Hydroxycyclohexadienyl radical

In the case of diaryl sulfoxides the formation of both the aryl radical and the hydroxycyclohexadienyl radical was observed optically. Veltwisch and coworkers45 studied also the reaction of OH radicals from radiolysis of aqueous solutions of mixed (alkyl phenyl) sulfoxides (PhSOR). They found the formation of both alkylsulfinic and phenylsulfinic acids. 

This indicates that there is a bimolecular-type first step in which the hydroxycyclohexadienyl radicals are formed. 

In a consecutive step, a first-order process, the hydroxycyclohexadienyl radicals decompose to give the sulfinic acids as is given in reaction 30. The rate constants /c, and k2 are also given in Table 1. In this case kt was calculated from the build-up of the hydroxycyclohexadienyl radicals. 

The variations of obs with temperature are ah very similar, but the absolute values of obs depend on the substituent on the aromatic ring. To explain these results, Ashton et al. [82] proposed the following mechanism whereby OH adds reversibly to form a 71-complex as a precursor of the hnal cr-bonded hydroxycyclohexadienyl radical ... 

Hydroxy radicals produced in an aqueous medium [Eq. (9.134)] readily attack benzene to form the 94 hydroxycyclohexadienyl radical (Scheme 9.19) ... 

The HO-toluene adduct corresponds to the methyl-substituted o-, m- and p-hydroxycyclohexadienyl radical isomers. The kinetics of formation and of unimolecular dissociation of HO-aromatic adducts have been studied extensively [30], The benzyl radical -CH2 is converted to the aldehydic product 0-CHO in the presence of NO via a series of reactions analogous to those involving simple alkyl radicals. Bandow et al. [144-146] have determined the yields of aromatic aldehydes to be < 12% of the overall reactions of toluene, xylenes, and trimethylbenzenes, and thus, the H-atom abstraction channel is relatively small but significant. In the case of 0-CHO, the... 

The terephthalate system follows the same principle. It has the advantage that the main product, 2-hydroxyterephthalate, is the only product which fluoresces and thus can be easily detected, even at low concentrations. Details of the mechanism have been elucidated (Fang et al. 1996). Hydroxyl radicals [fc( OH + terephthalate) = 3 x 109 dm3 mol-1 s 1] react preferentially (85%) at the 2-position [reaction (48)]. The resulting OH adduct is much more difficult to oxidize than many other hydroxycyclohexadienyl radicals (Chap. 6), and the more powerful oxidant IrCl62- is required for a quantitative oxidation [reaction (45)]. With O2 as the oxidant [reactions (46)-(50)], the yield of 2-hydroxyterephthalate is only... 

Brocks JJ, Beckhaus H-D, Beckwith ALJ, Ruchardt C (1998) Estimation of bond dissociation energies and radical stabilization energies by ESR spectroscopy. J Org Chem 63 1935-1943 Buxton GV, Langan JR, Lindsay Smith JR (1986) Aromatic hydroxylation. 8. A radiation chemical study of the oxidation of hydroxycyclohexadienyl radicals. J Phys Chem 90 6309-6313 Buxton GV, GreenstockCL, Helman WP, Ross AB (1988) Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals ( OH/O -) in aqueous solution. J Phys Chem Ref Data 17 513-886... 

Table 6.2. Rate constants of the oxidation of hydroxycyclohexadienyl radicals by IrClg2 and Fe(CN)63- (unit 109 dm3 mol 1 s 1) ...

Fig.8.2. Pulse radiolysis of IS O-saturated aqueous solutions of anisole. Spectrum of the anisole-derived hydroxycyclohexadienyl radicals. Inset shows their decay in the presence of 15% O2 and formation of the "plateau". (Source Fang et al. 1995a, with permission)...

Subsequent to the rapid attainment of this near-equilibrium situation, the radicals decay more slowly by both unimolecular and bimolecular processes (discussed below). For this simple case, it can be shown that when the decay process is much slower than the rates of the forward and reverse reactions, the observed rate constant for the disappearance of R- is given by the expression k0bS = forward [02] + Reverse- When fc0bs of the decay of the hydroxycyclohexadienyl radicals is plotted as a function of the 02 concentration, the slope represents the rate constant of the forward reaction, and the intercept that of the reverse reaction (Fig. 8.3). 

Aromatic rings are susceptible to OH attack. The oxidation is initiated by addition to the ring, which generates the hydroxycyclohexa-dienyl radical, shown in Eq. (77). For halogenated benzenes, OH- attack at an unsubstituted carbon is preferred. The hydroxycyclohexadienyl radical may disproportionate to produce phenol, shown in Eq. (78). 

