Oxidation-Reduction of Organic Compounds

An organic compound commonly is said to be reduced if reaction leads to an increase in its hydrogen content or a decrease in its oxygen content. The 

11 Alkenes and Alkynes II. Oxidation and Reduction Reactions. Acidity of Alkynes 

This is a very unsatisfactory definition because many oxidation-reduction or redox reactions do not involve changes in hydrogen or oxygen content, as the following example illustrates  

Redox reactions are better defined in terms of the concept of electron transfer. Thus an atom is said to be oxidized if, as the result of a reaction, it experiences a net loss of electrons and is reduced if it experiences a net gain of electrons. This simple definition can be used to identify oxidation or reduction processes at carbon in terms of a scale of oxidation states for carbon based on the electronegativities of the atoms attached to carbon. The idea is to find out whether in a given reaction carbon becomes more, or less, electron-rich. We will use the following somewhat arbitrary rules  

Elementary carbon is assigned the zero oxidation state. 

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Indeed the combination of the reactive intermediates in equations (la) and (2) forms the chemical basis of electron-transfer oxidation (reduction) of organic compounds in both stoichiometric and catalytic processes. ... 

In the present chapter we want to look at certain electrochemical redox reactions occurring at inert electrodes not involved in the reactions stoichiometrically. The reactions to be considered are the change of charge of ions in an electrolyte solution, the evolution and ionization of hydrogen, oxygen, and chlorine, the oxidation and reduction of organic compounds, and the like. The rates of these reactions, often also their direction, depend on the catalytic properties of the electrode employed (discussed in greater detail in Chapter 28). It is for this reason that these reactions are sometimes called electrocatalytic. For each of the examples, we point out its practical value at present and in the future and provide certain kinetic and mechanistic details. Some catalytic features are also discussed. 

Iron was one of the first metals employed for the reduction of organic compounds over 130 years ago. It is used in the form of filings. Best results are obtained with 80 mesh grain [765]. Although some reductions are carried out in dilute or concentrated acetic acid the majority are performed in water in the presence of small amounts of hydrochloric acid, acetic acid or salts such as ferric chloride, sodium chloride (as little as 1.5-3%) [765], ferrous sulfate [766] and others. Under these conditions iron is converted to iron oxide, Fe304. Methanol or ethanol are used to increase the solubility of the organic material in the aqueous medium [766] (Procedure 34, p. 213). 

Chapter 1 - Electrochemical Oxidation and Reduction of Organic Compounds,... 

Many organic compounds undergo reduction or oxidation at a DME. Consequently, polarographic techniques have been used extensively for determinations of organic compounds and for studying the mechanisms of their electrode reactions. In aqueous solution, the reduction of organic compounds is frequently a 2e process accompanied by protonation as in Equation 3.32 ... 

Rinehart, K.L., Jr. 1973. Oxidation and Reduction of Organic Compounds. Prentice-Hall, Inc., Englewood Cliffs, NJ, 1973. 

Most of the processes relate to the reduction of organic compounds, particularly the reduction of nitro-compounds, but oxidation methods have been utilised successfully, and in many cases substitution products and dye-stuffs have been prepared by electrolytic means. 

With the wide range of SSE s presently available, it should be possible to get an experimental value of Ein or E for almost any substrate, except possibly for those which are extremely difficult to reduce or oxidize or tend to form films. In the rare cases where an experimental value cannot be obtained, a reasonable value can often be inter- or extrapolated using known correlations between Hiickel MO parameters and oxidation or reduction potentials, or between gas phase ionization potentials and oxidation potentials 66 A very thorough discussion of structural effects on electrode reactions is available 24 as well as a comprehensive list of oxidation potentials of organic compounds 10 ... 

Some microorganisms can catalyze certain oxido-reduction reactions like the oxidation of iron and manganese in water, the oxidation of sulfur compounds, and oxidation-reduction of nitrogen compounds. Aerobic autotrophic bacteria of the type Thiobacillus can release soluble iron, copper, and sulfuric acid as sulphates into water. These organisms can be found everywhere in nature wherever an acidic environment is maintained in the presence of sulfide-containing minerals. 

Oxidation Potentials of Organic Compounds 7.422 Reduction Potentials of Oxidants (Electron Acceptors)... 

However, photocatalytic reduction of organic compounds is generally less important than their oxidation because the reduction potential of a Ti02(e ) is lower than the oxidation potential of a Ti02(h ). Also, most of the reducible substrates cannot compete kinetically with oxygen as an electron scavenger [15]. 

The preparation of paUadous oxide-palladium black and its use as a catalyst in the reduction of organic compounds have been studied by Shriner and Adams. Palladium black and coUoidal palladium have been widely used as hydrogenation catalysts. ... 

Another aspect of the advantage of bringing the depolarizer to the cathode is seen by comparing the results in Table LXXXV for smooth and spongy copper electrodes. The much larger effective area of the latter permits more intimate contact of the nitric acid with the cathode. It may be noted in this connection that in his extensive work on the electrolytic reduction of organic compounds, Tafel (1900) frequently used a prepared lead electrode, which had been roughened by electrolytic oxidation of the surface to lead dioxide followed by reduction to finely-divided lead by cathodic treatment. 

Electrochemical reactions serve as efficient and convenient methods for the synthesis of organoelemental compounds. There are four major methods for the formation of element (metal)-carbon bonds. The first method utilizes the anodic oxidation of organometallic compounds using reactive metal anodes. In the second method, the organic compounds are reduced using reactive metal cathodes. The third method involves the cathodic reduction of organic compounds in the presence of metal halides. The fourth one utilizes both the cathodic and the anodic processes. 

As a matter of fact, olefin-consuming reactions (by H2) may be a serious problem in some technical reactions. Palladium complexes and Co2(CO)g (commercial products) are typical catalysts. Problems may also arise in the Fischer-Tropsch reaction [19, 20] where iron oxides of a certain basicity (alkaline-metal doping) are being used to catalyze the formation of hydrocarbons according to (the simplified) eq. (15). More details are provided in Section 3.1.8. Since water is inevitably formed, carbon dioxide can also occur. On the other hand, it is doubtful whether the CO/H2O system will be used for directed reductions of organic compounds, since hydrogen is an extremely abundant industrial chemical. The water-gas shift reaction is thus to be avoided in the vast majority of cases. 

Adkins catalyst. A catalyst containing copper chromite and copper oxide. It is used for the reduction of organic compounds, usually at high temperatures and pressures. It is likewise used as a catalyst for dehydrogenation and for decarboxylation reactions. 

Recall some reactions that involve oxidation and reduction of organic compounds... 

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