Reduction reactions proton

Chemiosmotic model of ATP synthesis. The chemiosmotic model explains how energy from transport of electrons to O2 is transformed into the high-energy phosphate bond of ATP (see Fig. 21.1). Basically, the electron transport chain contains three large protein complexes (I, III, and IV) that span the inner mitochondrial membrane. As electrons pass through these complexes in a series of oxidation-reduction reactions, protons are transferred from the mitochondrial matrix to the cytosolic side of the inner mitochondrial membrane. 

The electrons undergo the equivalent of a partial oxidation process ia a dark reaction to a positive potential of +0.4 V, and Photosystem I then raises the potential of the electrons to as high as —0.7 V. Under normal photosynthesis conditions, these electrons reduce tryphosphopyridine-nucleotide (TPN) to TPNH, which reduces carbon dioxide to organic plant material. In the biophotolysis of water, these electrons are diverted from carbon dioxide to a microbial hydrogenase for reduction of protons to hydrogen ... 

The name amethyrin refers to the fact that the dull-purple color of the protonated form of the macrocycle is that of amethyst stones, Formal oxidation or reduction products are the aromatic [22]hexaphyrin(l.0.0.1.0.0) or the [26]hexaphyrin(l.0.0,1.0.0), respectively. However, none of these products could be observed either as reaction products or as direct products of oxidation or reduction reactions. 

The nature of the cathode material is not critical in the Kolbe reaction. The reduction of protons from the carboxylic acid is the main process, so that the electrolysis can normally be conducted in an undivided cell. For substrates with double or triple bonds, however, a platinum cathode should be avoided, as cathodic hydrogenation can occur there. A steel cathode should be used, instead. 

Fig. 3-4 Electron transport process schematic, showing coupled series of oxidation-reduction reactions that terminate with the reduction of molecular oxygen to water. The three molecules of ATP shown are generated by an enzyme called ATPase which is located in the cell membrane and forms ATP from a proton gradient created across the membrane.

In the same year, Evans and coworkers reported the electrochemical reduction of protons to H2 catalyzed by the sulfur-bridged dinuclear iron complex 25 as a hydrogenase mimic in which acetic acid was used as a proton source [201]. The proposed mechanism for this reaction is shown in Scheme 60. The reduction of 25 readily affords 25 via a one electron reduction product 25. Protonation... 

As described in Section 4-1. one important class of chemical reactions involves transfers of protons between chemical species. An equally important class of chemical reactions involves transfers of electrons between chemical species. These are oxidation-reduction reactions. Commonplace examples of oxidation-reduction reactions include the msting of iron, the digestion of food, and the burning of gasoline. Paper manufacture, the subject of our Box, employs oxidation-reduction chemishy to bleach wood pulp. All metals used in the chemical industry and manufacturing are extracted and purified through oxidation-reduction chemistry, and many biochemical pathways involve the transfer of electrons from one substance to another. 

In this section, we summarize the kinetic behavior of the oxygen reduction reaction (ORR), mainly on platinum electrodes since this metal is the most active electrocatalyst for this reaction in an acidic medium. The discussion will, however, be restricted to the characteristics of this reaction in DMFCs because of the possible presence in the cathode compartment of methanol, which can cross over the proton exchange membrane. 

This process involves a series of reactions, including dissolution, hydrogen reactions and chlorine withdrawal [20], The second type of reactions include reduction of protons at the catalyst by electron transfer yielding hydrogen radicals that are consumed by reaction or give elemental hydrogen otherwise. 

As with the phase diagrams and Pourbaix diagrams, the theoretical standard hydrogen electrode also allows us to calculate the relative energies of intermediates in electrochemical reactions. As an example, we investigate the oxygen reduction reaction (ORR). We look at the four proton and electron transfer elementary steps ... 

Raghuveer V, Manthiram A, Bard AJ. 2005. Pd-Co-Mo electrocatalyst for the oxygen reduction reaction in proton exchange membrane fuel cells. J Phys Chem B 109 22909-22912. 

Sometime probably two billion years before humans became interested in efficient catalysts for four-electron, four-proton reduction of O2 to H2O, the so-called oxygen reduction reaction (ORR),... 

The activation energy for the charge reduction reaction is due to two factors the bond stretching and distortions of the originally near linear complex, so as to achieve the internal proton transfer and the increase of energy due to the Coulombic repulsion between the two charged products, a repulsion that leads to a release of kinetic energy on their separation. 

Examining Table 2, one comes to the conclusion that only Ba2+ (H20)n where n > 1 can be produced by the association reactions of M2+ with H20. For all the other ions only the monohydrate will be obtainable. For ions with high IE(M) values, even the monohydrate, M2+H20 may not be obtained because of charge transfer reactions to H20 (see equation 22). Other protic solvents will lead to charge reduction by proton transfer at different values of r. Only NH3 has been examined.71 It leads to much more facile charge reduction than H20. Many of the doubly charged ions that were observed as hydrates could not be observed as the equivalent clusters of NH3. 

The charge reduction reactions occurred prior to any collisional activation in the second quadrupole Q2, which indicates that the activation energy for this reaction is very low. These results show that in the presence of a multiple charge even N-H hydrogens can become sufficiently acidic and engage in protonation of water molecules. 

