Electrochemistry Volume 7

A Review of Recent Literature

By H. R. Thirsk

The Royal Society of Chemistry

Copyright © 1980 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-870-7

Contents

Chapter 1 Organic Electrochemistry – Synthetic Aspects By J. Grimshaw, 1,
Chapter 2 Membrane Phenomena By N. Lakshminarayanaiah, 40,
Chapter 3 The Application of A.C. Impedance Methods to Solid Electrolytes By W. I. Archer and R. D. Armstrong, 157,
Chapter 4 The Electrical Double Layer By S. K. Rangarajan, 203,
Author Index, 257,

CHAPTER 1

Organic Electrochemistry – Synthetic Aspects

BY J. GRIMSHAW

This Report covers material published during 1975. Papers dealing with physical organic chemistry, such as reaction mechanisms, which have a bearing on electro-chemical synthesis are included. Studies of radical-ions by e.p.r. have been excluded, as have papers on electrochemically initiated polymerization, electro-coating, and related technical fields.

Abbreviations used throughout this chapter are as follows: AN, acetonitrile; DME, 1,2-dimethoxyethane; DMF, dimethylformamide; DMSO, dimethyl sulphoxide; HMPT, hexamethylphosphoric triamide; THF, tetrahydrofuran:

1 General

The coverage of electro-organic synthesis in the Techniques of Chemistry series was completed in 1975. Anodic oxidation was surveyed in another book and chapters on electrochemistry have appeared in textbooks on the chemistry of quinones and of hydrazo-, azo-, and azoxy-groups. Other books have covered electrode kinetics, experimental electrochemistry, and electrochemical data for organic, organometallic, and biochemical substances. Reviews have appeared on general synthetic reactions, the synthesis of cyclic compounds, electroreduction, oxidation, the synthesis and reactions of organometallic compounds, and industrial electrosynthesis, including indirect electrochemical processes and reactor design. The use of ion-exchange membranes in electrochemical cells has been reviewed. Electrochemistry in thin layers of solution is discussed in a critical review and the application of electrochemistry to physical organic problems is discussed. IUPAC have published recommendations for sign conventions and the plotting of electro-chemical data.

A process for the purification of HMPT by fractional freezing, vacuum distillation, and drying over calcium oxide has been described, and the electrochemical properties of several other solvents have been evaluated. Oxydipropionitrile shows a large potential range for reductions at a mercury cathode but no reactions were studied in this solvent, and a possible limitation is that electrogenerated bases will cause elimination to give acrylonitrile. Ethylene carbonate is liquid at 40°C and shows a good range for oxidation and reduction: nitromethane and 1,2-dichloroethane are both satisfactory solvents for oxidation processes. Triethyl-n-hexyl-ammonium triethyl-n-hexylboride is a new ambient-temperature molten-salt solvent with a useful working range for reduction, but the solvent readily undergoes oxidation. A mixture of aluminium chloride (2 moles) and ethylpyridinium bromide (1 mole) is molten at ambient temperatures and forms a strong Lewis acid solvent that is useful for oxidation processes. Quinones solubilized in micelles formed in aqueous sodium dodecylsulphate show well-defined diffusion waves on polarography.

Advances in electronic apparatus for electrochemistry have been reviewed and new designs proposed for function generators and integrators. New designs for laboratory electrolysis cells are available. One of these is formed in a rolled sandwich construction of the two electrodes and a separator cloth. The reaction solution is pumped through a tube packed with the sandwich, so that the substrate is in contact with both the cathode and anode. If the latter situation can be tolerated, then this cell design gives high flow rates and current densities. Porous Teflon has been proposed as a diaphragm material.

Tungsten bronzes have been studied as electrode materials for use in both aqueous and aprotic solvents. They have a large reduction and oxidation range. Attempts have been made to improve the qualities of graphite as cathode material by coating it with mercury and by attaching, with chemical bonds, a surface layer of (S)-(-)-phenylalanine methyl ester, bonded through the amine nitrogen. The latter forms a chiral electrode surface which promotes the reduction of ketones to carbinols with partial asymmetric induction. Acetophenone afforded 1-phenyl-ethanol for which αD (c=3, CHCl3) was -7.2°. However, other workers were unable to repeat this claim of asymmetric reduction. The properties of platinized silica particles as a fluidized-bed electrode for the Kolbe reaction have been examined.

Experimental and theoretical studies have been made on the effect of adsorption of neutral molecules on electrochemical reactions. Cryptate complexes of alkali-metal ions are reduced at very negative potentials, but the potassium ion complex of kryptofix-[2,2,2] is strongly adsorbed at a mercury cathode from dilute solutions, which limits the use of this ion in conducting salts.

General studies on the properties of redox reactions have included a study of the reversible oxidation and reduction of four polynuclear hydrocarbons at pressures up to 2000 atm. The change in partial molar volume which accompanies the redox reaction can then be determined, and this gives information on the solvation changes which accompany electron transfer. When redox potential is determined by cyclic voltammetry, it is usually assumed that the ratio of diffusion coefficients for the redox species is sufficiently close to unity that its logarithm can be taken as zero. In a critical study of some aromatic radical-cations, the diffusion coefficient for the parent molecule was always found to be greater than for the cation, but the ratio could be taken as unity with sufficient accuracy. A linear relationship has been shown between values of the electron affinity and the polarographic half-wave potentials for some cyclic anhydrides. Polarography has been used to detect short-lived radicals and radicalcations that are generated by pulse radiolysis from anthracene, naphthalene, benzene, and acetone.

Further papers have appeared on the use of convolution potential sweep voltammetry in the determination of electrochemical reaction mechanisms, including the acetophenone pinacolization, the intramolecular cyclization of 1,3-dibenzoylpropane, and the coupling of 4-methylbenzylidenemalononitrile. The technique can also be used to determine standard electrode potentials where one part of the couple is unstable.

Ultraviolet spectroscopy has been used to study the intermediates in electrochemical reactions, and there is a developing interest in the application of resonance Raman spectroscopy to the detection of intennediates. Thin carbon films deposited on germanium prisms form optically transparent electrodes suitable for i.r. spectroelectrochemistry.

The contrasting colours of radical-ions and their neutral substrates have been made the basis of electrochromic display systems. Electrochemiluminescence continues to be examined.

2 Reduction

General. — Reduction of acetophenone in a chiral solvent, (S)-(+)-Me2NCH2-CH(OMe)CH(OMe)CH2NMe, gives the same ratio of meso to ([+ or -])-pinacols and the same degree of asymmetric induction in the ([+ or -])-pinacol as is obtained by photo-reduction of acetophenone in the same solvent. This strongly suggests that dimerization occurs by the same step in the two reactions; i.e., by combination of two radicals PhCH(OH)CH3. Cobalt(III) trisacetonylacetonate is destroyed on cathodic reduction, and the reaction in the presence of trimethyl-(-)-menthyl-ammonium perchlorate as supporting electrolyte was found to exhibit enantio-selectivity. The magnitude of this enantioselectivity varies systematically with potential and with electrolyte concentration.

A series of papers on mechanistic electrochemistry in liquid ammonia has appeared. Liquid ammonia has a low dielectric constant, very low acidity, and is a suitable medium for reduction. In the absence of added protonating agents, nitrobenzene and nitrosobenzene are reduced by two reversible one-electron steps to the radical-anion and the dianion. In the presence of isopropyl alcohol as a weak acid, the dianion of nitrobenzene adds one proton and rapidly decomposes to nitrosobenzene. The dianion of nitrosobenzene adds one proton to give an anionic species which can be reversibly oxidized to the parent nitrosobenzene. In the presence of strong acids such as ammonium ions, both compounds are reduced to phenylhydroxylamine. Quinoline is reduced in two one-electron steps, and the radical-anion dimerizes to a dianion, which can be re-oxidized to the parent quinoline. Diethyl fumarate, cinnamonitrile, and acrylonitrile show similar electrochemical behaviour in liquid ammonia to that in aprotic solvents. Dimerization occurs by combination of radical-anions, and the rate is increased by the presence of potassium ions.

