Oxidation-Reduction Reactions in Organic Chemistry

For example, converting a carboxylic acid to an aldehyde is a reduction because the oxygen content is decreased  

In these examples we have used the symbol [H] to indicate that a reduction of the organic compound has taken place. We do this when we want to write a general equation without specifying what the reducing agent is. 

The reverse of each reaction that we have just given is an oxidation of the organic molecule, and we can summarize these oxidation-reduction reactions as shown below. We use the symbol [O] to indicate in a general way that the organic molecule has been oxidized. 

For example, replacing hydrogen atoms by chlorine atoms is an oxidation  

Note the general interpretation of oxidation-reduction regarding organic compounds. 

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The oxidation numbers given in Table 3 can be used to classify organic reactions as either oxidation-reduction reactions or metathesis reactions. Because electrons are neither created nor destroyed, oxidation cannot occur in the absence of reduction, or vice versa. It is often useful, however, to focus attention on one component of the reaction and ask Is that substance oxidized or reduced Assigning oxidation numbers to the individual carbon atoms in a complex molecule can be difficult. Fortunately, there is another way to recognize oxidation-reduction reactions in organic chemistry. 

The preceding examples, which were purposely restricted to well-known reactions of carbonyl and related functions, illustrate the large ambiguities associated with oxidation-reduction notions in organic chemistry. As suggested earlier, these ambiguities cer-... 

Oxidation-reduction reactions in water are dominated by the biological processes of photosynthesis and organic matter oxidation. A very different set of oxidation reactions occurs within the gas phase of the atmosphere, often a consequence of photochemical production and destruction of ozone (O3). While such reactions are of great importance to chemistry of the atmosphere - e.g., they limit the lifetime in the atmosphere of species like CO and CH4 - the global amount of these reactions is trivial compared to the global O2 production and consumption by photosynthesis and respiration. 

The oxidation of alcohols to aldehydes, ketones or carboxylic acids is one of the commonest reactions in organic chemistry, and is frequently achieved by transition metal complexes or salts. However, in most cases the precise mechanisms are not known, and the intermediates not fully characterised. In general, metal complexes of the alcohols are formed as transient intermediates in these reactions, but we shall not deal with these extremely important reactions in any great detail. The precise mechanisms depend upon the accessibility of the various one- and two-electron reduction products of the particular metal ion which is involved in the reaction. However, we will outline a brief indication of the mechanism. The first step involves the formation of an alcohol complex of the metal ion (Fig. 9-14). This might or might not deprotonate to the alkoxide form, depending upon the pH conditions of the reaction, the pK of the alcohol and the polarising ability of the metal ion. 

Classification of Solvents. Solvent classification helps to identify properties useful in solvent selection for individual applications for example, the study of acid-base reactions, oxidation-reduction reactions, inorganic coordination chemistry, organic nucleophilic displacement reactions, and electrochemistry. 

The transfer of a single electron between two chemical entities is the simplest of oxidation-reduction processes, but it is of central importance in vast areas of chemistry. Electron transfer processes constitute the fundamental steps in biological utilization of oxygen, in electrical conductivity, in oxidation reduction reactions of organic and inorganic substrates, in many catalytic processes, in the transduction of the sun s energy by plants and by synthetic solar cells, and so on. The breadth and complexity of the subject is evident from the five volume handbook Electron Transfer in Chemistry (V. Balzani, Ed.), published in 2001. The most fimdamental principles that govern the efficiencies, the yields or the rates of electron-transfer processes are independent of the nature of the substrates. The properties of the substrates do dictate the conditions for apphcability of those fimdamental... 

A. Oxidation-reduction reactions versus electron transfer reactions in organic chemistry and electrochemistry... 

A. Oxidation-Reduction Reactions Versus Electron Transfer Reactions in Organic Chemistry and Electrochemistry... 

Many redox reactions in organic chemistry involve single electron transfers, i.e. are radical reactions. We have already looked at a number of radical reactions, for example those that resulted in overall substitution. We will now study a few more radical reactions that do not conveniently fit under that heading. The addition of a single electron to an atom amounts to the reduction of that atom s oxidation state by one unit. 

Two important types of reactions in organic chemistry are oxidation and reduction. In oxidation reactions, the oxidized species loses electron density. 

Chromium can exist in several oxidation states from Cr(0), the metallic form, to Cr(Vl). The most stable oxidation states of chromium in the environment are Cr(lll) and Cr(Vl). Besides the elemental metallic form, which is extensively used in alloys, chromium has three important valence forms. The trivalent chromic (Cr(lll)) and the tetravalent dichromate (Cr(Vl)) are the most important forms in the environmental chemistry of soils and waters. The presence of chromium (Cr(Vl)) is of particular importance because in this oxidation state Cr is water soluble and extremely toxic. The solubility and potential toxicity of chromium that enters wetlands and aquatic systems are governed to a large extent by the oxidation-reduction reactions. In addition to the oxidation status of the chromium ions, a variety of soil/sediment biogeochemical processes such as redox reactions, precipitation, sorption, and complexation to organic ligands can determine the fate of chromium entering a wetland environment. 

Addition Reactions. The addition of nucleophiles to quinones is often an acid-catalyzed, Michael-type reductive process (7,43,44). The addition of benzenethiol to 1,4-benzoquinone (2) was studied by A. Michael for a better understanding of valence in organic chemistry (45). The presence of the reduced product thiophenyUiydroquinone (52), the cross-oxidation product 2-thiophenyl-1,4-benzoquinone [18232-03-6] (53), and multiple-addition products such as 2,5-(bis(thiophenyl)-l,4-benzoquinone [17058-53-6] (54) and 2,6-bis(thiophenyl)-l,4-benzoquinone [121194-11-4] (55), is typical ofmany such transformations. 

The quinone-hydroquinone system represents a classic example of a fast, reversible redox system. This type of reversible redox reaction is characteristic of many inorganic systems, such as the interchange between oxidation states in transition metal ions, but it is relatively uncommon in organic chemistry. The reduction of benzoquinone to hydroquinone... 

It is convenient to divide organic chemical reactions between acid-base and oxidation-reduction reactions as in inorganic chemistry. In acid-base reactions the oxidation states of carbon do not change, e.g. in hydrolysis, where reaction is, for example,... 

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