A Century of Organic Electrochemistry

A Century of Organic Electrochemistry

(Parte 1 de 5)

A Century of Organic Electrochemistry

Henning Lund* Department of Chemistry, University of Aarhus, DK 8000 Aarhus C, Denmark

Available electronically March 12, 2002.

The electrochemistry of organic compounds in the 20th century is built on the work done in the 19th century. The invention of the battery, the ‘‘Volta pile,’’ in 1800,1 was essential for the development of electrolysis as such experiments require a current running during an extended period. Volta, however, did not regard the chemical reactions observed at the electrodes as essential for the function of the battery and his position in science was so high that no preparative electrolyses were made until Faraday’s experiments in the eighteen thirties. Faraday was the first who made an electroorganic synthesis2 by electrolyzing an acetate solution and obtaining a gaseous product, ethane. The anodic oxidation of salts of fatty acids to hydrocarbons with loss of carbon dioxide was developed by Kolbe3 to become the first useful electroorganic synthesis, and it was later in the century extended by Brown and Walker4 to electrolysis of salts of monoesters of dibasic acids resulting in dimerization to esters of dibasic acids. The first electrochemical reduction of an organic compound seems to be the reductive dehalogenation of trichloromethanesulfonic acid to methanesulfonic acid5 at a zinc electrode.

During the second half of the 19th century preparative organic electrochemistry bloomed and a very optimistic view on the synthetic possibilities of the new method both for laboratory and technical applications was expressed. In this ‘‘classical’’ period several oxidations and oxidative substitutions as well as reductions of nitro compounds, carbonyl derivatives and dehalogenation reactions were performed; however, in most cases mixtures of products were obtained. Many of these investigations were made by the groups of Elbs, Gattermann, Haber, Lob, and Tafel. Their publications appeared mostly in Ber. Dtsch. Chem. Ges., Z. Elektrochemie, and J. Prakt. Chem.

In 1898 Haber6 published a classical paper on the stepwise reduction of nitro compounds; in this he realized that by using a constant current density the effective reduction potential would gradually become more negative and that it was essential for a selective reaction to keep the potential at the working electrode constant. However, control of the potential could only be made manually and there was no easy way to find the optimal potential for a given reaction.

20th Century

The organic electrochemistry of the 20th century may be divided into three periods: from 1900 to 1940, from 1940 to 1960, and since 1960.

1900 to 1940.—The situation in the first period was not much different from the last decade of the 19th century with regard to technique, but the optimism had almost disappeared during the period and only few publications appeared on organic electrochemistry between 1910 and 1940. A few groups continued doing research, mainly those of F. Fichter, S. Swann, and H. D. Law. Surveys of the work done in that period may be found in the book by Fichter7 and the two reviews by Swann.8

Only a few electroorganic reactions became industrial processes, and some of the more important ones were indirect reactions where the electrochemical step was a regeneration of an inorganic reagent. As early as 1900 it was realized9 that in many cases it was an advantage to separate the electrochemical step which was the reoxidation of Cr~I! to Cr~VI! from the chemical oxidation in which Cr~VI! was used to oxidize naphthalene or anthracene to the quinones. Another process in which this method was used was the bleaching of raw montan wax, and this reaction has been running commercially for more than 70 years by Hoechst ~now Clariant! on a 10,0 ton-per-year ~t/yr! scale. Montan wax is extracted from lignite and consists of esters of long-chain acids and alcohols; after hydrolysis of the esters the alcohols are oxidized by Cr~VI! to the acids.

A production of sorbitol and mannitol by cathodic reduction of glucose was established by Atlas Powder company10 on a 1400 t/yr scale, but some years later the electrolytic reaction was replaced by high-pressure catalytic hydrogenation.

1940-1960.—In that period electroanalytical methods, such as polarography and voltammetry at solid electrodes, were applied to the study of many organic molecules, and the results used for analysis and in a few cases for guidance of electrolysis at controlled potential. Also the use of aprotic media was begun, both for cathodic and anodic reactions.

Polarography.—Polarography, current-voltage curves obtained at the dropping mercury electrode ~DME!, was invented by Heyrovsky11 and was in the beginning mostly used for analysis of inorganic compounds; however, a publication on the polarography of nitrobenzene appeared early.12 The classical photographically recording ‘‘polarograph’’ was constructed by Heyrovsky and Shikata.13

Between 1925 and 1940 some organic compounds such as fructose,14 aromatic and halogenated carbonyl compounds,15 acetylacetone,16 and cystine17,18 were investigated by polarography.

