Solvated electron cathodic generation

Comparison of the optical absorption curves shown in Fig. 5 reveals that solvated electrons are generated in alkali metal salt solutions during cathodic polarization. In fact, the particles that form during electrolysis are identical to those appearing under conditions for which the formation of solvated electrons is well proved, i.e., on dissolution of alkali metals and on pulse radiolysis of pure solvent and/or of the salt solutions. 

It follows from Section 4 that solvated electrons are generated during cathodic polarization of inert metal electrodes, e. g., in liquid ammonia and hexamethylphosphotriamide solutions of alkali metals salts. 

Benkeser and Tincher 128>, on the other hand, reduced acetylenes preferentially to trans olefins using solvated electrons generated at a platinum cathode by electrolytic reduction of lithium chloride in methylamine [lithium metal is formed from lithium ion at the cathode in this electrolysis its dissolution in methylamine generates the solvated electron and regenerates lithium... 

The electrochemical generation of hydrogen in aqueous acid or alkaline solutions reduces unactivated alkynes 46 a b). This process is similar to catalytic hydrogenation, however, and does not involve electron transfer to the substrate. The electrochemical generation of solvated electrons in amine solvents or HMPA has also been effective in reducing these compounds 29). The focus of this section, however, is the electrolysis of these difficult to reduce alkynes and alkenes at mercury cathodes with tetraalkyl-ammonium salts as electrolytes. Specific attention is also given to competitive reductions of benzenoid aromatics and alkynes or alkenes. 

In very pure nonpolar dielectric liquids, electron injection currents at very sharp tips follow the Fowler-Nordheim voltage dependence (Halpem and Gomer, 1969), just as is the case in solid insulators, and in a gas, as described before. In a study of the electrochemical behavior of CNT cathodes (Krivenko et al., 2007) direct experimental proof was found of electron emission into the liquid hexamethylphosphortriamide, which was chosen because it is a convenient solvent for the visualization of solvated electrons at room temperature the solution will show an intense blue coloration upon the presence of solvated electrons. Electron spin resonance showed prove of a free electron. Electrogenerated (as opposed to photogenerated) solvated electrons have been used in the synthesis of L-histidinol (Beltra et al., 2005), albeit that in that work the electrons were generated electrochemically from a solution of LiCl in EtNH2, which is a solvent that is easier to handle than liquid ammonia (boiling points at atmospheric pressure are 17 °C and -33.34 °C, respectively). 

In cathodic addition reactions solvated electrons, radical anions or anions are generated at the cathode and added to activated or unactivated double or triple bonds. This broad spectrum of reactions is partially treated a) in section 8.2 when group conversion generates a reactive intermediate which undergoes addition reactions 1 S5a,b and b) in section 12.2 when olefins are coupled via addition of a cathodically generated radical anion to an activated double bond. 

Benzene is reduced in 95% current yield to a mixture of 23% cyclohexadiene, 10% cyclohexene and 67% cyclohexane. HMPTA as a solvent additive seems to play a dual role. Firstly it is selectively adsorbed at the cathode surface, thereby preventing hydrogen evolution from the protic solvent. Thus it permits the attainment of a potential sufficiently cathodic for the generation of the solvated electron. It secondly stabilizes the solvated electron, thus suppressing its reaction with protic solvents (eq. (130) ). With decreasing HMPTA concentration in the electrolyte the current efficiency for reduction decreases and hydrogen evolution dominates. In pure ethanol the current efficiency is less than 0,4%. 

Ammonia has been employed mostly for cathodic reactions, but some oxidations [330,333] have been carried out in this medium, although the potential range in the anodic direction is quite small. The anodically limiting reaction is oxidation to nitrogen and protons [340] the cathodically limiting reaction is the transfer of electrons to the solvent, which occurs at about —2.3 V (versus Hg pool electrode) in a saturated solution of TBAI. In the elecltrolytic generation of solvated electrons the potential is determined by the surface concentration of electrons and no external reference electrode is needed. 

We report here on the cathodic formation and follow-up reactions of anionic species of hydrocarbons. Other generation techniques, such as reduction by solvated electrons or indirect electrolysis, are discussed in detail in Chapter 29. 

Naphthols are hydrogenated to the corresponding decanols. )6-Naphthol gives trcms-2-decanol (70% yield) in an acidic solution [99], whereas or-naphthol is hydrogenated in HMPA to cw-l-decanol (47% yield) [100]). The latter is regarded as a hydrogenation induced by cathodically generates solvated electrons. 

