Photoperoxidation

The photosensitized results are from I.B.C. Matheson and J. Lee 118h It is seen that the quantum yields in photosensitized oxidation depend on the concentrations of luminol and base, and on temperature. At higher temperature (50°) and low luminol concentrations, the quantum yields reached those of hemin-catalyzed hydrogen peroxide oxidation of luminol in aqueous-alkaline solution. Primary products of the photosensitized oxidation are singlet oxygen (1Ag02) or a photoperoxide derived from methylene blue, but neither of these is directly responsible for the luminol chemiluminescence. 

The reaction mechanism for the aerobic oxidation of the pz to seco-pz can be attributed to a formal 2 + 2 cycloaddition of singlet oxygen to one of the pyrrole rings, followed by cleavage (retro 2 + 2) of the dioxetane intermediate to produce the corresponding seco-pz (160). This mechanism is shown in Scheme 29 for an unsymmetrical bis(dimethylamino)pz. Further photophysical studies show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product, 159. As a consequence, the kinetics of the process is shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. 

Although thiophene derivatives, e.g., tetraphenylthiophene XXI and XXII70 do not appear to form photoperoxides sulfurated 9,10-diphenyl-anthracenes XXIII and cerothiene XXIV derivatives have been reported to form transannular peroxides of the type A on irradiation in the presence of oxygen.209,210... 

Formation of the photo peroxide of azanthracenes has been studied extensively. Acridine and 9-phenylacridine seem not to form photo-peroxides,63 but from 2-phenyl-l-azanthracene07,66 a labile peroxide is obtained. The benzacridines photoperoxides are rapidly transformed into... 

A photoperoxide in the isoindole series has been reported. Upon irradiation of a solution of 1,2,3-triphenyl isoindole (CXIX) in the presence of oxygen, 1,2,3-triphenylisoindole peroxide (CXX)285 is formed. 

C. Kinetics of photoperoxidation reaction. In the sensitized photooxygenation by Type lib mechanism the 02 (JAg) acts as dienophile and adds to give the endoperoxide in concerted addition reaction similar to Diels-Alder... 

The P values for photoperoxidation vary with substrates over five Otders of magnitude p = 55 M for relatively unreactive cyclohexene and p = 3 X 10-4 M for reactive dimethylanthracene and rubrene. P values are also sensitive to the nature of solvent. For autoperoxidation of anthracene in solvents benzene, bromobenzene and CS2, respective values are 0.42,0.12,0.002. 

The quantum yield for photoperoxidation reaction of anthracene in CC14 is expressed by (Sec 11.3.1)... 

The reaction has been studied for the photoperoxidation of hydrocarbon l-methyl-2 pentene in MeOH -f EtOH solution using (3-carotene as the quencher of the reaction. For estimation of rate constants, following procedure has been followed i... 

These data do not apply directly for anthracene + CC14 reactions, but just give a procedure to obtain the photoperoxidation rates. The lifetime of Ot Ag) in CC14 = 700 ps.M The flow sheet for the photoreaction between anthracene and solvent CC14 is summarised as given on page 345. 

Tolyltetrazolonimide. See under Amino-tolyltetrazoles A265-R Total Acidity as Nitric Acid A88-R Total Acidity as Sulfuric Acid A89-L Total Actual Acidity ApO-L Total Nitric as Nitric Acid A88 A89 Total Sulfuric as Sulfuric Acid A89 Transannular Photoperoxide of Anthracene A458-R... 

On kinetic grounds it is not possible to distinguish between the schemes (1) and (2) since both give the same type of dependence on [A]. Some information can be obtained by using suitable quenchers for the reaction. These quenchers may either be competitors and undergo photoperoxidation themselves (chemical type) or quench the reaction by deactivating the intermediates (physical type).1840... 

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