The two books in 1941 on polarography by Kolthoff and

Lingane19 and the new edition of Heyrovsky’s book20 ~in German! had a great effect on polarographic research. Examples of compounds investigated polarographically in the nineteen forties are phenylsubstituted olefins and polynuclear aromatic hydrocarbons,21 aliphatic and aromatic halogen compounds,2 nitro compounds,23 diazonium salts,24 and N-heterocycles.25,26

A seminal paper by Lingane27 demonstrated that the potentials found by polarography could be used for selective reductions at a controlled potential at a macroelectrode, and that preparative electrolysis at the potential of the limiting polarographic current could establish the electrode reaction in polarography. Lingane demonstrated that 9-~2-iodophenyl!acridine could selectively be reduced to the 9-~2-iodophenyl!dihydroacridine without reducing the carboniodine bond. The introduction of potentiostats28,29 made it easier to perform electrochemical reactions at controlled potential.

In the nineteen fifties and nineteen sixties polarographic investigations of many types of organic compounds were made and the* Electrochemical Society Active Member.

results used for analysis and/or for a guidance for controlled potential electrolysis, mainly reductions at a macro-mercury electrode. Several books were published on polarography30-34 and in Analytical Chemistry Wawzonek wrote a survey of polarographic publications on organic electrochemistry every second year. Later in the century other electroanalytical techniques became more popular, although still valuable polarographic investigations were made, foremost by Zuman.

A theory for fast polarography, oscilloscopic polarography, at a dropping mercury electrode was developed by Matsuda and Ayabe;35 the electrode reactions were divided into reversible, quasi-reversible and irreversible according to the rate constant of the heterogeneous electron transfer. The problems and their solutions are similar to those encountered in linear sweep ~LSV! and cyclic voltammetry ~CV!.

Anodic voltammetry in aqueous or aqueous-alcoholic media is limited by the available potential range. The use of

acetonitile/NaClO4 as medium was introduced by Lund36 in investigations on the electrochemical oxidation of alcohols and aromatic

hydrocarbons. Preparative oxidations at controlled potential were made, sometimes in the presence of pyridine as base/nucleophile. The conclusion was that the oxidation consisted of a loss of electrons from the aromatic system which then, by loss of protons or reaction with a nucleophile, formed the product~s!. This was in contrast to the prevailing ideas that anodic oxidations proceeded through electrolytically generated reagents, such as peroxides.

Industrial processes 1940-1960.—The most important industrial application of organic electrolysis introduced in the period 1940- 1960 was the electrochemical fluorination ~ECF! of organic substrates, and the electrochemical manufacture of fluorinated compounds has since continued to be an important industrial process. The oldest one, the Simons ECF37 is an electrolysis of the organic compound in anhydrous hydrogen fluoride ~aHF! at a nickel anode, and it results in a perfluorination of the substrate. All hydrogens in the compound are replaced by fluorine, double bonds are saturated, some functional groups are retained, but a certain degradation of longer chains occurs.

The mechanism of the fluorination has been discussed during the years; the involvement of radical cations,38 fluoride ions, elemental fluorine, fluorine radicals,39 and high-valence nickel fluorides40 have been suggested and the importance of a layer of partly fluorinated polymers at the anode has been underlined. Recently it has been proposed that the fluorination goes through NiIV fluoride which in a two-electron three-center bond~s! binds to the substrate. The NiIV may be reduced in a process in which the adsorbed substrate is oxidatively fluorinated or, alternatively, the substrate may be oxidized without fluorination to give carbocations and reaction products from these or give tar; the reactive NiIV is continuously regenerated at the anode.41

The Phillips Petroleum Company has developed an electrochemical fluorination using an electrolyte of KF 2HF with a porous carbon anode. The process is thought to involve electrolytic generation of elemental fluorine, and the fluorination is believed to take place within the pores of the anode. Fluorination of hydrocarbons gives partly and fully fluorinated products. The method is a useful complement to the Simon process.

Later, other electrochemical partial fluorination reactions were developed. In these reactions the substrate is oxidized to a radical cation or cation which reacts with fluoride ion, so it is essential to perform the oxidation in a medium in which the nucleophilic reac- tivity of the fluoride ion is not diminished too much by solvation.