Hydrocarbons. — A patent has been issued for the reduction of aromatic steroids in a mixture of liquid ammonia and THF at a steel cathode (Scheme 1). Trioxan has been suggested as a very useful solvent for the related reduction of benzene to cyclohexadiene at a mercury cathode. The reduction of naphthalene in AN to 1,4-dihydronaphthalene has been patented. 3-Hydroxyphenalenone (1) behaves in a manner like that of naphthalene on reduction in an aqueous buffer at a mercury cathode (Scheme 2), to give a dihydro-derivative. Related to the reduction of benzenoid compounds is the electrosynthesis of 2,5-dihydrothiophen-2-carboxylic acid by the reduction, over a mercury cathode, of the lithium salt of thiophen-2-carboxylic acid.

A full paper has appeared describing the advantages of drying solvents over alumina actually in the electrolysis vessel, so as to stabilize the dianions from aromatic hydrocarbons. Under these conditions anthracene, benzanthracene, chrysene, coronene, and perylene show reversible behaviour on cyclic voltammetry due to the formation ofradical-anions and dianions. Cyclo-octatetraene shows two reversible one-electron reduction steps under these conditions, and the rate of charge transfer for addition of the first electron depends on the supporting tetra-alkylammonium cation, being 103 times faster for Me4N+ than for Bu4N+. The activation barrier for addition of the first electron, due to a conformational effect, is not as important as previously considered; electrolyte double-layer effects are greater than any conformational effects.

Polarography of 1-phenylhex-1-yne in DMF shows a single four-electron wave during which hexylbenzene is formed. However, under the conditions of preparative electrolysis, the isomerization of acetylene to allene becomes important, and the dominant process is reduction of the allene (Scheme 3).

Activated Olefins. — A number of patents have appeared on the conversion of acrylonitrile into adiponitrile. A mechanistic study of the hydrodimerization of ethyl cinnamate and diethyl fumarate in DMF at room temperature and lower shows that the reaction proceeds in both cases via a radical-anion dimerization step. Alkali-metal ions (Li+, Na+, K+) greatly increase the rate of dimerization of dialkyl fumarates, ethyl cinnamate, and cinnamonitrile in DMF due to ion pairing with the radical-anions and then rapid dimerization of the ion pairs. The unsaturated nitrites (2; R = H) and (2; R = Me) undergo irreversible one-electron reduction, with dimerization and then cyclization, in DMF, with or without an added proton source (Scheme 4); reaction between two radical-anions is proposed as the dimerization step. The related nitrites (2; R = But) and (2; R =Ph) show reversible radical-anion formation in DMF and further reduction to the dianion (Scheme 5). On addition of a proton donor, two-electron reduction to the dihydro-compound occurs at the potential of the first wave, and an ECE mechanism has been proposed. Electroreduction of αβ-unsaturated nitriles in acidic aqueous solution leads to the production of amines.

Co-electrodimerization of carbonyl compounds with acrylonitrile in aqueous buffers leads to γ-hydroxy-nitriles, while a similar reaction with acrylic acid leads to γ-lactones (Scheme 6).

Carbonyl Compounds. — The mixed electrolytic reduction of 1,4-dimethylpyridinium methylsulphate and acetone leads to mixed coupling products (Scheme 7) along with products from reduction of the pyridine compound. Patents have been issued for the electrolytic preparation of pinacols from simple aliphatic ketones, and in one process the corresponding secondary alcohol is used as the solvent.

Two strikingly similar stereo- and enantio-selective hydrodimerization reactions have been described. Reduction of benzoin gives the racemic pinacol formed by threo-coupling between two molecules of the same enantiomeric configuration (Scheme 8). Hydrodimerization of the racemic tricyclic enone (3) also gives the pinacol by threo-coupling, between two molecules of identical enantiomeric configuration (Scheme 9). No other stereoisomers of the pinacol are formed in each case, although the enone also gives a mixture of ketols. Pinacol formation has been recorded during the reduction of thiophen-2,5-dicarboxaldehyde, 2-benzoyl-thiophen, 2-formylselenophen, 2-acetylselenophen, and acetylferrocene.

Studies on the rate of the hydrodimerization of benzaldehyde in sulpholan, using a rotating ring-disc electrode, have been interpreted as showing that there is dimerization of the radical-anion. The radical-anion of 4-nitrobenzaldehyde reacts too slowly for a rate constant to be determined using this technique. 4-Cyanobenzaldehyde undergoes dimerization by the same mechanism at high current densities but by an ECE mechanism at low current densities, where the chemical step is reaction between the radical-anion and a neutral molecule.

Reduction of amino-desoxybenzoins is dependent on the pH of the solution. If the pH is sufficiently acid that the amino-function is protonated, then cleavage of the carbon-nitrogen bond occurs, as shown in Scheme 10. In more alkaline solutions this reaction is suppressed, and reduction of the carbonyl group to secondary alcohol occurs, giving a mixture of stereoisomers. Griseofulvin is reduced to dihydrogriseofulvin in aqueous buffer solutions.

Examples have been given of the reduction of carboxylic acid to primary alcohol in acidic aqueous buffers, reduction of pyridine-2-carboxylic acid and -2,6-dicarboxylic acid, and the reduction of an amide function (see Scheme 11). The reduction of oxalic acid in aqueous solution to glyoxalic acid is the subject of a patent.

Nitro- and Nitroso-compounds. — Reduction of the two nitro-groups in 2,4-dinitro-phenol and 2,4-dinitrotoluene to amine has been examined. 3-Nitro-4-hydroxy-coumarin is also smoothly reduced to the corresponding amino-compound. Reduction of α-nitrocinnamic acid methyl ester in acid solution gives (+ or -])-phenyl-alanine. Reduction of 2-nitro-2 -isothiocyanatobiphenyl causes electrochemically initiated intramolecular cyclization (Scheme 12), and the product depends on the pH of the solution. In acidic solution, condensation between the generated hydroxylamino-function and the isothiocyanato-group to give (4) is rapid, but in neutral or alkaline solution this condensation is suppressed, and the isothiocyanato-group undergoes reduction. Further reactions then lead to the dihydrobenzo-[c]cinnoline (5).

A detailed mechanistic study of the reduction of nitrosobenzene in DMF is available. The anion from 1,1-dinitroethane in aqueous alkaline solution undergoes reversible one-electron reduction. At more negative potentials an irreversible reduction process occurs.

Other Nitrogen-containing Compounds. — X-Ray crystallography has been used to define the structure of the dihydroquinaldine dimer (6) obtained from cathodic reduction of quinaldine (Scheme 13). The old process for reduction of indoles to their dihydro-derivatives at a lead cathode in 20% sulphuric acid has been revived in a recent patent.

Reduction of the C=N function in aqueous medium to its dihydro-derivative has been observed for a number of heterocyclic systems. Thus reduction of 7-methyl-guanosine (7) in acid medium leads to reduction of the imidazole ring (Scheme 14). Reduction of 1,4-benzodiazepines leads to their dihydro-derivatives. The cyclopropyl ring in prazepam (8) remains intact during this process, as shown in Scheme 15. Reduction of lorazepam (9) in a buffer of pH 10.4 can be terminated at the dihydro-stage, but the initial product loses water in a slow step (see Scheme 16), so that the product that is isolated corresponds to apparently simple replacement of a hydroxy-group by hydrogen. This product will undergo further reduction of the C=N function.

Reduction of the C=N function is the electrochemical step in a potentially useful preparation of amines from amides. The amide is first converted into its O-methyl ether (10), which is reduced in AN at a mercury cathode, an amine being the final product (Scheme 17).