Mantaka A, Marketos DG, Stein G. Continuous gamma and pulse radiolysis of aqueous benzene solutions some reactions of the hydroxycyclohexadienyl radical. J Phys Chem 1971 75 3885-3889. 

Biphenyls, formed via dimerization of the hydroxycyclohexadienyl radical intermediates are by-products in these reactions. Competition between these reactions is dependent on the Fe(III) concentration, since phenols are formed via electron transfer oxidation of the intermediate by Fe(III) [reaction (51)]. It should be emphasized, however, that the yields of phenols in these reactions are generally not impressive. The difficulty can mainly be attributed to further oxidation of the phenol product under reaction conditions. 

The reaction of OH radicals with benzoate has been found to produce the three isomeric hydroxycyclohexadienyl radicals (Eiben and Fessenden, 1971). Moreover, addition to each para- and metaposition was found to take place with similar rate, whereas addition to each ort/io-position proceeds at only about half that rate. 

The majority of the reaction proceeds via the addition channel (b) to give a methyl hydroxycyclohexadienyl radical. Cyclohexadienyl radicals are resonance stabilized and are relatively unreactive. Typical alkyl radicals add O2 rapidly with rate constants of the order of 10 12, in contrast the reaction of cyclohexadienyl and methyl cyclohexadienyl radicals with O2 proceed with rate constants of (2 - 5) x 10 16 cm3 molecule 1 s 1 [67,68]. It can be calculated that in one atmosphere of air the cyclohexadienyl radicals have a lifetime of... 

Among benzene derivatives, halogen-substituted compounds have been extensively studied and in the structure-reactivity studies " carried out on the reaction of OH and SO with the ortho and meta isomers of dichloro and dibromobenzenes and mono-bromotoluenes, the formation of substituted hydroxycyclohexadienyl radical was observed to be the major reaction channel. The bimolecular rate constants obtained for the reaction of OH with substituted halobenzenes are in the range (1.7 to 9.3) x 10 dm mol s. The rate constants obtained are found to follow the Hammett relationship for the reaction of OH with substituted halobenzenes and the p was found to be -0.5, indicating that OH radicals react by addition to the benzene ting. 

The stabilization of hydroxycyclohexadienyl radicals by -CHj and halogen groups is apparent from the bathochromic shifts in the spectra of OH adducts of chloro- and bromo-benzenes (325 nm) and toluene (317 nm) relative to benzene (313 nm). This is in accordance with the work of Chen and Schuler where the phenyl substituent leading to a more intense and a strong red shift in the transient absorption spectra was seen in the reaction of OH radical with biphenyl. 

The reaction mechanism reported in the reaction of OH with benzaldehydes is shown in Scheme 2. The OH radical attacks the ring [reaction (2)] and the -CHO group [reaction (4)]. The attack of OH on the ortho-position of benzaldehyde was considered negligible because of the lack of formation of salicylaldehyde under steady-state conditions. The two hydroxycyclohexadienyl radicals (structures 2 and 3) were oxidized to the corresponding phenols in the presence of an oxidant [reaction (3)], while the ipso OH adduct (structure 4) was not. 

Bhatia K, Schuler RH. (1974) Oxidation of hydroxycyclohexadienyl radical by metal ions. J Phys Chem 78 2335-2338. 

Sulfur-centered radical cations derived from aromatic thioethers (Ar-S-Ar) have been investigated much less extensively. An important feature of one-electron oxidation of aromatic thioethers is the lack of dimeric radical sulfur radical cations (ArS.. S-Ar) because of the spin delocalization onto the aromatic ring. Oxidation of thioanisole (Ar-S-CHj) by OH radicals was studied using pulse radiolysis. At neutral pH, OH addition led to the prompt formation of monomeric sulfur radical cations and hydroxycyclohexadienyl radicals (see Scheme 8). 

The pf -value for the hydroxycyclohexadienyl radical formed by addition of OH to an aromatic ring is above 14, similar to /3-hydroxy-alkyl radicals as might be expected from the electron distribution. 

All phenols react very rapidly with OH radicals via addition and the adducts undergo water elimination to form phenoxyl radicals (as discussed in the chapter on transient phenoxyl radicals). All phenols also react very rapidly with H atoms via addition to the ring to form hydroxycyclohexadienyl radicals (equation 2). 

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