The preferred media are R3N nHF or R4N nHF,42 but other media have also been used.43 A review on electrochemical partial fluorination has recently appeared.4

Another industrial production, although on a much lower scale, also began operating about 1950, the anodic methoxylation of furan and furan derivatives to 2,5-dihydro-2,5-dimethoxyfurans.45 Such compounds are derivatives of 1,4-dicarbonylbutene and as such are readily transformed to other compounds. A number of heterocyclic compounds were prepared from 1,4-dicarbonyl-2-butenes. The reaction was discontinued after some years but later reintroduced on a larger scale by BASF and Otsuka Chemical

1960-2001.—After about 1960 organic electrochemistry underwent a rapid development. A characteristic feature of the period was that new electroanalytical techniques made it possible to investigate the mechanism of the electrode reactions more efficiently than previously. Another development is the introduction of indirect electrolysis using organic and organometallic mediators and a growing interest of the electrochemistry of bioorganic systems. Furthermore, new industrial applications of organic electrochemistry appeared. A few areas of the development can be mentioned here, but can only be sketchily described.

Electroanalytical techniques.—In the years after 1960 the further development of electroanalytical methods made it possible to obtain a more detailed understanding of the different steps in the electrochemical reactions. Both the theory for different types of reactions and the instrumentation were developed, and the introduction of simulation of the various kinds of electroanalytical signals was a great step forward.46,47 A commercially available simulation program, Digisim, is now available.46 A monograph on the theory of the electroanalytical techniques has been published,48 and discussions of the methods for reaction studies has recently appeared.49-51

One of the most widely used techniques is cyclic voltammetry; publications dealing with the influence of the rate of the heterogeneous electron transfer and the rate of follow-up reactions on the shape of the curves were published by Saveant and Vianello52 and by Nicholson and Shain.53 Sometimes it is an advantage to include the whole curve in the interpretation by calculation of the convolution integral which transforms the voltammogram into a polarogram-like curve.54,5

When the electron transfer is followed by a rapid follow-up reaction, a high sweep-rate is needed to outrun the reaction, and for microelectrodes of conventional diameter ~about 1 m! the voltage drop caused by the ohmic resistance is difficult to compensate electronically. By the use of ultramicroelectrodes ~diam 2-10 m! higher sweep-rates may be employed.56 Using suitable ohmic-drop compensation it is possible to make an ohmic-drop-free electrochemical investigation in the megavolt per second range which corresponds to the development of a diffusion layer having only a few nanometers’ thickness.57-59 However, unfortunately the ratio between the capacitative and Faradaic current is proportional to the square root of the sweep rate so at very high sweep rates the Faradaic current is just a small ‘‘bump’’ on the large capacitative current. At high sweep rates the rate of the heterogeneous electron transfer may set a limit for the use of ultramicroelectrodes.

Another application of ultramicroelectrodes is in the scanning electrochemical microscope ~SECM!. It consists of a microelectrode ~radius 1-25 nm!, with the distance between the tip and the substrate being piezo-controlled. At a distance between the tip and the con-

Journal of The Electrochemical Society, 149 ~4! S21-S33 ~2002!S22 ductive substrate similar to the radius of the tip, a molecule reduced at the tip may diffuse to the substrate and be reoxidized, before it reacts in other ways; this gives an increase in the current as the apparent concentration is increased. If a fast follow-up reaction occurs, the changes from the expected current at different distances give information on the rate of the follow-up reaction. The experiments have some analogies to rotating ring-disc measurements. Using SECM the rates of fast reactions such as the dimerization of acrylonitrile in DMF60 and the rate of the oxidative dimerization of 4-nitrophenolate61 were determined.

The SECM allows the observation of the electrochemical behavior of a single molecule by trapping a small volume of a dilute solution of the electroactive species, e.g., a ferrocene derivative, between the tip and the conductive substrate. An anodic current was observed when the molecule moved in or out of the electrodesubstrate gap.62

The SECM has also been used to investigate the ‘‘reverse electron transfer,’’ in which the electron is transferred from the reduced form to the oxidized in an ‘‘uphill’’ reaction. Such thermodynamically unfavored reactions are common in living systems, and the driving force here is believed to be a proton electrochemical potential gradient across a membrane. A model for such a membrane is two immiscible solutions, and the role of the proton to produce a potential drop across the interface is taken over by a supporting ion common to both solutions. Using ferrocyanide and 7,7,8,8- tetracyanoquinodimethane ~TCNQ!, the forward and back rate constants for electron transfer between water and dichloromethane have been measured.63,64

(Parte 1 de 5)