Historically, synthetic electroorganic chemistry made its debut in 1801 with the electrochemical oxidation of alcohol. Several works have been compiled detailing the many electroorganic reactions that have been discovered over the years. Perhaps the majority of these reactions have their chemical analogies; however, there exists a growing body of reactions which remain unique because of the nature of the product formed and/or the mode of formation. Undoubtedly, these unique reactions are in part the result of the electrochemically reaction variables operating in the heterogeneous systems. A number of these reactions have been identified as proceeding from ion-radical intermediates. It is increasingly apparent that the electrochemical method offers the most convenient way for generating ion radicals. The study of these interesting species becoming a new fast growing frontier for organic chemists .
In the same article defining his new terms: electrode, anode, cathode, electrolyte, ion, anion, cation, Faraday wrote of his work on the electrolysis of alkali metal acetate:
“When a very strong solution (of potassium acetate) was used, more gas was evolved at the anode than at the cathode in the proportion of 4 to 3 nearly; that from the anode was a mixture of carbon dioxide and carbonic acid; that from the cathode pure hydrogen. When a much weaker solution was used, less gas was evolved at the anode than at the cathode, and it was contained carbureted hydrogen (chiefly ethane) as well as carbonic oxide and carbonic acid.”
Hermann Kolbe re-examined the electrochemical oxidation of salts of carboxylic acid with the intention of proving the existence of radicals. Today the “Kolbe electrolysis” is the most widely known organic electrode process and its mechanistic subtleties still command a sizeable portion of the literature of the field.
Herman Kolbe developed a electrochemical method of oxidative decarboxylation of carboxylates to form hydrocarbons at platinum which is one of the most widely used reactions in organic electrosynthesis.
RCOONa = RCOO – + Na+ ..............(1)
RCOO – – e – = [ RCOO.] = R. + CO2 …........(2)
R. + R. = R-R (at Pt anode) ..............(3)
In Hofer-Moest reaction, platinum is substituted by carbon anode. The intermediate after loosing an electron, becomes a carbocation which further reacts with nucleophiles (Nu–) present in its vicinity:
R. - e – = R+
R+ + Nu – = R-Nu (at carbon anode)
Another variant is Crum-Brown-Walker’s oxidation of monocarboxylate monoesters to diesters :
COOR COOR
2 ( CH2 ) n -2 e – ( CH2 ) n + 2 CO2
COO– COOR
Monocarboxylate monoester Diester
This is the commercial process for the preparation of sebacates from adipates because in spite of loss of a substantial portion of molecular weight as CO2, it is more economical than alternatives.
Haber[13] and Russ examined the electrochemical properties of gold, platinum and other less noble metal electrodes with respect to organic reductions and oxidations. The effects of electrode history and pretreatment techniques were noted qualitatively by these workers.
The concept of controlled potential electrolysis has had a great impact on the modern electroorganic synthesis.
Its first proponent, Fritz Haber, as early as 1898, noted the importance of electrode potential. Thus in the reduction of nitrobenzene he found phenylhydroxylamine at low negative potentials, and aniline at more negative potentials. He wrote:
“The electric current up to this time has been regarded in electroorganic as mean of reaction whose results are determined through current density, current duration and occasionally, through the material of electrode. This view is incomplete, for oxidation and reduction processes depend mainly on potential of the electrode at which they take place. The current density, current duration and electrode material are important only in so far as they determine the electrode potential and its changes in the process of electrochemistry.”
Haber’s important concept was rather difficult to apply in the laboratory for many years following the publication of his pioneering work. Almost all research work was carried out by controlling current density and duration for the simple reason that manual control of the electrode potential requires continuous attendance of the experimenter. It was in the period (1910 to 1940) that F. Fichter and his co-workers in
In 1942, A.Hickling solved the difficulty of manual potential control by constructing a device which automatically control the potential of the working electrode, named “potentiostat”. The “potentiostat” has revolutionized synthetic electroorganic chemistry- the method of controlled potential electrolysis generally leads to fewer side products, and where mechanistic studies are facilitated and are made considerably more meaningful. In addition, with the device it is possible to carry out a specific oxidation or reduction of a particular functional group in a complex molecule without affecting other sensitive functional groups in the molecule.
In the “classical period” of electrochemistry around the turn of the century results were obtained empirically as they were in other procedures for any other organic synthesis. Such a procedure is still a possibility; there now exists many kind of techniques which may help the investigator to choose the optimum conditions for electrosynthesis.
A number of electroanalytical methods e.g. Polarography, Cyclic Voltammetry, Derivative linear sweep voltammetry, Voltammetry at ring disk electrodes etc. can be used to elucidate the mechanism of the electrochemical reaction and to determine some of the rate constants of the individual steps. Such a knowledge makes it possible to select the reaction parameters on the rational ground.
The most immediately useful technique is Polarography with which names of Nobel laureate J. Heyrovsky [16] and Ilkovic [17] are associated. Polarography involves recording of current potential curves (polarograms) at Dropping Mercury Electrode (DME) and is usually employed for the study of reduction processes and not of oxidations, since mercury itself is readily oxidizable. Polarography can provide a considerable amount of mechanistic information with relatively little effort. The number of polarographic waves indicates immediately the number of discrete electrode processes undergone by the electroactive substance and the wave heights indicate the number of electrons involved in the overall, for each wave.
Peter Zuman used the technique of Polarography to clarify the reactions occurring in the course of reduction of organic compounds. Kolthoff introduced the technique in
Another important technique is Cyclic Voltammetry, first practiced by Sevcik and was formerly known as cyclic triangular wave voltammetry. Cyclic voltammetry is a simple technique which makes use of a stationary microelectrode to which a triangular voltage sweep is applied, while the current is recorded as a function of potential. The technique provides information regarding reduction and oxidation potentials; formation of intermediates and most importantly the information concerning rapidity (reversibility) of electron transfer across the electrode-solution interface, which helps in elucidating the kinetics and mechanism of the reaction taking place at the electrode. Nicholson and Shain have reviewed the use of cyclic voltammetry as a diagnostic tool for providing information about reaction mechanism.
Polarography and cyclic voltammetry provide useful information about the electrochemical behavior of new systems which may be profitably employed in formulating a judicious plan and thereby achieving an electroorganic synthesis with greater certainty and precision.
No comments:
Post a Comment