The cathode potential necessary for the production of solvated electrons is rather negative the standard potential of the hydrated electron has been calculated to be —2.68 V versus NHE. Also, in other solvents compatible with its formation, very negative potentials must be used for example, in liquid ammonia the generation of ens is achieved at —2.47 V versus Ag/AgN03 (0.1 M) [306], but the dissolution standard potential measured in HMPA was found to be —3.44 V versus Ag/AgC104 (0.1 M) [307]. Similarly, in methy-lamine f — 50°C), a potential of —2.90 V versus Ag/AgN03 was reported [308]. 

The Birch and Benkeser reactions of some unsaturated organic compounds [318 and references therein], which consist of a reduction by sodium or lithium in amines, can be mimicked electrochemically in the presence of an alkali salt electrolyte. The cathodic reaction is not the deposition of alkali metal on the solid electrode but the formation of solvated electrons. Most of the reactions described were performed in ethylenediamine [319] or methylamine [308,320]. A feature of these studies is variety introduced by the use of a divided or undivided cell. In a divided cell, the product distribution appears to be the same as that in the classic reduction by metal under similar conditions. In contrast, in an undivided cell the corresponding ammonium salt is formed at the anode it plays the role of an in situ generated proton donor. Under such conditions, the proton concentration... 

General Conditions for Cathodic Generation of Solvated Electrons.167... 

Over the past 10-15 years a new trend has been developed in theoretical electrochemistry the electrochemistry of solvated electrons. In this review theoretical concepts of the electrochemical properties of solvated electrons and the results of experimental studies are considered from a unified position. Also discussed are energy levels of localized (solvated) and delocalized electrons in solutions and methods for their determination conditions of electrochemical formation of solvated electrons and properties of these solutions equilibrium on an electron electrode . The kinetics and mechanisms of cathodic generation of solvated electrons and of their anodic oxidation are discussed in detail. In the last sections participation of solvated electrons in ordinary electrode reactions is discussed, and the possibilities of cathodic electrosyntheses utilizing solvated electrons are considered. 

Electrochemical generation of solvated electrons was first observed in 1897 by Cady who found that when sodium solutions in liquid ammonia are electrolysed the blue coloration intensity increases at the cathode. All information on cathode generation of solvated electrons remained at this qualitative level for over half a century until Laitinen and Nyman made the first attempt to quantitatively investigate the kinetics of this process. This work, however, remained isolated for a long time and only after 20 years, with the awakening of interest in the chemistry of solvated electrons, were systematic studies into the kinetics of electrode reactions of solvated electrons started, almost simultaneously by three groups of researchers in Southampton Tokyo and Moscow In Moscow these studies... 

Electron photoemission from solvated electron solution (in solvents such as hexa-methylphosphotriamide and liquid ammonia the solvated electrons are fairly stable) to vapour phase has been studied by Delahay and co-workers (whose works are reviewed in Ref. ). According to them, this process proceeds in three stages solvated electron photoionization diffusion of generated delocalized electrons to the solution s surface and emission proper, i.e. transition of electrons to the vapour phase where they are transferred from the cathode surface (i.e., from the solution) to the anode by the external electric field. 

Another significant discrepancy between the theory and experiment is that even the computed values of the reorganization energy, to say nothing of its experimental estimation, are too small to be used for explaining the rate of the cathodic generation of solvated electrons (see Sect. 7). All this indicates that some factors affecting the electrochemical behaviour of solvated electrons are still unknown. 

Obtaining solvated electrons by dissolving alkali metals and by electrochemical generation is of special interest. Back in the last century, the dissolution of alkali metals in liquid ammonia gave the very first evidence of obtaining solvated electrons. Electrochemical (cathodic) generation of solvated electrons is a process in which electrons are transferred from the electrode into solution under the action of high cathode potentials. 

These methods are linked in that the solvents which dissolve alkali metals yielding solvated electrons and solvated cations are most commonly used for electrochemical generation of solvated electrons. One of the first experiments on such a generation was performed in 1897 by Cady who noticed an increase in the intensity of the blue coloration at the Pt cathode in a dilute sodium solution in liquid ammonia at —34 °C... 

Cathodic generation of solvated electrons in general competes with other cathodic reactions, say, in alkali metal salt solutions, with electrolytic deposition of the alkah metal on the electrode. Therefore, it is necessary to find a criterion which could be used to estimate how effective the cathodic generation will be, if it is possible at all, for the given system. 

