Transferring Electrons Organic Compounds

XII. MECHANISTIC STUDIES OF ELECTRON TRANSFER, CONCERNING ORGANIC COMPOUNDS OF GROUP 14 ELEMENTS... 

Rate Constants for Electron Transfer Between Organic Compounds and the Mercury Electrode... 

Another factor ensuring high current density is the rapid removal of solvated electrons from the electrode due to intense convection in the solution, which is caused by a decrease in its density at the cathode surface. This phenomenon is associated with an increase in solution volume caused by the introduction of electrons into it. Unlike electrostriction that accompagies the solvation of ordinary ions, the formation of solvated electrons increases the volume by 65-96 ml/mol for liquid ammonia and by about 80 ml/mol for hexamethylphosphotriamide . As a result, according to Avaca and Bewick the current densities for the generation of solvated electrons can by 2500 or more times exceed the rate of mass transfer of organic compounds to the electrode. 

In chemisorption, unshared electron pairs or p electrons from organic compounds interact with the metal orbitals to form a coordinate-type bond. The interaction proceeds in the presence of heteroatoms (P, Se, S, N, and O) that possess lone-pair of electrons and/or aromatic rings in the adsorbed molecules [16,17]. Chelate forms through a coordinate covalent bond by electron transfer from organic compounds to metal. The chemisorption has higher activation energy than electrostatic adsorption. It is a temperature-dependent phenomena, it occurs slowly, and is not reversible. 

Figure 6.13 Electron transfer between organic compounds. Correlation of rate constant with standard free-energy change (AG ) for reactions of amine donors with aromatic hydrocarbons in acetonitrile (see text). After D. Rehm and A. Weller, Ref. [27,c].

When cells of higher plants are under anaerobic conditions (deprived of oxygen) carbon dioxide release persists for some time and at least part of this carbon dioxide arises from a type of anaerobic respiration known as fermentation in which the electrons and protons are transferred to organic compounds such as pyruvic acid and acetaldehyde. The reduction of these compounds results in the appearance in the cell of compounds like lactic acid and ethanol. Clearly, this fermentation which can occur in the cells of higher plants closely resembles the alcoholic fermentation of yeast first studied in detail by Pasteur in 1870 and the production of lactic acid in manunalian muscles under conditions of an oxygen deficit, a process studied in detail by Fletcher and Hopkins in 1907. 

There are three types of electron transfers, firstly the generation of an electron electrochemically, by y-irradiation, or by photolytic dissociation, secondly the transfer of an electron from an inorganic or organic compound, referred to as a nucleophilic homolytic leaving group (Zollinger, 1973 a), and thirdly a transfer from a transition metal or transition metal ion complex. In this section we will discuss the fundamental aspects of these three types. In the following sections and in Chapter 10, specific examples and synthetic applications will be summarized. 

Valette-Hamelin approach,67 and other similar methods 24,63,74,218,225 (2) mass transfer under diffusion control with an assumption of homogeneous current distribution73 226 (3) adsorption of radioactive organic compounds or of H, O, or metal monolayers73,142,227 231 (4) voltammetry232,233 and (5) microscopy [optical, electron, scanning tunneling microscopy (STM), and atomic force microscopy (AFM)]234"236 as well as a number of ex situ methods.237 246... 

As in chemical systems, however, the requirement that the reaction is thermodynamically favourable is not sufficient to ensure that it occurs at an appreciable rate. In consequence, since the electrode reactions of most organic compounds are irreversible, i.e. slow at the reversible potential, it is necessary to supply an overpotential, >] = E — E, in order to make the reaction proceed at a conveniently high rate. Thus, secondly, the potential of the working electrode determines the kinetics of the electron transfer process. 

Conversely, the use of elevated temperatures will be most advantageous when the current is determined by the rate of a preceding chemical reaction or when the electron transfer occurs via an indirect route involving a rate-determining chemical process. An example of the latter is the oxidation of amines at a nickel anode where the limiting current shows marked temperature dependence (Fleischmann et al., 1972a). The complete anodic oxidation of organic compounds to carbon dioxide is favoured by an increase in temperature and much fuel cell research has been carried out at temperatures up to 700°C. 

Besides water, the most common weak base is ammonia, NH3, whose proton transfer equilibrium with water appears in Section 16-. Many other weak bases are derivatives of ammonia called amines, hi these organic compounds, one, two, or three of the N—H bonds in ammonia have been replaced with N—C bonds. The nitrogen atom in an amine, like its counterpart in ammonia, has a lone pair of electrons that can form a bond to a proton. Water does not protonate an amine to an appreciable extent, so all amines are weak bases. Table 17-4 lists several examples of bases derived from ammonia. 

The enzymes are protein molecules having globular structure, as a rule. The molecular masses of the different enzymes have values between ten thousands and hundred thousands. The enzyme s active site, which, as a rule, consists of a nonproteinic organic compound containing metal ions of variable valency (iron, copper, molybdenum, etc.) is linked to the protein globule by covalent or hydrogen bonds. The catalytic action of the enzymes is due to electron transfer from these ions to the substrate. The protein part of the enzyme secures a suitable disposition of the substrate relative to the active site and is responsible for the high selectivity of catalytic action. 

In the presence of substances that react with Ag , the 340 nm absorption decays more rapidly and the rate constant of reaction can be calculated from this decay. It was found in this way that free silver atoms are indeed a strongly reducing species. They reduce Fe to Fe and Cu to Cu (note that these reactions would not occur with the silver atoms at the surface of a compact electrode ) organic compounds containing electrophilic groups such as CICH2COOH or CH3NO2 are reduced by Ag via electron transfer... 

The Taft relation Ej 2 = P 2a + x, which was found to hold for organic compounds and some transition metal complexes can also be of use here (37). Phenyl compounds do not fit the relation. This is probably due to a mesomeric effect that depends on the dihedral angle between the phenyl and the NCS2 planes. For bulky substituents deviations are also found which could be caused by widening of the CNC angle, changing the hybridisation of the N. The low values of p indicate that the M.O. s involved in the electron transfer have little ligand contribution. 

Polarographic studies of organic compounds are very complicated. Many of the compounds behave as surfactants, most of them exhibit multiple-electron charge transfer, and very few are soluble in water. The measurement of the capacitance of the double layer, the cell resistance, and the impedance at the electrode/solution interface presents many difficulties. To examine the versatility of the FR polarographic technique, a few simple water-soluble compounds have been chosen for the study. The results obtained are somewhat exciting because the FR polarographic studies not only help in the elucidation of the mechanism of the reaction in different stages but also enable the determination of kinetic parameters for each step of reduction. 

Minero, C., Mariella, G., Maurino, V., and Pelizzetti, E. (2000) Photocatalytic transformation of organic compounds in the presence of inorganic anions. 1. Hydroxyl-mediated and direct electron-transfer reactions of phenol on a titanium dioxide-fluoride system. Langmuir,... 

Back electron transfer takes place from the electrogenerated reduc-tant to the oxidant near the electrode surface. At a sufficient potential difference this annihilation leads to the formation of excited ( ) products which may emit light (eel) or react "photochemical ly" without light (1,16). Redox pairs of limited stability can be investigated by ac electrolysis. The frequency of the ac current must be adjusted to the lifetime of the more labile redox partner. Many organic compounds have been shown to undergo eel (17-19). Much less is known about transition metal complexes despite the fact that they participate in fljjany redox reactions. 

The immediate product of the electron transfer reaction with an organic compound is unstable. 

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