Downhill reactions

The very fact that the A-to-D conversion is a downhill process implies that a chain reaction may take place in the solution, in parallel to the electrode process (Scheme 2.12). After initiation by an electron (or a hole) coming from the electrode, the propagation loop involves the conversion of B into C and the oxidation of the latter by A. When > c, the solution electron transfer is a downhill reaction, whereas for , B < , c, it is an uphill reaction. It may, nevertheless, interfere in the latter case since the entire process is pulled by the B/C reaction. As sketched in Scheme 2.10, the interference of the solution electron transfer is more important for slower B/C conversion. More precisely, the factor governing the interference of the solution electron transfer is the same as in the ECE-DISP problem discussed in Section 2.2.4 (kecPA/ (Fv/ R-T)1/2. Apparently, disconcerting phenomena take place upon interference of the solution electron transfer, such as dips in the current-potential trace when (Figure 2.25a ) and trace crossing... 

If the energy levels of the products are lower than the energy levels of the reactants (a downhill reaction), then the reaction is energetically favored and this equation gives a negative value of AG, corresponding to a decrease in the energy of the system. 

More reactive acid derivatives are easily converted to less reactive derivatives. A downhill reaction O O... 

Figure 10 shows the proposed ubiquinol oxidation and electron bifurcation mechanism at Qp site. (A) In the absence of the ubiquinone, the side chain of Glu-271 is connected to the solvent in the mitochondrial intermembrane space via a water chain. (B) As a reduced ubiquinol molecule binds to the site, the side chain of Glu-271 flips to form a hydrogen bond to the bound ubiquinone. (C) Now, the ISP, which is moving around the intermediate position by thermal motion is trapped at the b" position by a hydrogen bond to the bound ubiquinone. (D,E) Coupled to deprotonation, the first electron transfer occurs. Since the Rieske FeS cluster has a much higher redox potential (ca. +300 mV) than heme bl (ca. 0 mV), the first electron is favorably transferred to ISP. This yields ubisemiquinone, (F,G). After ubisemiquinone formation, the hydrogen bond to the His-161 of ISP is destabilized. The ISP moves to the c position, where the electron is transferred from the Rieske FeS cluster to heme c. Now unstable ubisemiquinone is left in the Qp pocket. The redox potential of the deprotonated ubisemiquinone is assumed to be several hundred millivolts. Now the electron transfer to the heme bl is a downhill reaction. (H) Coupled to the second electron transfer, the second proton is transferred to Glu-271 and subsequently to the mitochondrial intermembrane space. The fully oxidized ubiquinone is released to the membrane. 

The oxidation of acetate by O2 is a downhill reaction, which is catalysed by Ti02 in the presence of light absorbed by the semiconductor. Corresponding reactions are performed with stable oxides, primarily with Ti02 particles. The basic processes are illustrated in Fig. 2.34 for dispersed small semiconductor particles as typically used in this application. The holes produced by light excitation are used for the oxidation of the acetate, whereas electrons are transferred to O2. 

Figure 21-9 is critical Reactions which go from a more reactive functional group to a less reactive functional group ("downhill reactions") will occur readily. 

In the first part of Chapter 11 systems have been described in which solar energy is used for producing electrical energy or a storable fuel. Both processes lead to an increase of free energy, i.e. AG > 0 (uphill reaction). On the other hand, a photocatalytic reaction is a downhill reaction (AG < 0), where light excitation is only used to speed up a reaction which is thermodynamically possible in the dark but is kinetically inhibited. Examples are reactions involved in pollution control or in the synthesis of some organic compounds these are discussed below. 

If we insert in Equation (46) the value of the viscosity of water at 298 K we obtain/Cd = 0.7 x 10 ° dm mol s" and similar values are obtained by inserting experimental values of diffusion coefficients and plausible values of i ij in Equation (44). This is therefore the velocity constant which might be anticipated for any thermodynamically favourable (or downhill ) reaction between two solute species which is not retarded by the need for activation energy or other requirements. In view of the crude nature of the model, this prediction is not likely to be valid to much better than a power of ten, so that lO dm mol" s is a more realistic prediction. Equations (47) and (48) predict that reactions between oppositely charged ions will be somewhat faster than those in which at least one reactant is uncharged, and those between ions of like charge somewhat slower. 

This predicts that a = j for reactions with zero overall free energy change, while a < T for downhill reactions, and a > t for uphill reactions. If AG° can be measured or estimated, the observed value of a for a limited... 

ApK = p sH — P BH where SH is the carbon acid and B the base with which it reacts ApK is positive for all the reactions listed of ethyl a-methyl-acetoacetate and sodium propan-2-one-l-sulphonate (except for the reaction of the latter with hydroxide ion), and k /kP increases with decreasing ApK. However, a new factor emerges if we consider the much stronger carbon acid ethyl nitroacetate, for which both positive and negative values of ApK (i.e., both uphill and downhill reactions) are accessible. Here there is clear evidence of a maximum isotope effect in the neighbourhood of ApK = 0, with a decrease for reactions which are either markedly uphill or markedly downhill. 

Sacrificial H2 production Downhill reaction AG<0 Election donors (Abundant compounds)... 

In later studies [22] the authors have used the Pd(II) complex of NBD, Pd(NBD)Cl2 as a catalyst for the reverse reaction, i.e. the opening of QC to NBD. This downhill reaction can reach quantum yields of the order of 100 implying a chain mechanism. Since carbomethoxy substituted QC derivatives reacted slower, an electron transfer mechanism, where electron transfer is retarded by electron withdrawing substituents, as depicted in Scheme 6, was proposed. 

In practice the fact that both redox couples are exchanging electrons readily with the electrode will mean that the downhill reaction (AG = -0.77F)... 

Downhill" Reactions Generating More Electrophilic Species... 

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