Phenols, reaction with singlet oxygen

Tratnyek, P.G. and Hoigne, J. Oxidation of substituted phenols in the environment a QSAR analysis of rate constants for reaction with singlet oxygen. Environ. Sci. Tecbnol., 25(9) 626-631, 1991. 

Haag and Mill (1987) reported rate constants and correlation equations for oxidation of a series of phenylazonaphthol dyes, where oxidation of the naphthol and naphtholate ion are accounted for mainly by reaction with singlet oxygen, with some contribution from R02. Naphtholate anion is about ten times more reactive than naphthol toward oxidants. The correlation equations for these compounds are similar to those used for phenol oxidations (Tratnyek and Hoigne, 1991) ... 

Surface water photooxidation ty, = of 77-3840 h in water, based on reported reaction rate constants for RO2 radical with the phenol class (Mill Mabey 1985 selected, Howard et al. 1991) ti/j 58 d, estimated for photooxidation via peroxy radicals and ty, = 2600 yr for reaction with singlet oxygen in water (Howard 1991)... 

Scully FE, Jr, Hoigne JR. 1987. Rate constants for reactions of singlet oxygen with phenols and other compounds in water. Chemosphere 16 681-694. 

Aromatic hydroxylation.1 The reaction of l-isopropylidene-2-indanone (1) with singlet oxygen (Rose bengal) in CH3CN at —35° results in an unstable product 2. which on deoxygenation with P(OC2Hs)3 affords the phenol 3 in 55% yield. The initial reaction is presumed to form an endoperoxide (a) by a [4+2]cycloaddition, which rearranges to 2. 

Figure 7 Direct photochemical reaction pathway of Triclosan (5-chloro-2-(2,4-di-chlorophenoxy)phenol) in natural water (3-100 iM of Triclosan in pH-bujfered solutions), irradiated with a Hg-vapour lamp. Only the triclosan phenolate anion (present in water at pH >8) is susceptible to significant photochemical transformation and will also react through indirect photolysis (not shown) with singlet oxygen ( O2) and OH radicals to form 2,4-dichloro-phenol (2,4-DCP). The photoproduct 2,8-dichlorodibenzo-p-dioxin (2,8-DCDD) belongs to a notoriously persistent group of chemicals commonly referred to as dioxins ...

Scully, F. E., and Hoign6, J. (1987) Rate Constants for Reactions of Singlet Oxygen with Phenols and other Compounds in Water, Chemosphere 16, 681-694. 

A more extensive evaluation of the reaction of singlet oxygen with substituted phenols has been reported " and some of the data are compiled in Table 6.14. At a given pH, the overall constant would be a composite of both the neutral and charged forms ... 

Figure 6.15 Influence of pH on the rate of oxidation of aqueous solutions of phenols by singlet oxygen. [Reproduced from F. E. Scully and J. Hoigne, Rate constants for reactions of singlet oxygen with phenols and other compounds , Chemosphere 16, 681. Copyright 1987, with permission from Elsevier.]...

TABLE 6.14 Reaction of Singlet Oxygen with Phenols—Rate Constants... 

Energy transfer, discussed above, is one example of the direct reaction of the excited triplet state. It is also possible that the excited triplet could react directly through electron or hydrogen-atom transfer. The photochemical reactivity of a series of phenols (methyl and methoxy substituents) in the presence of natural DOM and ketone sensitizers can be best explained by the latter processes. Reactions were carried out at pH 8 where phenols existed primarily (>98%) in the undissociated form, which were shown (p. 225) not to be particularly reactive with singlet oxygen. 

Phenols with bulky ortho- and para-substituents, eg, phenoHc antioxidants, do not undergo this reaction however, they scavenge radicals generated by thermolysis of diacyl peroxides and other peroxides. Diacyl peroxides react with potassium superoxide, KO2, forming singlet oxygen (207). 

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