In contrast with the work just described, an extensive study of the reduction of N-benzylidene-4-toluidinein a solvent mixture of MeOH-MeOAc-H2O indicates that the products are the stereoisomeric hydro-dimers as well as the dihydro-compound. Those benzodiazepines which give only the dihydro-compound can be regarded as Schiff’s bases from benzophenone and an alkylamine; it would be useful to extend the study of Schiffs bases to detect any systematic variation of hydrodimer, dihydro-product yields. The yield of hydro-dimer from N-benzylidene-4-toluidine depends upon the availability of protons in the double layer and is increased by using a hydrophobic supporting electrolyte. Typical results are 44% hydro-dimer using KOAc and 58% hydro-dimer using BU4PBr as electrolyte and a mercury cathode, the yield decreasing at copper, lead, or glassy carbon. The ratio of ([+ or -])-: meso-hydro-dimer is in the range 0.9-1.1:1.

Benzyltrimethylammonium salts give bibenzyl as the principle product from reduction in HMPT at an aluminium cathode. The yield of bibenzyl falls when the water content of the solvent rises above 0.25 mol l-1, and toluene becomes the principal product. Use of a platinum cathode in HMPT leads to mixtures of bibenzyl and toluene; toluene is the only product from a platinum cathode in DMSO, DMF, or DME. Toluene is formed by reduction of the intermediate benzyl radical to the carbanion and then proton abstraction. Under these conditions the solution becomes sufficiently basic to promote the Sommelet–Hauser rearrangement of the substrate to form NN‘-dimethyl-o-toluidine.

(Continues…)Excerpted from Electrochemistry Volume 7 by H. R. Thirsk. Copyright © 1980 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Electrochemistry Volume 9

A Review of Recent Literature

By D. Pletcher

The Royal Society of Chemistry

Copyright © 1984 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-077-0

Contents

Chapter 1 The Electrochemistry of Porous Electrodes: Flow-through and Three-phase Electrodes By N. A. Hampson and A. J. S. McNeil, 1,
Chapter 2 Semiconductor Electrochemistry By L. M. Peter, 66,
Chapter 3 Spectroelectrochemistry By J. Robinson, 101,
Chapter 4 The Electrochemistry of Transition-metal Complexes By C. J. Pickett, 162,
Chapter 5 Organic Electrochemistry – Synthetic Aspects By J. Grimshaw, 222,
Chapter 6 State Gas Sensors and Monitors By D. E. Williams and P. McGeehin, 246,

CHAPTER 1

The Electrochemistry of Porous Electrodes: Flow-through and Three-phase Electrodes

BY N. A. HAMPSON AND A. J. S. McNEIL

1 Introduction

In the first of these reviews, we dealt with the recent literature of the flooded porous electrode and in doing this we specifically excluded flow-through electrodes and electrodes with a three-phase interface. We now consider these two important categories of porous electrodes.

Electrochemical technology is mainly concerned with arranging for electrochemical reactions to be carried out economically. This is frequently a very major problem, for although the electrochemical approach to a reaction generally offers a clean and effective method, often there is a limitation due to the heterogeneous electrode reaction. This limits the effective reaction rate. Even if the reaction is driven by the application of a considerable overpotential the rate of the desired reaction may be forced past the mass-transport-limited plateau so that the current goes into a side reaction and the process becomes both inefficient and non-selective. Flooded porous electrodes have limited use in electrochemical reactions where electrolyte flow is a prerequisite for successful processing and continuous operation. The development of reactors to fulfil these requirements has occurred during the last two decades. Fixed-bed reactors have been developed from the principles derived for catalytic reactors in which the catalyst is supported on a porous substrate upon which the reaction occurs. Chemical engineers have been concerned with the flow of fluids through such reactors and the hydrodynamics are well established. Indeed, the fluid-flow-through porous bed is not unnaturally the priority of the F electrochemical engineer who is concerned mainly with getting the maximum output from the reactor. The reactor is therefore arranged so that the mass-transport-limited plateau spreads across the whole length of the electrode bed. Other porous electrode arrangements are attempts to simplify or improve this basic flow-through principle, especially with provision for gas interaction to form the three-phase interface porous electrode. This latter arrangement is crucial for the operation of fuel cells and metal–air batteries. With changes in external (economic) conditions, these developments have slowed down after a burst of activity in the late 1970s. However, with the ultimate specificity offered by the electrochemical technique it is expected that an active interest in this area will be re-established. It is hoped that this review might mark the beginning of a new interest in these systems.

The layout of the review was suggested by the obvious connection of the three-dimensional electrodes with the porous flooded format. Flow-through porous systems are clearly linked as are trickle beds, fluidized beds, and three-phase interfacia1 systems. These latter have not been reviewed in any depth before and consequently a large amount of published work, especially from the Eastern Bloc countries, is included.

2 Flow-through Electrodes

Introduction. — While there is considerable literature concerning flow-through electrodes, there are relatively few reviews which guide the reader. A relatively recent review by Newman and Tiedemann referred to much of the literature up to 1977. Earlier reviews. dealt with the general extension of what might be termed conventional electrochemistry by its combination with chemical-engineering principles to produce new electrochemical technologies and to provide further insight into the ‘classical’ electrochemical industries, such as electrochemical energy conversion. Later reviews have provided evidence for the growth of a thriving electrochemical-engineering discipline, given additional impetus by industry’s need to deal with low reactant concentrations at ever-increasing rates. This growth has stimulated further systematization in associated areas.

In an early review paper, Kalnoki and Brodal showed that flow-through porous electrodes engender improved reaction rates by comparison with either solid electrodes over which electrolyte flowed, or flooded porous electrodes in static electrolytes. Clearly the flowing solution enhances mass transport at the high-specific-area electrode. Conversely a dilute solution can be handled effectively by using low flow rates and extended porous electrodes. The fluidized-bed electrode developed by Fleischmann and his group is a close relative of the flow-through porous electrode.

Cell Dispositions. — There are clearly three possible dispositions of electrodes (see Figure 1): with the counter electrode beside the working electrode the solution may be flowing normally towards (a), away from (b) or parallel to (c), the counter electrode. If both anode and cathode are porous then it might be necessary to keep the anolyte and catholyte separate from a common feed. A downstream counter electrode tends to distribute the reaction in the porous matrix since the front of the electrode reacts under the least favourable mass-transport conditions; an upstream counter electrode produces a maximum reaction rate at the front of the electrode. The parallel flow format may have advantages for slow electrochemical reactions but it is said to be the most difficult to analyse.

Factors Affecting Reaction Rates. — Hydrodynamics. The processes which affect the reaction rate are the transport of mass to the electrode surface and the transport of charge across the interphase. The transport of mass to the surfaces of a porous matrix (packed bed) is determined by the solution velocity via the appropriate mass-transfer coefficient. The mass-transfer coefficient furthermore depends upon the other solution characteristics, most conveniently expressed using the dimensionless hydrodynamic constants. Examples of these relationships for characteristic solution ranges are available.