Binding of cations by cryptands and/or crowns hinders the cathodic reduction of cations. Thus, in propylene carbonate the presence of excess cryptand-222 markedly shifts the half-wave potential for the formation of alkali metal amalgams A maximum shift of about 1 V was observed for sodium ions. On adding 18-crown-6 to glyme and ethylenediamine, deposition of sodium is replaced by electrochemical generation of solvated electrons. 

As mentioned earlier, for generation of solvated electrons it is necessary that the potentials of the cathode should be high enough. This can be realized for a number of systems (see the Scheme). Alkali metal cations serve as counterions in the... 

In the systems listed under A-1, thermodynamically it would be more advantageous on solid electrodes if the cathodic generation of solvated electrons proceeds... 

In the systems of subgroup A-2, cathodic reduction of a cation is thermodynamically disadvantageous, as a rule, for the salts of lithium, rubidium, and cesium, in whose solutions solvated electrons can be generated. In sodium salt solutions electrodeposition of the metal takes place. Potassium salt systems occupy an intermediate position. 

All solvents suitable for generation of solvated electrons are thermodynamically unstable at high cathode potentials. However, cathodic reduction of a solvent kinetically slows down. In Group A solvents the chemical reaction of solvent and solvated electrons is also hindered the reaction rate markedly increases as the temperature is raised and/or use is made of catalysts. 

From what has been said it is clear that cathodic generation of solvated electrons is possible in a number of systems hence, account should be taken of the electrochemical formation of solvated electrons in all cases where the cathode potentials are negative enough for this process to proceed. 

Vast material on the properties of solvated electrons in liquid ammonia has been accumulated by now . i23-i27> Ammonia was the first solvent for which it was shown that the properties of solvated electrons obtained by different methods (by dissolving alkali metals, by pulse radiolysis, and by cathodic generation) were identical (see Fig. 4). 

Cathodic generation of solvated electrons is a convenient method for studying their association, since it enables the concentration of solvated electrons to be varied independently over a wide range while preserving the cations concentration well in excess. 

Table 5 compares the standard potential of the electron electrode in hexamethylphosphotriamide (5 °C) with the standard potentials of alkali metals (25 °C). Data for liquid ammonia are also given. In both solvents the rubidium electrode potential serves as a reference point since it depends very little on the solvent. It is seen from the Table that in both solvents the standard equilibrium potential of the electron electrode is more positive than that of a lithium electrode and is close to the potentials of other alkali metals. In the course of experiment, cathodic production of dilute solutions (10 — 10 mol/1) of solvated electrons takes place and this makes the electron electrode equilibrium potential more positive compared to the standard value. In case of hexamethylphosphotriamide the same happens when electrons are bound in strong non-paramagnetic associates by the cations of all alkali metals except lithium (see Sect. 4). This enables one to assume that under the conditions of the experiments the electron-electrode equilibrium potential in liquid ammonia and hexamethylphosphotriamide is more positive than the equilibrium potential of all alkali metals. This makes thermodynamically possible primary cathodic generation of solvated electrons in solutions of all alkali metal salts in the two solvents. 

The results of the studies of this process in different media are summarized in Table 6. When investigated by cyclic voltammetry, one usually starts with solutions that initially do not contain solvated electrons solvated electrons are then obtained during the cathodic sweep of potential. In other methods, the necessary bulk concentration of solvated electrons was attained by dissolving the alkali metal or by preliminary cathodic generation at an auxiliary electrode. 

As two kinds of particles capable of undergoing oxidation are observed both solvated electrons and alkali metal anions may be assumed to be formed also during cathodic generation of electrons in anhydrous solutions of alkali meta iodides in ethylenediamine. 

By the beginning of 7O s only one work on the kinetics of electrochemical generation of solvated electrons was known Later, however, a number of papers reflecting the increased interest in this problem appeared. A brief annotation of the works pertaining to the cathodic generation of solvated electrons in liquid ammonia and methylamine and also in hexamethylphosphotriamide is given in Table 7. 

In some cases the generation of solvated electrons proceeds under diffusion control. A study of the cathodic process under these conditions yielded information on equilibrium standard potentials of the electron electrode (Sect. 5), and for methylamine — on the competition of electron generation and alkali metal deposition processes. Also, information has been obtained on the stoichiometry of the associates formed by electrons and on the tendency of various systems to association. 

Table 7. Kinetics of cathodic generation of solvated electrons 00... 

Beyond the dependence on solution composition and electrode material the cathodic curve is located in the generation potential range of solvated electrons. It consists of two Tafel portions having different slopes 120-140 mV (lower) and 60 mV (upper)... 

Cathodic generation of solvated 1976 electrons according to two parallel 1978 processes, i.e., electrochemical... 

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