Electron-transfer reactions homogeneous

Electron transfer reactions involving alkali metals are heterogeneous, and for many purposes it is desirable to deal with a homogeneous electron transfer system. It was noticed by Scott39 that sodium and other alkali metals react rapidly with aromatic hydrocarbons like diphenyl, naphthalene, anthracene, etc., giving intensely colored complexes of a 1 to 1 ratio of sodium to hydro-... 

Theories of Elementary Homogeneous Electron-Transfer Reactions Sacher, E. Laidler, K. J. 3... 

This chapter attempts to survey the studies which have been made on the various electron transfer reactions, occurring between metal ions (of the same element) in homogeneous solution. These reactions include the types known as exchange reactions... 

Marcus RA (1965) On the theory of electron-transfer reactions. VI. Unified treatment for homogeneous and electrode reactions. J Chem Phys 43 679... 

Coming back to thermal homogeneous dissociative electron transfer reactions, the question arises whether the electron-donor molecule reacts as a single electron donor or as a nucleophile in an Sn2 reaction. We will review this long-debated question in Section 7, including the most recent developments. 

The first attempt to describe the dynamics of dissociative electron transfer started with the derivation from existing thermochemical data of the standard potential for the dissociative electron transfer reaction, rx r.+x-,12 14 with application of the Butler-Volmer law for electrochemical reactions12 and of the Marcus quadratic equation for a series of homogeneous reactions.1314 Application of the Marcus-Hush model to dissociative electron transfers had little basis in electron transfer theory (the same is true for applications to proton transfer or SN2 reactions). Thus, there was no real justification for the application of the Marcus equation and the contribution of bond breaking to the intrinsic barrier was not established. 

Another interesting case concerns a homogeneous electron transfer reaction, namely, the initiation step of the classical Kornblum-Russell reaction153155 of 2-nitropropanate ions with 4-nitrobenzyl chloride (Scheme 14). 

Electron-transfer reactions at ITIES resemble electron-transfer reactions across biological membranes, which adds a special interest. Also, in contrast to homogeneous electron-transfer reactions, they allow a separation of the reaction products. So it is disappointing to report that only very few experimental investigations of electron-transfer reactions at ITIES have been performed. This is mainly due to the fact that it is difficult to find systems where the reactants do not cross the interface after the reaction in addition, side reactions with the supporting electrolyte can be a problem. 

Here, i is the faradaic current, n is the number of electrons transferred per molecule, F is the Faraday constant, A is the electrode surface area, k is the rate constant, and Cr is the bulk concentration of the reactant in units of mol cm-3. In general, the rate constant depends on the applied potential, and an important parameter is ke, the standard rate constant (more typically designated as k°), which is the forward rate constant when the applied potential equals the formal potential. Since there is zero driving force at the formal potential, the standard rate constant is analogous to the self-exchange rate constant of a homogeneous electron-transfer reaction. 

COUPLING OF ELECTRODE ELECTRON TRANSFERS WITH HOMOGENEOUS CHEMICAL REACTIONS... 

It should be noted that the conditions that make possible the occurrence of an ECE mechanism, involving the reduction of C at the electrode surface, involve the possibility of another mechanism in which the second electron is transferred from B to C rather than from the electrode as pictured in Scheme 2.5. This homogeneous electron transfer reaction may be viewed as a disproportionation reaction insofar that A has one oxidation number more than B and C, and D, one oxidation number less. 

When k eCp/kc <1, the rate-determining step is the forward homogeneous electron transfer reaction, meaning that the system behaves as just analyzed above. When, conversely, k ed /kc 1, the rate-determining step is the follow-up reaction, while the homogeneous electron transfer plays the role of a preequilibrium. The governing kinetic parameter is then... 

Back electron transfer is at the diffusion limit because the homogeneous electron transfer reaction is uphill, owing to the fact that the standard potential of the redox catalyst is necessarily chosen as positive of the reduction potential of the substrate. 