A further mass-transport factor arises from the transfer of material in the solution between different parts of the electrode. A concentration difference gives rise to simple diffusion. In addition, the non-uniform velocity in the matrix pores results in mixing of the fluid in the flow direction, which results in a dispersion of the concentration profiles (the axial dispersion). Dispersion coefficient relationships have been given for some simple cases by Sherwood et al. The axial dispersion must be taken into account in considering the mass-transfer coefficient and Newman and Tiedemann give an example of the effect. The mass-transfer coefficient is crucial to the behaviour of porous electrodes. Conventionally, the target is the correlation of the Nusselt number with the Péclet number, the Schmidt number, the porosity and the electrode depth. There has been some discussion of the definition of the mass-transfer coefficient; however, one can only measure [bar.k]m, the average value at limiting current conditions, where [bar.k]m = v/aL ln (C0/CL), where v is the superficial fluid velocity, a the specific interfacial area, L the electrode thickness, and C0 and CL the reactant concentrations at the entry and exit positions, respectively. It is thus more sensible to build up the theory from this operational standpoint. The Nusselt number ([bar.N]u = ε[bar.k]m/aD0, where ε is the porosity volume fraction and D0 the diffusion coefficient of reactant in the feed solution), has been correlated with the Péclet number (Pe) to give the well known curve due to Newman for a fixed Schmidt number, characteristic of a deep bed of spheres on a simple cubic lattice. The various theoretical implications of this model are related to the established mass-transport theory and experimental flow-through porous beds operating at the limiting current. The correspondence between theory and experiment was significant and this was recognised by Newman and Tiedemann. The magnitudes of the experimental unknowns, and indeed how the simple spherical model should be modified for any particular bed geometry, was not really clear. Transient methods rather than steady-state methods were not found to improve the situation. In fact, transient methods clearly involve difficulties with double-layer charging and the fact that it is only just possible under steady-state conditions to reduce the ohmic potential drop within the electrode to a sufficiently low value so that the concentration of the electroactive reactant at the wall is zero as required by the establishment of the limiting condition.

The above discussion of mass transport has been in terms of the solution velocity. Frequently the pressure across the porous electrode is a convenient measure of the solution dynamics. A relationship for fully developed flow in which the pressure drop ΔP (proportional to L, the bed length) taken into the dimensionless group ΔP/aLρv2 (where ρ is the fluid density) is shown as a function of the modified Reynolds number Re’ = dvv/(1 – ε), where dP is the equivalent sphere diameter and v is the kinematic viscosity, has been given by Bird et al. This relationship is rather simplistic as it does not take into account internal structures of equivalent spheres, which in itself is an abstraction.

Electrochemistry. The advantage of the porous electrode lies in the high rates of reaction, and consequently reaction is usually arranged to be under limiting conditions. The mass-transport-limited current plateau is bounded at the low-overpotential side by the charge-transfer-controlled region, and on the high-overpotential side by the transition to a new reaction, usually the hydrogen-evolution reaction (HER) or the oxygen-evolution reaction (OER). The most satisfactory method of arranging the potential is to design the process so that one end of the porous electrode is at the low-overpotential side of the limiting-current plateau and the other end coincides with the other limit of the plateau (so that the current going into intruding side reactions is limited). This condition imposes an ohmic limitation on the reactor and for this situation Newman and Tiedemann have generated equations which enable reactor characteristics to be estimated in an approximate sense, and applied these to the cases of copper recovery, lead removal from acid solutions of PbSO4, and desalination.

Theory of Porous Flow Electrodes. — The literature as far as 1976 has been well reviewed. Since 1976 no in-depth review on porous flow-through electrodes has been written, although there seems to be a tendency to take a more general approach and treat the electrode/electrolyte system as an electrochemical ‘black box’. 1977 marked a peak in interest in porous flow-through electrodes; since then, however, the annual number of published papers has decreased.

Flow-by Porous Electrodes. — Alkire and Ng have carried out an engineering analysis of a packed-bed electrochemical cell, the electrolyte flowing in the axial direction and the current in the radial direction (the packed-bed electrode confined within a thin cylindrical porous separator surrounded by a concentric counter electrode). Experiments were made using a sectioned porous electrode (packed copper spheres) in order to measure the axial-current distribution at and below the limiting current. Copper sulphate (acid) was used as the electrolyte. Difficulties arose because the presence of flow channelling caused the mass-transfer coefficient to differ from its reported value. Empirical calculations were established in order to estimate the mass-transfer coefficient. The exchange current density was estimated to be ca. 0.07 mA cm-2 in 1mmol dm-3 CuSO4. Comparisons of theory and experiment have been made in the cases of collection efficiency, axial-current distribution, electrode polarization, and reactor current. Agreement was obtained for a range of situations of various geometrical dimensions, porosities, flow rates and reactant concentrations. Various criteria were generated for the volumetric reaction rate. The approximate method of solution of the simplified model provided an accurate representation of the reactor assuming that axial dispersion and channelling effects were absent. When the latter were present, modification of the mass-transfer coefficient was found to give an accurate procedure. However, this treatment by Alkire and Ng of the two-dimensional potential and current distributions is clearly limited in generality. The same is true of the Tentorio and Ginelli investigation which considered the mass-transfer and current relationships at reticulate three-dimensional copper electrodes (metallization of polyurethane foams forms the cathode in a filter press cell) at which copper electrodeposition is occurring. They assumed that the only current component is in the direction normal to the electrolyte flow. Storck et al. have developed a mathematical model to describe the behaviour of three-dimensional electrodes operating under limiting-current conditions. An analytical solution for the three-dimensional potential distribution is based on several assumptions. A number of these are conventionally made (highly conductive metal phase, supporting electrolyte, and uniform porosity), although it is also assumed that axial dispersion is negligible and that the limiting current is given by ZF[bar.k]C, where C is the local concentration of electroactive species and [bar.k the mass-transfer coefficient. Integration of the resulting Poisson equation is done by Fourier-transform methods. Potential and overpotential variations are obtained and these are applied to the design of a three dimensional structure. In a further paper Enriquez-Granados et al. describe an experimental study of the efficiency of three-dimensional electrodes operating at limiting current conditions using the dispositional characteristics of the theoretical study. The packed bed was of spherical nickel particles with the ferri-/ferro-cyanide reduction as the single-electron process. The experimental potential distributions measured by means of a moving probe were found to be in excellent agreement with the theoretical predictions. This investigation rests on the assumptions of the theoretical part and in this respect most of the important reaction parameters are satisfactorily dealt with.

Flow-through Porous Electrodes. — Newman has summarized the more promising arrangements for flow-through electrodes in terms of their applications, with special reference to simultaneous reactions. The paper is based on another in which a one-dimensional model operates below and above the limiting current of a cathodic metal deposition. The calculations assume only one reactant species and that a simultaneous side reaction might occur; the model includes the effects of axial dispersion and diffusion. Ohmic, mass-transport, and kinetic limitations are shown to predict non-uniform reaction rates. Results are compared with the experiments of others for copper deposition from sulphate solutions with an intruding HER. Very satisfactory agreement between model predictions and experimental data has been obtained for overall reactor performance and deposit distributions. The calculations include a new set of dimensionless parameters.

Trainham and Newman have also used thermodynamics in order to estimate the minimum concentration attainable in a flow-through porous-electrode reactor. The physical technique employed treated the reactor as an electrochemical cell at equilibrium and hence found the minimum exit concentration for the ion in question. The calculated concentration reductions obtained by the flow of the solution through the reactor are compared with experimentally attained reductions in the cases of copper, silver, lead and mercury and for the oxidation of ferrous ions. The calculations are recognised as only leading to a lower limit, for it is obvious that kinetics and mass transport will reduce the bed efficiency.

A further paper by Trainham and Newman considers the effect of electrode placement and finite matrix conductivity using a one-dimensional model for a flow-through porous electrode. The effluent concentration is predicted as a function of matrix conductivity and electrode length for upstream and downstream placement of the counter electrode and current collector relative to the fluid inlet of the working electrode. The dimensionless numbers developed in the previous paper were used to compare the performance as a function of counter-electrode placement. It is emphasized repeatedly that for the most efficient (low exit concentration) operation the porous-electrode potential range must straddle the limiting-current plateau. However, the intrusion of side reactions causes the simple picture to be inadequate and the distributions of current and potential in the presence of the side reactions are all required if the particular system is to be optimised, for example, for metal-ion removal. The paper includes boundary conditions for the four regimes treated and the appropriate parameters for the electrodeposition of Cu and Ag, from which the dependence of the effluent concentration on the parameters of interest was calculated.

