Photochemical reactions, peroxides

Photochemical Reactions. The photochemistry of chlorine dioxide is complex and has been extensively studied (29—32). In the gas phase, the primary photochemical reaction is the homolytic fission of the chlorine—oxygen bond to form CIO and O. These products then generate secondary products such as chlorine peroxide, ClOO, chlorine, CI2, oxygen, O2, chlorine trioxide [17496-59-2] CI2O2, chlorine hexoxide [12442-63-6] and... 

The first step in the peroxide-induced reaction is the decomposition of the peroxide to form a free radical. The oxygen-induced reaction may involve the intermediate formation of a peroxide or a free radical olefin-oxygen addition product. (In the case of thermal and photochemical reactions, the free radical may be formed by the opening up of the double bond or, more probably, by dissociation of a carbon-hydrogen bond in metal alkyl-induced reactions, decomposition of the metal alkyl yields alkyl radicals.)... 

Gaseous hydrogen peroxide is a key component and product of the earth s lower atmospheric photochemical reactions, in both clean and polluted atmospheres. Atmospheric hydrogen peroxide is believed to be generated exclusively by gas-phase photochemical reactions (lARC, 1985). Low concentrations of hydrogen peroxide have been measured in the gas-phase and in cloud water in the United States (United States National Library of Medicine, 1998). It has been found in rain and surface water, in human and plant tissues, in foods and beverages and in bacteria (lARC, 1985). 

Disulfides (RS-SRJ are not as useful as peroxides in their photochemical reactions, partly because carbon-sulfur bond cleavage (to give R" and RSi) competes more effectively with sulfur-sulfur bond cleavage than does C-O with 0-0 cleavage for peroxides, but largely because thiyl radicals are less reactive than their oxygen analogues. 

The decomposition of liquid water and the following reactions are the results of a typical chemical effect. In this case, however, overall water splitting does not occur because oxygen is not obtained but hydrogen and hydrogen peroxide are. On the other hand, it is impossible to decompose water by photochemical reaction under illumination with a xenon lamp. Although it is possible to decompose water by photocatalytic reaction using a desirable photocatalyst and photoirradiation, it is difficult to decompose in practice because of rapid backward reaction, the formation and accumulation of intermediates onto the surface of photocatalyst,10) and other reasons. 

The products of the photochemical reaction of oxygen and hydrogen in a flow system are ozone, hydrogen peroxide, and water. Mechanisms for the formation of these products are discussed below. 

Salomon J, Elad D (1973) Photochemical reactions of nucleic acid constituents. Peroxide-initiated reactions of purines with alcohols. J Org Chem 38 3420-3421 Salomon J, Elad D (1974) Ultraviolet and y-ray-induced reactions of nucleic acid constituents. Reactions of purines with amines. Photochem Photobiol 19 21-27 Samuni A, Neta P (1973) Hydroxyl radical reaction with phosphate esters and the mechanism of phosphate cleavage. J Phys Chem 77 2425-2429... 

In a systematic ESR and CIDEP study of various alkoxy substituted phenols by photochemical reactions with ketones and with organic peroxides, we have shown (15) that molecular oxygen is not the active radical reactant in the oxidation. Rather, molecular oxygen is necessary to produce peroxy and alkoxy radicals, ROO and RO-which then add onto the phenyl ring to initiate the oxidation processes. The precursor radical R - can be derived from as many ways as one can imagine, both via photo and thermal reactions. 

In addition to the direct absorption as a biological hazard, UV can have additional indirect effects on organisms.26,27 A number of UV photochemical reactions occur in solutions, both within cells and in the external aquatic environment. In the presence of UV, water itself is hydrolyzed, producing hydroxyl ions. Related reactions involving dissolved substances and mediated by UV lead to the formation of peroxides, super oxide, and other radicals. These reactive products are toxic by causing oxidative damage to biological molecules.28-31... 

Kharasch, M. S., J. Kuderna, and W. Nudenberg Reactions of Atoms and Free Radicals in Solution. XXXIII. Photochemical and Peroxide-induced Addition of Cyclohexanone to 1-Octene. J. org. Chem. 18, 1225 (1953). 

Sun Y, Pignatello JJ. Photochemical reactions involved in the total mineralization of 2,4-D by iron(3 +)/hydrogen peroxide/UV. Environ Sci Technol 1993 27 304-310. 

Laser flash photolysis has been used to study the primary photochemical reactions involving the excited state of the photosensitizers as well as the photochemically generated intermediate species. A sequence of reactions leading to oxygen photoreduction, with the concomitant formation of hydrogen peroxide, is proposed in every case. 

Sun, Y. and Pignatello, J.J. (1993) Photochemical reactions involved in the total mineralization of 2,4-D by iron(3+)/hydrogen peroxide/UV. Environ. Sci. Technol. 27, 304-310 Toepfer, B., Gora, A. and Li Puma, G. (2006) Photocatalytic oxidation of multicomponent solutions of herbicides Reaction kinetics analysis with explicit photon absorption effects. Appl. Catal. B Environ. 68,171-180... 

Photochemical reactions of HFA with perfluorinated carbon-oxygen compounds have been reported (271, 272). HFA serves as a mild source of CO in the reaction with bis(trifluoromethyl) peroxide (271) to yield bis(trifluoromethyl) carbonate with perfluoromethyl oxalate, CF3 radicals are the reactive species to yield perfluoromethyl acetate (272). 

Under conditions of total absorbance of the UV radiation by H2O2, a theoretical maximum amount of 2.63 mmol hydrogen peroxide per liter can be decomposed by irradiation with the specified lamp by consuming an energy Eel of 10 kW h. Example 3-7 convincingly demonstrates the relationship of photons to stoichiometric calculations of photochemical reactions (Tab. 3-4, section D). Additional examples of calculations concerning photons and energy of lamps can be found in Wohrle et al. (1998). 

In summary, the main products derived from these photochemical reactions are O3, peroxides (e.g., PAN, PBN, and H202), hydroperoxides, aldehydes, ketones, alcohols, nitro compounds (alkyl and benzyl nitrates and nitrites), and acids such as sulfuric, nitric, and nitrous acid. 

In addition to their importance as precursors of vitamin A, carotenes can also act, at least in vitro (and under conditions of low oxygen tension), as antioxidants, trapping singlet oxygen generated by photochemical reactions or lipid peroxidation of membranes (Burton and Ingold, 1984). Studies with... 

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