Redox reaction sequences

A typical redox reaction sequence between an (usually organic) reductant R and iron(III) (hydr)oxide can be schematically expressed as... 

THE REDOX BEHAVIOR OF NATURAL SYSTEMS 11.2.1 Redox Reaction Sequences and Redox Ladders... 

For this modelling the Monod kinetic approach was chosen. In this work the primary redox reaction sequence (1A)-(5A) was set up introducing appropriate threshold values (half satmation concentrations) as reaction limiting factors. The contribution of a given electron acceptor i to the overall DOC degradation is given by... 

Mechanism of Action. Under photolysis, acidic, and electron bombardment conditions, the transformation of o-nitrobenzyl alcohol or its derivatives involves an internal redox reaction sequence followed by liberation of the deprotected alcohol or amine (eq 1). Analogously, the photorearrangement of esters of o-NBA, obtained through its reaction with acid chlorides or anhydrides, also induces an internal redox reaction (eq 2). 

A number of electronic and photochemical processes occur following band gap excitation of a semiconductor. Figure 5 illustrates a sequence of photochemical and photophysical events and the possible redox reactions which might occur at the surface of the SC particle in contact with a solution. Absorption of light energy greater than or equal to the band gap of the semiconductor results in a shift of electrons from the valence band (VB) to... 

There is no direct experimental evidence for the intermediate 2.30 in the reaction sequence of Scheme 2-19. In the corresponding diazotization of 2-aminophenazine the proportion of the quinone diazide (isomer of 2.31) amounted to only 16%, but 30% unsubstituted phenazine was also found. The phenazine may have resulted from the overall redox reaction. 

S.3.3 Electrocatalytic Modified Electrodes Often the desired redox reaction at the bare electrode involves slow electron-transfer kinetics and therefore occurs at an appreciable rate only at potentials substantially higher than its thermodynamic redox potential. Such reactions can be catalyzed by attaching to the surface a suitable electron transfer mediator (45,46). Knowledge of homogeneous solution kinetics is often used to select the surface-bound catalyst. The function of the mediator is to facilitate the charge transfer between the analyte and the electrode. In most cases the mediated reaction sequence (e.g., for a reduction process) can be described by... 

The following examples of reagent sequences, which include the reagent Ammonium monovanadate — p-anisidine described in the second part of the book can also be classified as redox reactions. 

This analogy to a surface redox mediated process is significant. In a way very similar to the reaction sequence (1.14), the standard potential of the redox surface system Pt(H20)/Pt-0Hads (0.80 V with respect to RHE) determines the active (reduced) site population at any cathode potential E, and consequently is the critical parameter in determining the ignition potential for the ORR process. 

The previous analysis indicates that although the voltammetiic behavior suggests that the aqueous phase behaves as a metal electrode dipped into the organic phase, the interfacial concentration of the aqueous redox couple does exhibit a dependence on the Galvani potential difference. In this sense, it is not necessary to invoke potential perturbations due to interfacial ion pairing in order to account for deviations from the Butler Volmer behavior [63]. This phenomenon has also been discarded in recent studies of the same system based on SECM [46]. In this work, the authors observed a potential independent ket for the reaction sequence. 

P. Mitchell (Nobel Prize for Chemistry, 1978) explained these facts by his chemiosmotic theory. This theory is based on the ordering of successive oxidation processes into reaction sequences called loops. Each loop consists of two basic processes, one of which is oriented in the direction away from the matrix surface of the internal membrane into the intracristal space and connected with the transfer of electrons together with protons. The second process is oriented in the opposite direction and is connected with the transfer of electrons alone. Figure 6.27 depicts the first Mitchell loop, whose first step involves reduction of NAD+ (the oxidized form of nicotinamide adenosine dinucleotide) by the carbonaceous substrate, SH2. In this process, two electrons and two protons are transferred from the matrix space. The protons are accumulated in the intracristal space, while electrons are transferred in the opposite direction by the reduction of the oxidized form of the Fe-S protein. This reduces a further component of the electron transport chain on the matrix side of the membrane and the process is repeated. The final process is the reduction of molecular oxygen with the reduced form of cytochrome oxidase. It would appear that this reaction sequence includes not only loops but also a proton pump, i.e. an enzymatic system that can employ the energy of the redox step in the electron transfer chain for translocation of protons from the matrix space into the intracristal space. 