(Continues…)Excerpted from Electrochemistry Volume 9 by D. Pletcher. Copyright © 1984 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Electrochemistry Volume 2

A Review of the Literature Published during 1970

By G. J. Hills

The Royal Society of Chemistry

Copyright © 1972 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-017-6

Contents

Chapter 1 Organic Electrochemistry — Synthetic Aspects,
Chapter 2 Electrochemistry of Molten Salts,
Chapter 3 Solid Metal Electrode Reactions,
Chapter 4 Ionic Double Layers and Adsorption,
Chapter 5 Membrane Phenomena,
Author Index, 287,

CHAPTER 1

Organic Electrochemistry — Synthetic Aspects

BY A. BEWICK AND D. PLETCHER

1 Introduction

Judged solely by the number of papers reviewed this year, it would seem that there remains a very high level of interest and activity in the area of organic electrosynthesis. Other criteria would, however, suggest that organic electro-chemists are beginning to turn to other fields, particularly biochemistry. There is certainly evidence that in industry the current economic depression has served to sharpen the criticisms that exciting, specific electrosynthetic reactions have failed to materialize. This disenchantment must, however, be rather premature since much of the published work is still being carried out by workers who are ill qualified in at least one of the essential areas, organic chemistry or electrochemistry. As a result, there are still many examples of electrolyses in which the electrochemical conditions have been completely uncontrolled or in which product analysis has not been made. There has been a definite trend towards better balanced and higher quality work as the interaction between organic chemists and electrochemists has increased, but the number of truly ‘bilingual’ workers is still very low.

It is also apparent that there is still no coherent understanding of the detailed mechanisms by which the various electrolysis conditions determine the efficiency and specificity of electrosynthetic reactions. Some of the changes produced by variation of solvent, electrolyte, and electrode material are not quantitatively, or in some cases qualitatively, understood and further systematic studies are required. On the other hand, it is now well established that electrochemistry is an excellent technique for the generation of the reactive intermediates of organic chemistry and their participation in electrode processes is well characterized.

It is a necessary preliminary to the development of new industrial scale electrosynthetic processes that electrochemical techniques should become established as routine tools among synthetic chemists at the laboratory level. Already during the past ten years a sufficient number of laboratory syntheses in which electrochemical methods offer distinct advantages have been developed. The missing factor which would lead to more widespread acceptance of these has been the lack of a textbook covering electrochemical techniques from the viewpoint of the organic electrochemist. Although a number of books are now appearing, these are all heavily slanted towards particular aspects. The most recent of these, by Mann and Barnes, gives an adequate summary of the reactions of organic compounds in non-aqueous solvents but the treatment tends to be rather non-critical and non-stimulating, and there is little guidance on the applications of electrochemical techniques. Another new review, which has appeared in a context where it could make considerable impact on the synthetic chemist, was restricted in content and failed to demonstrate the range of techniques of modern organic electrochemistry. A number of other reviews have also appeared.

2 Oxidations

A. Aromatic Hydrocarbons. — Dietz and Larcombe have reported the use of cyclic voltammetry to identify some new carbonium ion intermediates in the anodic oxidation of certain aromatic hydrocarbons. For a number of years it has been known that the dicarbanion formed by the reduction of aromatic hydrocarbons in aprotic media will abstract a proton from the environment to form a carbanion and this intermediate may, at more anodic potentials, be reoxidized to a radical, as outlined in Scheme 1. Dietz and Larcombe have shown that there is a parallel process in the oxidation of those aromatic hydrocarbons which form stable cation-radicals. When studying the cyclic voltammetry of 9,10-diphenylanthracene (DPA), they observed that if the potential sweep included the second irreversible oxidation process, a new reduction peak was observed at more negative potentials. They attributed this peak to the reduction of the cation (1) formed in the sequence shown in Scheme 2, where Nu is an unidentified nucleophile, perhaps the solvent, traces of water, or the anion of the inert electrolyte. This behaviour was observed in several solvent-lectrolyte systems (not acetonitrile) and in all cases the new peak occurred at a potential 82 [+ or -] 6 mV more negative than the DPA-DPA·+ couple; the similarity in reduction potential is expected since the lowest unfilled orbital of the cation (1) is non-bonding. A reduction peak attributable to the reduction of a nucleophiledication adduct is also observed in the cyclic voltammetry of 1,3,6,8-tetraphenylpyrene and perylene. In every case this peak is irreversible, probably since the equilibrium (Scheme 3) lies well to the right, as it must do if the cation-radical is stable.

In the case of perylene, a further reversible couple is observed slightly negative to the potential of the hydrocarbon-cation-radical couple. The authors propose that this couple is due to system (2)/(3) in Scheme 4.

Jeftic and Adams have elucidated the overall reaction mechanism for the oxidation of benzo[a]pyrene at a Pt electrode in a series of aprotic solvents. In a detailed and careful study using cyclic voltammetry, a rotating disc electrode, coulometric and spectroscopic techniques, and product isolation, the authors consider the initial electron-transfer step, the intermediates in the reaction and their relative stability, and the final products. As may be seen from the Schemes 5 and 6, the major products are the dimer, and derived polymers, and a mixture of benzopyrene quinones formed by a complex series of electron transfers and hydrolyses by trace quantities of water present in the solvents.

The oxidation of 6-acetoxybenzo[a]pyrene is also discussed, the quinones being the only products; since this reaction does not proceed via a benzopyrene cation-radical, no dimerization or polymerization is observed.

A long series of papers on the oxidation of anthracenes has been produced by Parker. His main techniques are cyclic voltammetry, coulometry, and product identification and his two continuous themes are that (a) electrochemical substitution reactions generally take place by an e.c.e. mechanism and (b) many added reagents have the dual and competing roles of base and nucleophile to reactive intermediates produced at the electrode.

The oxidation of anthracene was studied in acetonitrile containing water, alcohols, acetic acid and acetate i0n. The products from the oxidation in the presence of water were shown to depend on the concentration of water in the system; at low water concentrations the product isolated was anthraquinone and as the water concentration was increased bianthrone became the major product. Finally, at very high water contents, a trimer compound is also produced, in which a 9,10-dihydroanthracene moiety separates two anthronyl units. Coulometry and cyclic voltammetry are used to confirm that the n-value for the oxidation actually decreases as the water content of the solvent is stepped up, and it is suggested that two factors may be important in explaining this surprising effect. They are (a) the relative importance of the water as a nucleophile and as a base may change with its concentration and (b) changes in the adsorption isotherms for anthracene and reaction intermediates may occur as the solvent becomes less anhydrous.

The product from the oxidation of anthracene in acetonitrile containing ethanol or acetic acid is bianthrone. In some earlier short communicaiions* it had been assumed that the bianthrone was formed by reaction between an intermediate and trace water in the system, although the n-value for the reaction and the actual mechanism had been disputed. Parker, however, points out that this mechanism is unlikely since it is difficult to believe that trace water could compete with a nucleophile as strong as an alcohol when the alcohol is present in a large excess and, anyway, the work in the presence of low concentrations of water had shown anthraquinone and not bianthrone to be the major product. Instead, he suggests that the mechanism for the production of bianthrone is that shown in Scheme 7, and supports this view by showing that, in acetonitrile containing ethanol or acetic acid, the 9-substituted anthracenes undergo a quantitative one-electron oxidation to bianthrone. The oxidation of anthracene in acetonitrilealcohol is reported to be an excellent preparative method for bianthrone.

The oxidation of 9,10-dihalogenoanthracenes’ and 9-phenylanthracene in acetonitrile has also been investigated. These species give, as the primary electrode intermediates, cation-radicals with sufficient stability to allow more detailed information to be obtained about the subsequent chemical reactions and the mechanism of substitution reactions. For example, the e.c.e. nature of the oxidation of 9,10-dihalogenoanthracenes may be definitely proved by a rotating disc experiment. A preparative scale electrolysis on the dihalogeno-anthracenes forms the bis-halohydrins (Scheme 8) which decompose to anthraquinone during isolation. The mechanism for the formation of anthra-quinone may be modified by the addition of a halogen acceptor such as cyclohexene. In the absence of an added nucleophile, the cation-radical formed by the oxidation of 9-phenylanthracene simply dimerizes to yield the 10,10′ dimer, while in the presence of ethanol or acetic acid the electrolysis product is the 9-substituted-9-phenyl-10-anthrone formed by the reactions set out in Scheme 9.

