Aromatic rings radical hydroxylation

While radical species can hardly abstract H atom from benzene rings, some of them, in particular HO radical, are easily trapped by aromatic molecules. Radical hydroxylation most often follows Fenton-type chemistry that involves generation of HO in the course of homolytic decomposition of HjOj induced by an iron(II) salt and addition of HO to an aromatic nuclear to give hydroxycyclo-hexadienyl radical I. This radical may dimerize, be oxidized to phenols (Cu(II) is one of the most effective oxidants for this), or undergo an acid-catalyzed collapse to radical cation [15, 28]. Scheme 14.3 shows the classical mechanism suggested by Walling [28]. 

At elevated temperatures, methylene carbons cleave from aromatic rings to form radicals (Fig. 7.44). Further fragmentation decomposes xylenol to cresols and methane (Fig. 7.44a). Alternatively, auto-oxidation occurs (Fig. 1.44b ). Aldehydes and ketones are intermediates before decarboxylation or decarbonylation takes place to generate cresols and carbon dioxide. These oxidative reactions are possible even in inert atmospheres due to the presence of hydroxyl radicals and water.5... 

The uncatalysed p-coumaric acid oxidation led to the formation of intermediates (not shown here) almost similar to those of the catalysed reaction, without formation of dihydroxylated aromatic compounds, such as 3,4- dihydroxybenzaldehyde. This result shows that the catalyst may promote the hydroxylation of aromatic ring by enhancing the formation of hydroxyl radicals in the reaction mixture. 

In addition, phenols are formed by the reaction of hydroxyl radical addition to the aromatic ring of oxidized alkylaromatic hydrocarbon [56]. 

Finally, it makes possible the oxidation of hydrocarbon to a significant depth, and when the RH molecule contains several methyl groups, the catalyst allows all these groups to be transformed into carboxyls. This last specific feature is insufficiently studied so far. Perhaps, it is associated with the following specific features of oxidation of alkylaromatic hydrocarbons. The thermal decomposition of formed hydroperoxide affords hydroxyl radicals, which give phenols after their addition at the aromatic ring... 

Gierer, J., Yang, E. and Reitberger, T. (1992). The reactions of hydroxyl radicals with aromatic rings in lignins, studied with creosol and 4-methylveratrol. Holzforschung, 46(6), 495-504. 

The aromatic-hydroxyl radical reaction has been studied by Davis et They reported rate constants for benzene and toluene and concluded that hydroxyl additions to the aromatic ring compete favorably with the abstraction of hydrogen atom from the alkyl substituent. Doyle et al recently published hydroxyl reaction rate constants for a series of alkylbenzenes. 

The effect of the hydroxyl radical (HO ) on luminol chemiluminescence has also been intensively studied Although detailed mechauisms for the reactiou of hydroxyl radicals with hydrazides remaiu uuknowu, two differeut processes are assumed to be involved oxidation of the hydrazide group and addition of hydroxyl radical to the aromatic ring (Scheme 19) ° . 

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