Only transformations in the longest linear sequence (LLS) are considered. The term skeleton constructions refers to C-C and C-O bond formations (notwithstanding redox reactions) that directly introduce native structural features of the bryostatins without further modification. The term other functional group manipulations refers to steps that indirectly introduce native structural elements, the interconversion of functional groups (e.g., the introduction and removal of auxiliaries) and miscellaneous transformations that do not involve skeleton construction... 

Upon increasing the pH, first this species is present in increasing concentrations, but eventually the non-participating ligand is replaced with another cysteine molecule and the redox inactive Cu(RS)2 is formed. Consequently, the reaction rate decreases sharply The following reaction sequence was proposed for the redox reaction ... 

Two additional systems were exploited in order to confirm the involvement of free-radical processes during vindoline oxidations. These were the enzyme peroxidase and photochemistry. Horseradish peroxidase (HRP) oxidized both vindoline and 16-O-acetylvindoline in the presence of hydrogen peroxide. Vindoline was converted to the enamine dimer 59 (78). During the reaction, the following sequence of redox reactions occurs ... 

Leach-proof sol-gel entrapment can be exploited to carry out one-pot reactions with mutually destructive reactants while still allowing these reagents to activate or participate in desired reactions. For instance, three different one-pot redox reactions can be carried out in sequence in one pot over two separate sol-gel matrices doped with an oxidant (pyridinium dichromate) and with a reducing species (RhCl[P(C6H5)3]3) without their mutual destruction and with no need for separation steps (Figure 5.12).24... 

After formation of an O-coordinated ketyl radical anion and a cis coordinated tyrosin via hydrogen abstraction, a rapid intramolecular one-electron redox reaction occurs with release of the product aldehyde and formation of the fully reduced active site containing a Cu(I) ion, which then reacts with 02 to give H202 and the active enzyme. The above sequence represents Nature s mechanistic blueprint for coordination chemists. 

Because of the precise control of the redox steps by means of the electrode potential and the facile measurement of the kinetics through the current, the electrochemical approach to. S rn I reactions is particularly well suited to assessing the validity of the. S rn I mechanism and identifying the side reactions (termination steps of the chain process). It also allows full kinetic characterization of the reaction sequence. The two key steps of the reaction are the cleavage of the initial anion radical, ArX -, and conversely, formation of the product anion radical, ArNu -. Modeling these reactions as concerted intramolecular electron transfer/bond-breaking and bond-forming processes, respectively, allows the establishment of reactivity-structure relationships as shown in Section 3.5. 

In heterogeneous redox reactions similar reaction sequences are observed usually an encounter (outer-sphere or inner-sphere) surface complex is formed to facilitate the subsequent electron transfer. 

The powerful biological machinery of energy conversion proceeds via redox reactions in aqueous media that involve electron and proton transfer between molecular entities. - Nature devised concerted sequences of these processes that generate electrochemical potential gradients across cell membranes and thereby enable the storage and the release of electrical energy. 

This enables one to use aliphatic systems as precursors to the radicals X-Y whose solvolytic (= redox) behavior can then be studied. Equations 2a, c describe what may be called oxidative solvolysis . This reaction sequence, the first step of which is in many cases induced by the OH radical, is of great importance in radical (and radiation) chemistry. It extends from /8-elimination reactions of monomeric radicals [6, 7] to the mechanism of DNA strand breakage [8]. An example for Eq. 2 in which it is shown that the radical XY can be produced by either step a or b is given in section 3.3. 

The first half of the sequence, i.e. Cys-Ile-Ala, was also prepared and the redox reaction of the cluster with four Z-Cys-Ile-Ala-OMe ligands was examined. 

A water-soluble Cj-symmetrical trisadduct of Cjq showed excellent radical scavenging properties in vitro and in vivo and exhibits remarkable neuro-pro tective properties [7,8]. It is a drug candidate for the prevention of ALS and Parldnsoris disease. Concerning the reaction mechanism, nucleophilic additions and radical additions are closely related and in some cases it is difficult to decide which mechanism actually operates [92]. For example, the first step in the reaction of f-eo with amines is a single electron transfer (SET) from the amine to the fullerene. The resulting amines are finally formed via a complex sequence of radical recombinations, deprotonations and redox reactions [36]. 

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