In a paper with Eberson, Parker has extended the work, previously reported as short communications, on the steric factors which control whether, during the oxidation of aromatic hydrocarbons and the debromination of 9,10-dibromoanthracene, certain heterocyclic compounds act as nucleophiles or bases. It is shown that cyclic voltammetry may be used to distinguish the two reactions.

Oxidation of anthracenes in acetonitrile containing acetate ion, or methanol containing methoxide ion, has been shown to produce the 9,lO-disubstituted-9,10-dihydroanthracene and Parker has reported a study of the stereochemistry of these reactions.’ He found that in all cases the trans isomer was favoured in the anode reaction, and the ratio of trans to cis varied between three and infinity for different anthracenes; conversely the chemical oxidation of the anthracenes, by lead tetra-acetate, lead to a 50 : 50 mixture. Although in the case of the acetate ion the stereochemical preference could be explained by formation of a cyclic acetoxonium ion, this is not so in the case of methoxylation and it would appear that the electrode must affect the stereochemistry of the products.

A number of papers have considered the anodic oxidation of alkyl-substi-tuted aromatic hydrocarbons. In the non-nucleophilic medium methylene chloride-tetrafluoroborate, the products of controlled potential oxidation of durene, mesitylene, and p-xylenehave been studied. The major product from durene is the diphenylmethane (4); it is isolable in organic yields as high as 85 % and it is thought to arise by electrophilic attack of the trimethyl-benzyl cation on a further durene molecule. The major product from the oxidation of mesitylene is the biphenyl (9, i.e. coupling between nuclei has occurred. Some trimeric nuclear coupled product and polymeric material is also formed. The author suggests that this coupling of nuclei arises from the one-electron oxidation of the substrate (Scheme lo), while the side-chain-nucleus coupling observed for durene occurs via an initial two-electron oxidation of the substrate as shown above. However, the marked difference between the products from mesitylene and durene is surprising and further work on these systems is clearly warranted. p-Xylene oxidation yields mainly polymer but some side-chain-nucleus coupled product is also produced.

Two papers have reported studies of the oxidation of alkyl-substituted benzenes in acetic acid and both conclude that when the hydrocarbon is not discharged at a less positive potential than the anion of the inert electrolyte, the products are best explained by two co-existing mechanisms involving oxidation of the anion and of the hydrocarbon. The first paper reports the oxidation of toluene in acetic acid containing acetate, nitrate, or tolylsate ions while the second paper considers the oxidation of mesitylene in the presence of nitrate ion. In both papers the conclusions are reached mainly from a careful analysis of the products, although the interpretation of the results in the former paper is complicated by the use of constant current electrolyses. The evidence for the discharge of the hydrocarbon is the isolation of some nuclear-substituted acetates which occur in fairly high yields at low conversions. However, the major products are formed by substitution in the side-chain and the isolation of bibenzyls is evidence for benzyl radicals as intermediates. The presence of toluene in the nitrateacetic acid system does not change the current-potential curve of the inert electrolytesolvent system which is strong evidence for discharge of the nitrate ion. Thus the dual mechanism shown in Scheme 11 is postulated. Nuclear substitution is favoured by the presence of free acetate ion when an acetate assisted concerted mechanism has been proposed. It is also pointed out that the situation is different when the hydrocarbon is oxidized at potentials well below that of the anion; in these cases nuclear substitution is favoured.

The oxidation of an aqueous emulsion of cumene at a platinum electrode has been shown to lead to fracture of a carbon-carbon bond and high yields of benzaldehyde (up to 80%) with some acetophenone. The optimum conditions were with sodium hydroxide as the base electrolyte and a current density of 2-4 A dm-2.

While carbonium ion rearrangements are well known in anodic oxidations, cation-radical rearrangements are less common. Miller and Mayeda believe that they have observed a sigmatropic rearrangement of a cation-radical during the oxidation of 1,1,3-triphenylindene at a platinum electrode in a non-nucleophilic solvent, liquid sulphur dioxide, which contained methanol as a trapping agent (see Scheme 12). In methanol as solvent, the phenyl shift does not occur during the reaction, indicating that the solvolysis is too rapid for the rearrangement to take place.

The oxidation of a substituted bibenzyl to a phenanthrene has been reported. This reaction was shown, by means of cyclic voltammetry, to proceed via initial formation of a dihydrophenanthrene (Scheme 13). Cyclic voltammetry on the dihydrophenanthrene showed that it forms a relatively stable cation-radical. Since this species is not oxidized to the dication until more positive potentials, it would seem that the phenanthrene must arise by an e.c.e. mechanism, i.e. the initial electron transfer is followed by loss of a proton to form a radical which can then be oxidized further. This reaction may be taken to be evidence that benzyl cations are generally formed via a radical intermediate.

Shono and Matsumura have reported that the anodic oxidation of arylcyclopropanes in methanol leads to opening of the cyclopropane ring and the formation of ethers. The products are more consistent with initial electron transfer from the aromatic ring rather than the cyclopropane system and this mechanism is supported by the slopes of E1/2 – σ* plots.

The oxidation of ethylenes with various degrees of phenyl substitution has been discussed by Eberson and Parker. They carried out cyclic voltammetry in acetonitrile and preparative electrolyses in an acetonitrileacetic acid mixture and they conclude that the basic mechanism and the products are very dependent on the structure of the olefin. The initial electrode process may involve one or two electrons. Bard and Phelps have, however, disputed their claim that tetra-p-anisylethylene undergoes oxidation in a single, reversible, two-electron step; the data are interpreted in terms of two, reversible, one-electron steps separated only by a few mV. Tetrakis-(p-NN-dimethyl-aminophenyl) ethylene is reported to show behaviour which is consistent with a direct two-electron transfer.

Miller et al. have extended the work, previously reported in a preliminary communication, on the electrophilic substitution of aromatic hydrocarbons by the anodically generated iodine cation; they have used a dual approach. In the first, they oxidized mixtures of iodine and the aromatic hydrocarbon at the potential required for oxidation of the iodine. This led to a mono-iodinated product, but the yield was not high due to the further oxidation of the product and to acetamidation of the aromatic compounds (Scheme 14). In the second approach they added the hydrocarbon after the complete anodic oxidation of the iodine and the yields of mono-iodinated products were then very high. They believe that the electrophilic species is the N-iodonitrilium salt

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and this view was supported by the isolation of the corresponding amide after addition of water.

B. Aliphatic Hydrocarbons. — The anodic oxidation of cyclohexene-chloride ion mixtures in acetonitrile has been reported. At low potentials, where the chloride ion but not the cyclohexene is oxidized, the major product isolated is formed via chlorine evolution and reaction between the chlorine and the cyclohexene (see Scheme 15). Although this is the only product which could be obtained by chemical oxidation of the mixtures, it was shown that at potentials where the cyclohexene is discharged and when suitable concentrations of the reactants were used, the reaction

[FORMULA NOT REPRODUCIBLE IN ASCII]

could be carried out to give reasonable current yields of 3-chlorocyclohexene; this product is further strong evidence for the carbonium ion as an intermediate in the oxidation of hydrocarbons in aprotic media. The success of this allylic substitution process demonstrates an important feature of electrode reactions: the heterogeneous nature of the process and the consequent need for diffusion of species to the electrode surface introduces the possibility of controlling reactions by controlling the flux of each reactant to the electrode surface. The use of this principle may make possible a range of syntheses which cannot be carried out in homogeneous solution.