Fuel cell applications Manganese dioxide as a new cathode catalyst in microbial fuel cells [118] OMS-2 catalysts in proton exchange membrane fuel cell applications [119] An improved cathode for alkaline fuel cells [120] Nanostructured manganese oxide as a cathodic catalyst for enhanced oxygen reduction in a microbial fuel cell [121] Carbon-supported tetragonal MnOOH catalysts for oxygen reduction reaction in alkaline media [122]... 

ELECTROCATALYTIC REDUCTION OF PROTONS AND HYDRIDE TRANSFER REACTIONS... 

The reduction of protons is one of the most fundamental chemical redox reactions. Transition metal-catalyzed proton reduction was reviewed in 1992.6 The search for molecular electrocatalysts for this reaction is important for dihydrogen production, and also for the electrosynthesis of metal hydride complexes that are active intermediates in a number of electrocatalytic systems. 

Cofacial ruthenium and osmium bisporphyrins proved to be moderate catalysts (6-9 turnover h 1) for the reduction of proton at mercury pool in THF.17,18 Two mechanisms of H2 evolution have been proposed involving a dihydride or a dihydrogen complex. A wide range of reduction potentials (from —0.63 V to —1.24 V vs. SCE) has been obtained by varying the central metal and the carbon-based axial ligand. However, those catalysts with less negative reduction potentials needed the use of strong acids to carry out the catalysis. These catalysts appeared handicapped by slow reaction kinetics. 

Theoretically, according to the mechanism of biological azo dye reduction, the processes of biological decolorization are oxidation-reduction reactions, in which transfer of electrons match with the proton flow by the help of coenzymes, such as NADPH/NADP+ and NADH/NAD+. The oxidation-reduction potentials of the couples of NADPH/NADP+ and NADH/NAD+ are -324 and -320 mV, respectively [25, 46]. The least AGo value of the conversion NADPH/NADP+ and NADH/NAD+ is 44 kJ [47]. Therefore, —93 mV, which is obtained from (1), could be considered as a rough limited ORP value for ordinary primary electron donors of the third mechanism of biological azo dye reduction. This was demonstrated by the results of many researches (Table 1). Hence, the observed failure of cyanocobala-min [30] and ethyl viologen [48] to act as a mediator is most probably due to their too low Ed values 530 and —480 mV, respectively. 

In the case of mono-ester substituted pyrroles (e.g., 68) wherein relatively unstable dianions likely to deprotonate ammonia might be produced, the authors instead utilized an excess of (MeOCH2CH2)2NH as a substitute for ammonia. It was felt that upon in situ formation of (MeOCH2CH2)2NLi, this base would be unable to protonate the dianion <00TL1331>. Remarkably, quenching the reduction reactions with benzoyl chloride affords P-keto esters (e.g., 69, R = COPh), a reaction that does not occur when conducted in liquid ammonia. 

Of course the basis for any biological hydrogen producing system is an enzyme that is capable of carrying out what is arguably the simplest chemical reaction, the reduction of protons to hydrogen 2H+ + 2e H2. All enzymes capable of hydrogen evolution contain... 

Lehnert and Tuczek further studied end-on terminal coordination by density functional theory (DFT) calculations on the compounds [Mo(N2)2(dppe)2], [MoF(NNH)(dppe)2], and [MoF(NNH2)(dppe)2]+, where dppe= 1,2-bis(diphenyl-phosphino)ethane.50 They proposed a reaction scheme, shown in reaction 6.13, for asymmetric dinitrogen reduction and protonation. The end-on model favored by Lehnert in reference 50, as shown in reaction 6.13, appears to be a less thermodynamically unfavorable pathway, at least to reach the M-NNH3 intermediate. Step 1 produces a metal-attached diazenido ion (NNH-), step 2 produces a hydrazido ion (NNH2 ), and step 3 produces a hydrazidium ion (NNHj). 

Since many of the transformations undergone by metabolites involve changes in oxidation state, it is understandable that cofactors have been developed to act as electron acceptors/ donors. One of the most important is that based on NAD/NADP. NAD+ can accept what is essentially two electrons and a proton (a hydride ion) from a substrate such as ethanol in a reaction catalysed by alcohol dehydrogenase, to give the oxidized product, acetaldehyde and the reduced cofactor NADH plus a proton (Figure 5.2). Whereas redox reactions on metal centres usually involve only electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer but... 

Material balances can be written for moieties which are conserved during the reaction, such as the atoms of a particular element or the total charge, or for reactant or product species if the stoichiometry is unambiguous. Oxidation-reduction reactions may be particularly troublesome. In the following situation, for example, one cannot write a material balance relating protons to water molecules. Consider the oxidation of O2 to H2O and the equilibrium dissociation of I O. 

Crampton24 has also demonstrated that for Meisenheimer complex formation, increased crowding at the reaction site caused by change from primary amines to piperidine results in rate reduction of proton transfer from the complex to the amine catalyst, and Hirst199... 

The formation of S-oxides has also been observed when oxidizing a variety of 5-substituted 2-tert-butyl-l,3-dithianes in wet acetonitrile. In an undivided cell, 4-substituted 1,2-dithiolane-l-oxides were oblained (Scheme 25) [113]. A coupled cathodic process, in this case, was the reduction of protons formed in the anodic reaction. 

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