Peroxygen species

Other peroxygen species can also be photolytically cleaved to yield the hydroxyl radical and another radical centre. For example, homolysis of peroxymonosulfate (HOOSO3) generates OH and SO4-. The concentration of the generated hydroxyl radical can be controlled by variation of the wavelength and the intensity used. The photolysis of hydrogen peroxide in the presence of alcohol produces EPR (electron paramagnetic resonance) spectra which indi-... 

Figure 3.40 Two competing pathways operating during the oxidation of aldehydes with peroxygen species.

Ketones may be oxidized by peroxygen species under a range of conditions. The most common reaction involves the formation of esters, or in the case of cyclic ketones, lactones. This is known as the Baeyer-Villiger reaction.232-235 This reaction can occur under acidic or alkaline conditions (Figure 3.49), first outlined by Criegee.236... 

A range of metal catalysts can be employed with peroxygen species for the effective oxidation of sulfur compounds. For example, branched-chain high molecular weight mercaptans are difficult to oxidize with hydrogen peroxide. However, this difficulty is overcome if the reaction is conducted with hydrogen peroxide in the presence of a copper(II) salt.395 The formation of a copper(I) mercaptide followed by its oxidation are believed to be the key steps. 

A number of other aromatic oxidations involving peroxygen species have been documented. Probably the best known of these is the hydroxylation of phenols using alkaline peroxydisulfates, known as the Elbs reaction.490 This... 

Other species In order to maintain bleach actives in the wash bath, it is essential that the other components of the formulation do not react with the peroxygen species. The majority of detergent components are compatible. 

Product solutions or effluents of peroxygen reactions may contain variable amounts of unreacted peroxide, usually in the form of hydrogen peroxide, percarboxylic acids, and/or organic peroxide. For reasons related to safety, waste treatment or product stability, it is usually necessary to destroy unreacted peroxide species in the product solution or effluent prior to discharge or workup, and certainly before any product concentration process. The remainder of... 

The facile oxidation of organic substrates in the presence of a Fenton system is believed to be due to the production of the hydroxyl radical, as mentioned earlier. Consequently, a number of techniques have been employed to generate the species independently of any iron centres. Two techniques worthy of a mention are photolysis and radiolysis. Photolytic activation can be used to cause homolysis of the peroxygen bond because peroxides have a relatively broad absorption band above 300 nm (Figure 2.13).29... 

A number of other peroxygen based methods are known for alcohol oxidations. These include both organic peroxides, such as i-butyl hydroperoxide, and inorganic species such as perdisulfates. In all cases, the presence of metal species to catalyse the reaction is required. 

Several different peroxygen-based systems exist, all of which are compatible with biological treatment. The peroxygen treatment can be used up- or downstream of the biotreatment unit, either as a pre-treatment to reduce toxicity and improve biodegradability or as a post/polishing treatment to remove remaining biorecalcitrant species from the effluent. All the peroxygen-based treatments can also be used as stand-alone processes for the total treatment of a waste stream. The most common AOPs are ... 

For primary alcohols, fewer peroxygen systems are available. In situ halogen, with or without photolytic activation, may be used for some substrates the carboxylic acid formed condenses with the starting alcohol under the acid conditions employed, sometimes giving the ester as main product [106]. The Fenton system may also be used in a few cases [107]. Also useful are methods based on nitroxides as catalytic active species. These nitroxides, which are 1-electron oxidants, are produced from fully a-substituted secondary amines (notably 2,2,6,6-tetramethylpiperidine) and a variety of primary oxidants. Such oxidants include hypochlorite [108], peracids [109], Caro s acid and H2O2. Where H2O2 itself is used, a second catalyst, such as tungstate, is probably required to catalyse the oxidation of the amine [110] (see section 9.5.1.7). 

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