(Continues…)Excerpted from Electrochemistry Volume 2 by G. J. Hills. Copyright © 1972 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Electrochemistry Volume 1

A Review of the Literature Published during 1968 and 1969

By G. J. Hills

The Royal Society of Chemistry

Copyright © 1970 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-007-7

Contents

Chapter 1 Electrolyte Solutions,
Chapter 2 Reversible Electrode Systems and Related Topics,
Chapter 3 The Conductance of Electrolyte Solutions,
Chapter 4 Organic Electrochemistry — Synthetic Aspects,
Chapter 5 Electrochemistry of Molten Salts,
Chapter 6 Double Layers,
Author Index 265,

CHAPTER 1

Electrolyte Solutions

BY A. K. COVINGTON AND T. H. LILLEY

PART I: Thermodynamic Properties of Electrolyte

Solutions 1 Introduction

In the period under review water has remained the most studied solvent system, although more thermodynamic studies are being made using the newer protic and aprotic solvents. A major preoccupation has been with ion-solvent interactions and particularly with ‘solvent structure effects’, a loose phrase which, as Atkinson has said, may be no more than a collective repository for our ignorance whilst we are still forced to retain a continuum theory of ion-ion interactions. In the following sections it will be noted that tetra-alkylammonium salts have been extensively studied by nearly all available techniques and much discussion has revolved around interactions between large tetra-alkyl-ammonium ions and the solvent. Whilst we are still a long way from a complete understanding of the factors involved, progress is being made particularly by modern spectrnscopic techniques and for this reason we devote a separate section entirely to discussion of this topic.

In order to improve their knowledge of non-ionic solution interactions a number of ‘electrolyte’ physical chemists have turned their attention to aqueous non-electrolyte solutions. Kineticists too, have become more concerned with attempts to understand the effect of environmental factors on ionic reactions. The proceedings of a symposium held in Newcastle in January 1968 which brought together those with a common interest in hydrogen-bonded solvents, have been published, as have lectures presented at an earlier meeting held in Bradford. ‘Solute-Solvent Interactions’ is the title of a collection of essays centred around, rather than on, this theme. A review of hydration effects and the thermodynamic properties of ions has recently appeared. A review essentially complementary to the present one has appeared in Annual Reports on the Progress of Chemistry for 1968.

‘Structure and Properties of Water’ is the title of a book by Eisenburg and Kauzmann and of a review by Ives and Lemon. Views on this subject remain subjective and often controversial. The translation of the proceedings of a conference held in Tbilisi in 1966 on ‘Water in Biological Systems’ has been published and contains articles by Gurikov on ‘Water Structure’ and by Samoilov on the ‘Theory of Hydration in Aqueous Solutions’. Franks has reviewed the role of water structure in disperse systems. A computer-compiled bibliography of papers covering the period 1957 — 68 on water structure and the physical properties of water is available 16 from Bell Telephone Laboratories, Murray Hill, New Jersey. Anomalous, meta or polywater so-called, has aroused considerable interest even though it was first reported by Deryagin in 1962. It is confidently expected to be the subject of a number of papers in 1970 whilst arguments continue about its observed physical properties and structure.

The viscosity of (ordinary) water up to 1400 kg cm-2 and moderate temperatures (2 — 30 °C) has been reported. Recent data are considered to be in error. Kay and co-workers have traced discrepancies in reported values for the dielectric constant (ε) of water at various temperatures to a doubtful capacitance correction. The preferred values are given by the equation:

log ε = 1.94409 – 1.991 x 10-3 T

A minimum has been reported 27 in the Kerr constants for H2O and D2O near 30 °C, which doubtless is related to structural features. Recalculations of the heat capacity of water at constant volume have been given which are important for testing models of liquid water. Energies of proton solvation have been derived from studies of the generation of protonated water molecules in a high pressure gaseous ion source.

Two other solvents of considerable current interest, dimethyl sulphoxide (DMSO) and dimethylsulphone, have been studied by neutron inelastic scattering and by X rays. Similarity of liquid and solid spectra shows a high degree of dipole association. The effect of these solvents on water structure was also studied. X-Ray studies are also reported of formamide and of potassium iodide solutions in formamide. It was concluded that concentrated solutions show ion-pair formation and doubly solvated cations.

Physical properties of solvents reviewed, or newly reported, include dimethyl sulphoxide, N-methyl acetamide and its mixtures with dioxan and benzene (dielectric constants), N-methyl propionamide (density and dielectric constants, 20 — 40 °C) and sulpholan (tetramethyl sulphone) and its mixtures with water and methanol (density, viscosity, and dielectric constant).

Gillespie has reviewed his recent work with the highly acidic solvent, fluorosulphuric acid, in which such interesting ionic species as I2+ (red), I42-, Se82+ (green), Se42+ (yellow) and Te42+ (red) have been shown to exist. A successful synthesis of perbromates has been reported by electrolytic oxidation of bromate, and rubidium perbromate has been isolated from the chemical oxidation of bromate with xenon difluoride. Zordan and Hepler present a critical Latimer-type compilation of data for manganese and its ions.

2 Single Electrolytes

A. Activity and Osmotic Coefficients. — Cation responsive glass electrodes (see Chapter 2) are now sufficiently well established as reliable electrodes that, with care, they can be used for precise determination of activity coefficients. Such measurements are always made relative to some chosen standard concentration in the same way as determination of activity coefficients from cells with transport and transport numbers. Hostetler, Truesdell, and Christ report activity coefficients for potassium chloride in the range 10 — 50 °C using potassium-sensitive glass electrodes. A rather complicated graphical method of evaluation was devised to eliminate the effects of e.m.f. drifts with time. Truesdell has reported similar determinations for sodium chloride, where such elaborate analysis is unnecessary because of the better behaviour of the sodium-responsive electrode. Because of interference from hydrogen ions, the pH of the solutions is important. Schindler and Waelti, using a small quantity of added ‘tris’ to buffer the solution, have confirmed accepted values for sodium chloride activity coefficients in the range 0.13 — 2.2 molal. A paper by Shatkay and Lerman also reports some measurements on sodium chloride solutions using sodium selective and silver-silver chloride electrodes. Activity coefficients of ammonium nitrate in liquid ammonia at -30 °C were obtained from hydrogen electrode concentration cell measurements and previously determined transport numbers.

Salomon reports activity coefficients from e.m.f. work for lithium chloride and bromide in propylene carbonate and lithium bromide in anhydrous DMSO. Lithium metal and amalgamated thallium-thallium halide electrodes were used. Butler and co-workers have determined activity coefficients for lithium chloride in anhydrous DMSO from e.m.f. measurements and also studied the same system by cryoscopy. Their results, however, are at variance with those of Garnsey and Prue, and Dunnett and Gasser. It is suggested that systematic errors are present in the latter two investigations, possibly due to heat transfer effects giving rise to spurious steady temperatures for periods up to thirty minutes. This view is contentious but the differences do not arise from use of the wrong cryoscopic constant. λc = 4.07 kg mol-1 K was determined from measurements with benzoic acid. Whereas the value used by Dunnett and Gasser was higher (4.36), their raw results of freezing point depression are up to 0.02° higher than those of Garnsey and Prue. The latter workers also report cryoscopic measurements on alkali-metal perchlorates and lithium chloride in sulpholan (tetrahydrothiophene-1,1-dioxide). The advantages of the large cryoscopic constant (λc = 64.1 ± 0.2 kg mol-1 K) are largely illusory for thermal buffering is poor and differences in the solidus and liquidus slopes are small. DMSO and sulpholan are solvents of different powers of ionic solvation even though they have the same dielectric constant, with the result that the opposite sequence of osmotic coefficients with cationic radius for the alkali-metal perchlorates is observed. These perchlorates are incompletely dissociated in sulpholan and there are large differences in the association behaviour of lithium chloride and bromide in this solvent. Another paper reviews the properties of sulpholan as a solvent for cryoscopy.

