretical results was verified by both... 

Providing that the potential is sufficiently negative, the kinetics of the reduction reaction in equation (2.125) can usually be rendered fast enough to tip the system into the diffusion-controlled regime, as was shown in the discussion of the Butler-Volmer equation in chapter 1. 

Figure 6.20 Quick EXAFS and XRD measurements recorded during the temperature programmed reduction of copper in a Cu/Zn0/Al203 methanol synthesis catalyst. The disappearance and appearance of peaks with increasing temperature in the series of EXAFS spectra corresponds to the conversion of oxidic to metallic copper. The intensity of the relatively sharp peak around 9040 eV, indicative of Cu metal, clearly illustrates the kinetics of the reduction process, as does the intensity of the (111) reflection of Cu metal in the XRD spectra (adapted from Clausen 44J).

NO 3-Reducing. Fig. 9.15 shows data on groundwater below agricultural areas. The sharp decrease of 02 and NO3 at the redox cline indicate that the kinetics of the reduction processes are fast compared to the downward water transport rate. Postma et al., 1991 suggest that pyrite, present in small amounts is the main electron donor for NO3 reduction (note the increase of SOJ immediately below the oxic anoxic boundary). Since NO3 cannot kinetically interact sufficiently fast with pyrite a more involved mechanism must mediate the electron transfer. Based on the mechanism for pyrite oxidation discussed in Chapter 9.4 one could postulate a pyrite oxidation by Fe(III) that forms surface complexes with the disulfide of the pyrite (Fig. 9.1, formula VI) subsequent to the oxidation of the pyrite, the Fe(II) formed is oxidized direct or indirect (microbial mediation) by NO3. For the role of Fe(II)/Fe(III) as a redox buffer in groundwater see Grenthe et al. (1992). 

The disproportionation of tertiary and primary alkoxy species 4 has been reported. The NMR spectrum of an equivalent molar mixture of LiAlH3(OBu ) and LiAlH(OBu )3 was the same as that of LiAlH2(OBu )2, suggesting disproportionation of the nonsymmetrical compounds (37). Kinetics of the reduction of several aromatic ketones in ether with reagents formed by the reaction of LAH with Bu OH were consistent with partial disproportionation of species 4 (38) ... 

The kinetics of the reduction of spinach plastocyanin PCu(II) by the optically active complexes 2,6-bis[3-(S)- or 3-(/ )-carboxyl-2-azabutyl] pyridine, here abbreviated to (S,S)- or (f ,/ )-ALAMP have been studied [107]. The latter enantiomer (A-configuration) reacts 1.6-2.0 times faster at different values of pH and temperature than the S,S form. Activation parameters have shown that the observed stereoselectivity is a consequence of the difference in activation... 

The kinetics of the reduction of 2,6-dichlorophenolindolphenol (DCPI), a common dye used for analysing ascorbic acid, by Fe and oxalate have been studied and indicate the rapid formation of an intermediate complex of Fe + and 204 , predominantly FeC204, prior to the reduction of DCPI. ... 

The kinetics of the reduction of perruthenate(VII) by [FefCbOe]" and [W(CN)g]" and the oxidation of ruthenate(VI) by [Mo(CN)g] and [Ru(Cb06] have been studied in aqueous alkaline solutions. The cross-reaction data have been treated according to the Marcus relations and yield a self-exchange rate constant of 10 s at 25.0 °C and 1.0 M ionic strength for the... 

James has investigated in some detail the kinetics of the reduction of silver ions by several developing agents. Without exception, the reduction was markedly catalyzed by metallic silver. 

The kinetics of the reduction of silver ions by p-phenylenediamine differ in important respects from those of the reduction by hydroquinone and hydroxylamine. Once more, the silver catalysis is marked and the reaction rate varies directly as the area of the catalyst surface, but the rate is directly proportional to the silver ion concentration (James, 7). 

The kinetics of the reduction of 1,2-dithiane with triphenylphosphine was studied in aqueous ethanol at various temperatures by UV spectroscopy <1991PS(60)215>. First-order kinetics was clearly observed and the reaction rate was found to depend strongly on the solvent polarity. 