More complicated reactions that combine competition between first- and second-order reactions with ECE-DISP processes are treated in detail in Section 6.2.8. The results of these theoretical treatments are used to analyze the mechanism of carbon dioxide reduction (Section 2.5.4) and the question of Fl-atom transfer vs. electron + proton transfer (Section 2.5.5). A treatment very similar to the latter case has also been used to treat the preparative-scale results in electrochemically triggered SrnI substitution reactions (Section 2.5.6). From this large range of treated reaction schemes and experimental illustrations, one may address with little adaptation any type of reaction scheme that associates electrode electron transfers and homogeneous reactions. 

Electron transfer reactions of metal ion complexes in homogeneous solution are understood in considerable detail, in part because spectroscopic methods and other techniques can be used to monitor reactant, intermediate, and product concentrations. Unfavorable characteristics of oxide/water interfaces often restrict or complicate the application of these techniques as a result, fewer direct measurements have been made at oxide/water interfaces. Available evidence indicates that metal ion complexes and metal oxide surface sites share many chemical characteristics, but differ in several important respects. These similarities and differences are used in the following discussions to construct a molecular description of reductive dissolution reactions. 

Some Comparisons Between the Energetics of Electrochemical and Homogeneous Electron-Transfer Reactions... 

Commonly, in the description of chemical reactions coupled to electron transfer, the homogeneous chemical reaction is indicated by C and the heterogeneous electron transfer by E. The order of C with respect to E then follows the chronological order in which the two events occur. Furthermore, while Ox and Red indicate the electro active species, other non-electro active species which result from the coupled chemical complications are indicated by Y, Z, W, etc. 

The theory of homogeneous electron transfer reactions in solution has been formulated in terms of models in which the transferring electron is localized at a donor site in the reactant and at an acceptor site in the... 

An improved and direct correlation between the experimental rate constant and [obtained using Eq. (49)] is observed if v = /zd is used instead of v = 1/Tt, the solvent-dependent tunneling factor is utilized, and only AG (het) of Eq. (8) is used in Eq. (49) (see triangles in Fig. 18). Furthermore, the inverse of the longitudinal solvent relaxation time Xi is not necessarily the relevant one to use as the frequency factor v (see empty circles in Fig. 18). Similar conclusions were reached by Barbara and Jerzeba for the electron transfer reaction in homogeneous solutions. Barbara and Jerzeba measured the electron transfer time... 

Earlier, in Sect. 8.3.1, a generalized mechanistic scheme for the reduction of simple alkyl halides was presented. What distinguishes aryl halides (ArX) from alkyl halides (RX) is the finite lifetime of the initially electrogenerated anion radical (ArX ). Thus, although ArX exhibits the same kinds of reactions as RX, a key difference is that the transient anion radical (ArX ) can undergo a homogeneous electron-transfer reaction with the aryl radical (Ar) (Eq. 4) ... 

The fact that the anion radical is an intermediate in this case falls in line with the observation that it is also an intermediate in the reduction of the same substrates by homogeneous or heterogeneous outer sphere electron donors and also that nitrobenzyl halides are quite easy to reduce (see Section 2, p. 66). In the other cases, the generation of the R radical has been assumed to proceed by halogen-atom transfer (158). It should, however, be noted that an outer sphere, dissociative electron-transfer reaction (163) would also... 

The electron formed as a product of equation (2.5) will usually be received (or collected ) by an electrode. It is quite common to see the electrode described as a sink of electrons. We need to note, though, that there are two classes of electron-transfer reaction we could have considered. We say that a reaction is heterogeneous when the electroactive material is in solution and is electro-modified at an electrode which exists as a separate phase (it is usually a solid). Conversely, if the electron-transfer reaction occurs between two species, both of which are in solution, as occurs during a potentiometric titration (see Chapter 4), then we say that the electron-transfer reaction is homogeneous. It is not possible to measure the current during a homogeneous reaction since no electrode is involved. The vast majority of examples studied here will, by necessity, involve a heterogeneous electron transfer, usually at a solid electrode. 

Note that all cations are initially in solution and will certainly be solvated to some extent. In addition, notice that the symbol for the electron is again subscripted to show that charge comes from an electrode rather than from a homogeneous electron-transfer reaction in solution (cf. the potentiometric titrations we discussed in the previous chapter). 

Consider an electron-transfer reaction that forms an intermediate which can undergo a homogeneous reaction, and where the product of the chemical... 

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