Russian workers reports some new determinations and some recalculations of osmotic coefficients from cryoscopic measurements for alkali-metal halides, chlorates, and bromates in aqueous solution. Bonner, Kim, and Torress have carried out cryoscopic studies of various solutes in ethylene carbonate and N-methyl acetamide (NMA). They note that the order of osmotic coefficients for the alkali-metal iodides (Li > Na > K > Rb > Cs) is the same for these two solvents as in water at any fixed concentration. Further the osmotic coefficient of bis-trimethylammonium iodide is nearly the same as in water. Since this can be considered as a dimer of tetramethylammonium iodide they conclude there is no structure enforced ion-pairing in aqueous solutions of tetra-alkylammonium salts.

Comparative (isopiestic) vapour pressure measurements are more popular than direct measurements but a few such determinations have been reported. Gardners has revised and extended previous work on the osmotic co-efficients of sodium chloride at high temperatures (125 — 270 °C). Previous data at 1 molal were low by 2% for reasons not ascertained. Using a differential manometric method, Bus, Steinberg, and de Boer have determined vap2ur pressures in the system D2SO4-D2O, and Campbell and Oliver in the systems lithium or sodium chlorate-dioxan-water. It was concluded that dioxan plays a major role in ionic solvation. By a gas displacement method the partial pressures of hydrogen chloride over saturated lithium chloride (13 molal) containing hydrochloric acid were determined indicating appreciable incomplete dissociation.

A useful discussion of important features for the design of isopiestic apparatus has been given by Luk’yanov, who describes in detail apparatus in which equilibrium attainment is enhanced by the device of rotating the container vessel on a central pivot at an angle of ca. 20° to the vertical. A method 60 of double isopiestic measurement where two volatile components are employed, is somewhat restricted in its application because of solubility restrictions.

A modified isopiestic apparatus employing glass flasks instead of the more usual silver or platinum dishes has been described by Wai and Yates and used to determine the water activity of concentrated perchloric acid solutions extending the range of study beyond 16 molal (62 — 75%).

Two papers report osmotic coefficients for bivalent perchlorates. Libus and Sadowska have studied manganese, cobalt, nickel, and copper perchlorates comparing their results with literature data for zinc and magnesium. Over the range 1.0 — 3.5 molal the osmotic coefficients of cupric and magnesium perchlorates are slightly lower than those of the other four. Pan and Ni report values for cadmium perchlorate and chloride. Platford has used the isopiestic method to study sodium and potassium tetraborates, sodium metaborate and fiuoroborate, and boric acid. The latter behaves like a non-electrolyte up to saturation. An apparatus suitable for studies up to 80 °C has been used to obtain data for potassium chloride and bromide and sodium sulphate, which can be compared with boiling-point elevation studies at reduced pressures (60 °C). Alkylsulphonium salts which are similar in behaviour to alkylammonium salts have been studied by Lindenbaum.

A number of papers report the use of vapour pressure osmometers to determine osmotic coefficients. Your reporters share the views of others who consider the accuracy of this method is often exaggerated especially if water is the solvent. Sometimes called the adiabatic isopiestic method, it depends on observation of the condensation of solvent on to droplets of solution and solvent placed separately on two identical thermistors.

Schwabe, Kretschmar, and Gartner report partition measurements of uranyl perchlorate between water and some organic media from which they derive activity coefficients for uranyl perchlorate much lower (3 orders of magnitude) than those determined from isopiestic measurements by Robinson and Lim. The new measurements are also supported by ultracentrifuge studies. Whilst the explanation of the abnormally high values determined isopiestically, may lie in hydrolysis, this also affects the new measurements. Since the actual experimental data are not given, it is not possible to comment further.

From distribution measurements of carboxylic acids and their sodium salts between water and butyl ether, Czeisler and Schrier have derived activity coefficients for the acids and salts in the aqueous phase. Dimer formation of the acids is assumed in the organic phase.

B. Volumes. — Determination of apparent and partial molal volumes from precision density measurements continues to attract considerable attention. In an important series of papers, Dunn describes a precise dilatometric technique and reports determinations of densities of sodium chloride, potassium chloride, bromide, and iodide and also tetra-n-butyl ammonium bromide at 25 °C in the range 0.001 — 1.0M. The third paper extends measurements to seven temperatures in the range 0 — 65 °C confirming the Debye-Hückel slope for volume at these temperatures for barium and calcium chlorides in addition to the alkali-metal salts mentioned above. It was noted that φov (apparent molal volume at zero concentration) increases with temperature reaching maximum values at 60 °C for the 1:1 electrolytes, 35 °C for calcium chloride and 45 °C for barium chloride. The effect is attributed to changes in the water structure on temperature increase. Structural effects are often invoked to explain any anomalous behaviour that is encountered. An entertaining paper by Holtzer and Emerson should be compulsory pre-reading for all those tempted to commit their fancies to print. Desnoyers and co-workers, and Millero and Drost-Hansen report precision determinations by float methods for 1:1 electrolytes in water. The latter paper gives data for 0.1 molal solutions at 1° intervals in the range 20 — 40 °C. A cubic fit to this density data is differentiated 79 to yield apparent molal expansibilities. The same authors with Korson find no evidence for reported anomalies in the temperature dependence of the apparent molal volume (or viscosity) of sodium sulphate near 32·5 °C. Ellis has continued his work on densities of electrolyte solutions at high temperatures (up to 200 °C).

Apparent molal volumes of aqueous solutions of sodium alkyl sulphates (C10 — C14) have been reported by Franks and co-workers. φv exhibits negative deviations from the limiting law, both above and below the c.m.c. at which there is a large positive volume change. Possible explanations of the first have been summarised by Franks and Smith who prefer the ‘co-operative hydrophobic hydration effects’ explanation. Prue and Pethybridge clearly distinguish the various schools of thought on the topics of ‘hydrophobic bonding’ and ‘hydrophobic interaction’ which are current in connection with the many recent studies of tetra-alkylammonium salts over the past few years. Other opinions on these topics can be found in the proceedings of a symposium on ‘Hydrogen-Bonded Solvent Systems’. Desnoyers and Arel find that plots of φv – 1.86 c+ versus c are linear for RNH3Br where R = H to octyl, which they take as evidence that dimerisation cannot explain a minimum and subsequent increase in φv for R ≥ C7. Millero and Drost-Hansen report measurements of φv for R4HCl where R = H to n-butyl at 1° intervals between 20 — 40 °C at one concentration only. Broadwater and Evans find that the behaviour of octane-1,8-bi-(tri-n-butylammonium)dibromide is similar to that of n-butylammonium bromide. The former is considered as a model for a cation-cation pair which is another possible explanation of observed ‘hydrophobic hydration’ effects. Conway and Laliberté have determined the isotope effect for alkali metal and tetraalkylammonium salts in light and heavy water. A positive effect V2°(D2O) > V2°(H2O) is found for structure-making ions (tetra-alkylammonium) and negative for structure breaking ions (sodium fluoride). Since the sizes of H2O and D2O are identical it is considered that the differences must reflect the degree to which the solutes affect the structure of the solvent, and therefore D2O is more structured than H2O. Darnell and Greyson using a dilatometer technique have investigated the effects of solutes on the temperature of maximum density of D2O(tmax 11.17 °C cf. 3.98 °C for H2O). tmax is reduced by all salts in both isotopic waters by an extent proportional to the solute concentration. Lithium chloride reduces tmax in both solvents and therefore is considered to be a structure breaker at this temperature (but not at 25 °C). With the exception of this salt, lowering is greater in H2O than in D2O, which might suggest that more structure is broken in H2O than D2O in spite of the fact that the latter is usually considered to be more structured. The preferred explanation for the observations that structure making effects are not found at tmax is unconvincing.

(Continues…)Excerpted from Electrochemistry Volume 1 by G. J. Hills. Copyright © 1970 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.