Equation (8) also describes the kinetics of the reduction of the cupric salt, ultimately to metallic copper, which is observed in the absence of an added substrate (Macgregor and Halpern, 16). This reaction proceeds by a mechanism in which step (11) is replaced by the rapid disproportionation of Cu+ to give metallic copper ... 

The reduction of aromatic nitro compounds is believed to proceed to an intermediate mixture of nitroso compounds and substituted hydroxylamines which are not isolated but condense to form an azoxy compound which, in turn, is reduced to an azo compound. Contributing evidence to substantiate this mechanism is that the reduction of a mixture of two aromatic nitro compounds leads to a mixture of azo compounds consistent with that predicted if each of the nitro compounds were reduced to a nitroso compound and a hydroxylamine and these, in turn, reacted with each other in all possible combinations. This observation also implies that the bimolecular reduction of nitro compounds is practical only from the preparative standpoint for the production of symmetrically substituted azo compounds. Spectrophotometric studies of the reaction kinetics of the reduction of variously substituted nitro compounds may, however, uncover reasonable procedures for the synthesis of unsymmetrical azo compounds. 

As early as 1905, Tafel [137] showed that the kinetics of the reduction of a proton strongly depended on the nature of the electrode material. Thirty years later, Horiuti and Ikusima used the word catalyst instead of electrode [48] and Butler pointed out the very important role of adsorbed intermediates in the evolution of hydrogen [33]. 

As discussed earlier, the two most important experimental factors in determining heterogeneous rate constants by CV are the precision in the measurement of AFp and the effectiveness with which the l u problem is dealt with. A detailed study of the kinetics of the reduction of benzo-nitrile in DMF—Bu4NBF4 (0.1 M) was carried out using derivative techniques and extensions of the correlations described by Tables 4—6 [42]. The study resulted in k° equal to 0.37 0.02 at 23.5°C for the reaction... 

HETEROGENEOUS KINETICS OF THE REDUCTION OF PERYLENE IN APROTIC SOLVENTS... 

Aalstad and Parker [46] also studied the kinetics of the reduction of perylene (PE) in DMF and acetonitrile... 

Reduction by sulfide seems a likely reaction for the formation of As(III). The ion product (Q) for reaction 3 was calculated from the measured concentrations of As(III), As(V), HS, and S042- at 30-31 m depth in October-November 1990 (in the presence of sulfide) log Q = 13.1-13.4 was obtained. The reduction of As(V) is incomplete, probably because of slow reduction processes (16). According to the reactions listed, complexes of As(III) with sulfide (such as AsS2" or H2As3S6 ) may control the solubility of As in presence of sulfide. The kinetics of the reduction of As(V) by sulfide and of the formation of sulfide species are poorly known. 

The kinetics of the reduction of Eu3+ by Yb2+ and oxidation of Eu2+ by V8+, Cr3+, Ti3+ and Ce4+ have been studied by King [193]. Meieb and Garner [194] have shown the exchange reaction Eu2+—Eu3+ to be rapid and first order with respect to both Eu2+ and Eu3+ as well as first order in chloride ion concentration. The rate is, however, independent of hydrogen ion concentration. 

Many aliphatic aldehydes exist primarily as hemiacetals in alcoholic solvents. It has been well understood for many years47 that the actual reducible species, or electrophore , in such media is not the hemiacetal but rather the small amount of the carbonyl compound itself (actually a hydrogen-bonded complex see above) present at equilibrium. Thus reduction is kinetically controlled that is, the overall rate of reduction is governed by the rate of conversion of the hemiacetal to the aldehyde. More recently, this has been confirmed and studied for formaldehyde and acetaldehyde in water at different pH levels48 and the kinetics of the reduction process have been studied for glucose, galactose and lactose49. 

The kinetics of the reductive dissolution mechanisms shown in Fig. 8.1 can be derived using the principle of mass action. The kinetic expression for precursor complex formation by way of an inner-sphere mechanism (Stone, 1986) is... 

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