Overoxidation

The term overoxidation refers to degradation of the conductivity and electroactivity of an oxidized conducting polymer by reaction with a nucleophile. This topic has recently been thoroughly reviewed,33 and so the treatment here will be brief. 

Hie electrochemical characteristics of overoxidation vary widely among polymers, solvents, and nucleophiles.129 Its rate depends on the degree of oxidation of the polymer (and therefore on the potential applied), and the concentration127 and reactivity of the nucleophile. Polypyrroles usually become overoxidized at lower potentials than polythiophenes because of their lower formal potentials for p-doping. In acetonitrile, the reactivity of the halides follows their nucleophilicity in aprotic solvents, 

The overoxidation of polyanilines has been studied most extensively in aqueous solutions.33 It occurs much more slowly than the overoxidation of polypyrroles and polythiophenes, requiring extensive cycling through the second oxidation wave (at ca. +0.7 V in Figs. 6 and 8), or many minutes 

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DMSO, molybdenum peroxide, benzene, reflux, 7-20 h, 60% yield. This method was used to m onoprotect 1,2-diols. The method is not general because oxidation to a-hydroxy ketones and diketones occurs with some substrates. On the basis of the mechanism and the results it would app>ear that overoxidation has a strong conformational dependence. 

This example shows that overoxidation of allylic alcohols may occur with DDQ. ... 

Oxidation of primary alcohols to aldehydes (Section 15.10) Pyridinium dichromate (PDC) or pyridinium chloro-chromate (PCC) in anhydrous media such as dichloromethane oxidizes primary alcohols to aldehydes while avoiding overoxidation to carboxylic acids. 

Another convenient method for the preparation of tertiary enamines involves the dehydrogenation of saturated bases with mercuric acetate (111-116). A trans-1,2 elimination occurs, which requires an antiperi-planar position of the nitrogen-free electron pair and the eliminated atom. A preferential elimination of the hydrogen atom from the tertiary carbon atom is supposed. Overoxidation can be avoided by adding disodium ethyl-enediaminotetraacetate to the reaction mixture (117). 

While the oxidation of tertiary amines has been used extensively for the generation of enamines, an example of overoxidation with formation of an acetoxyimonium salt has been reported (484). 

DDQ, 35% yield. The DDQ-promoted cleavage of phenolic MPM ethers can be complicated by overoxidation, especially with electron-rich phenolic compounds. 

I2, Met, Tyr, His, and Trp are susceptible to overoxidation with iodine if the reaction conditions are not carefully controlled. ... 

DDQ is often used to remove the MPM group from alcohols and can be used to cleave it from an amine, but in the following case overoxidation also occurs ... 

This reaction is highly exothermic the excessive temperature increase reduces ethylene oxide yield and causes catalyst deterioration. Overoxidation can he minimized hy using modifiers such as organic chlorides. 

In the past, periodate titrations have been of limited value for establishing the structure of quercitols or cyclohexanetetrols. The former show overoxidation, because of the fact that malonaldehyde is formed, and this compound undergoes further oxidation. Some isomers of the tetrols... 

In theory, periodate oxidation could have given a clear-cut answer as to the composition of the isomeric mixture of deoxy ribose phosphates. The 4-phosphate (73), devoid of vicinal diol groups, should be resistant to periodate the 3-phosphate (74) should reduce one and only one molar equivalent of the oxidant and yield one molar equivalent of both formaldehyde and the phosphorylated dialdehyde (75), whereas the 5-phosphate (76) could be expected to reduce one molar equivalent of periodate relatively rapidly, followed by a slower overoxidation reaction owing to the oxidation of malonaldehyde, formed as a result of the glycol cleavage. 

There are several available terminal oxidants for the transition metal-catalyzed epoxidation of olefins (Table 6.1). Typical oxidants compatible with most metal-based epoxidation systems are various alkyl hydroperoxides, hypochlorite, or iodo-sylbenzene. A problem associated with these oxidants is their low active oxygen content (Table 6.1), while there are further drawbacks with these oxidants from the point of view of the nature of the waste produced. Thus, from an environmental and economical perspective, molecular oxygen should be the preferred oxidant, because of its high active oxygen content and since no waste (or only water) is formed as a byproduct. One of the major limitations of the use of molecular oxygen as terminal oxidant for the formation of epoxides, however, is the poor product selectivity obtained in these processes [6]. Aerobic oxidations are often difficult to control and can sometimes result in combustion or in substrate overoxidation. In... 

These facts are different demonstrations of the same event degradation reactions occur simultaneously with electropolymerization.49-59 These reactions had also been called overoxidation in the literature. The concept is well established in polymer science and consists of those reactions between the pristine polymer and the ambient that promote a deterioration of the original polymeric properties. The electrochemical consequence of a strong degradation is a passivation of the film through a decrease in the electrical conductivity that allows a lower current flow at the same potential than the pristine and nondegraded polymer film did. Passivation is also a well-established concept in the electrochemistry of oxide films or electropainting. 

Reactions with other nucleophiles follow a similar mechanism. For the reaction of Cl with poly(3-methylthiophene) in acetonitrile, the reaction stops at structure 5 (Scheme 2).128 A fully conjugated, Cl-substi-tuted product 6 can subsequently be obtained by electrochemical or chemical dehydrogenation.128 With Br and alcohols, the overoxidation... 

Okada, and the mechanism for pitting, 272 Oligomers, 556 Overoxidation, 563 Oxidation... 

Electrochemical measurements on polyaniline (PANI) produce a picture of the charge storage mechanism of conducting polymers which differs fundamentally from that obtained using PTh or PPy. In the cyclic voltammetric experiment one observes at least two reversible waves in the potential range between —0.2 and -)-1.23 V vs SCE. Above -1-1.0 V the charging current tends to zero. Capacitive currents and overoxidation effects, as with PPy and PTh, do not occur The striking... 

An important problem encountered with polymer electrodes is that of overoxidation. It occurs after reversible charging of the electrode at high oxidation potentials and leads to polymer degeneration. The results of thorough studies show that such degenerative mechanisms are promoted by the nucleophilicity of the solvent. Especially the activity of water leads to the formation of quinone-type compounds, to the cleavage of C—C bonds, the liberation of CO2, and the formation of carboxylic acids Hence, there is a clear tendency to avoid both nucleophile solvents... 

In the following scheme, an oxidation pathway for propane and propene is proposed. This mechanism, that could be generalized to different hansition metal oxide catalysts, implies that propene oxidation can follow the allylic oxidation way, or alternatively, the oxidation way at C2, through acetone. The latter easily gives rise to combustion, because it can give rise to enolization and C-C bond oxidative breaking. This is believed to be the main combustion way for propene over some catalysts, while for other catalysts acrolein overoxidation could... 

Various experimental conditions have been used for oxidations of alcohols by Cr(VI) on a laboratory scale, and several examples are shown in Scheme 12.1. Entry 1 is an example of oxidation of a primary alcohol to an aldehyde. The propanal is distilled from the reaction mixture as oxidation proceeds, which minimizes overoxidation. For secondary alcohols, oxidation can be done by addition of an acidic aqueous solution containing chromic acid (known as Jones reagent) to an acetone solution of the alcohol. Oxidation normally occurs rapidly, and overoxidation is minimal. In acetone solution, the reduced chromium salts precipitate and the reaction solution can be decanted. Entries 2 to 4 in Scheme 12.1 are examples of this method. 

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