Aromatic-OH adduct

Xylene and more reactive aromatics XYL Aromatic-OH adduct from TOL ADDT... 

The aromatic-OH reaction proceeds initially either by OH addition to or abstraction from the aromatic ring to form a radical (Reaction la) or an aromatic-OH adduct (Reaction lb). Note that the "R" in all of the reactions presented subsequently refers either to a hydrogen atom or an alkyl group. The depiction is meant... 

Under atmospheric conditions, oxygen is expected to rapidly react with the aromatic-OH adduct, forming either an alcohol (Reaction 2a) or a peroxy radical (Reaction 2b) ... 

Several computational chemistry studies have focused on the reactions of O2 with aromatic-OH adducts [27,43-45]. These studies are in good agreement, indicating that the aromatic-OH adduct will add O2 to form the corresponding peroxy radical, as seen in (Reaction 2b). 

The aromatic OH adduct formed in (Reaction lb) can react with NO2 according to a series of different pathways (Reactions 3a-3e). Experimental data have indicated the formation of stable nitroaromatic products (stable product formed in... 

While several studies have also pointed out that nitroaromatic products are more characteristic of laboratory studies where high NOx mixing ratios are employed rather than the ambient environments where lower NOx mixing ratios exist, understanding the formation mechanisms of these nitroaromatic compounds is critical to properly defining the reaction mechanisms for aromatic compounds. The only computational study to date that probed the NO2 reaction with the aromatic-OH adducts is that of Andino et al. [27], and their work considered only the reactions of the most likely aromatic-OH adduct, as predicted from the computational results. However, theoretical work involving NOx tends to be complicated by symmetry problems, so that reaction energy profiles are difficult to obtain. 

Zetzsch has measured reaction rate constants for the reactions of the aromatic OH adduct with O2, NO2 and NO (Table 16) (units cm molecule" s" at 298 K) ... 

These data show that under atmospheric conditions the adducts will react predominantly with O2 to form hydroxycyclohexadienylperoxy radicals. The subsequent reactions of these peroxy radicals leading to ring-opening are still unclear. Several possible reaction pathways which result in formation of dicarbonyl and unsaturated dicarbonyl compounds are outlined in Fig. 10. Experiments on radical cycling (Zetzsch) indicate that the reaction of the aromatic-OH adducts with O2 produce HO2 radicals with high yield on a short timescale. The mechanism leading to this so-called "prompt HO2" formation in the aromatic compound oxidation systems is presently unclear. 

Becker has performed detailed product studies on the OH reaction with toluene and p-xylene. At present, product formation can best be explained by the mechanism shown in Fig. 10 where muconaldehydes (hexa-2,4-diene-1,6-dial) are proposed as direct products of the reaction of the aromatic-OH adduct with O2. Several of these muconaldehydes have been synthesised and their reactions with OH radicals investigated (Becker). These reactions result in the formation of unsaturated 1,4-dicarbonyl compounds, glyoxal, methylglyoxal and maleic anhydride, which have also been observed in the reaction of OH with toluene and p-xylene. It has been demonstrated that the unsaturated 1,4-dicarbonyl species react very rapidly with OH and photolyse, yielding products which possibly accelerate O3 formation in smog chamber type studies. Many of the aromatic oxidation products especially the unsaturated 1,6-dicarbonyl species are known either to be toxic or are potentially toxic with both carcinogenic and mutagenic properties [8]. 

DFT and ab initio calculations have been used to study the mechanism of the gas-phase oxidation of phenol by HO. Addition of HO to the ort/io-position forms P2, which subsequently combines with O2 at the ip o-position to form adduct P2-1-00. A concerted HO2 elimination from P2-1-00 forms 2-hydroxy-3,5-cyclohexadienone (HCH) as the main product and is responsible for the rate constants for the reaction between P2 and O2 to be about two orders of magnitude higher than those between other aromatic-OH adducts and O2. The HCH subsequently isomerizes to catechol, which is thermodynamically more stable than HCH, possibly through a heterogeneous process. Reaction of P2 with NO2 proceeds by addition to form P2-n-N02 ( = 1, 3, 5) followed by HONO elimination from P2-1/3-N02 to form catechol. The barriers for HONO elimination and catechol formation are below the separate reactants P2 and NO2, being consistent with the experimental observation of catechol in the absence of O2, while H2O elimination from P2-I/3-NO2 forms 2-nitrophenol (2NP). The most likely pathway for 2NP is the reaction between phenoxy radical and N02." ... 

X-ray ionization of o-vinylbenzaldehyde (136) in argon matrices leads to the quinoketene (137) via die radical cation, detected by IR spectroscopy.298 The product identity was confirmed by the independent preparation of (137) and (137+-) by the photo-stimulated ring-opening of 2-methylbenzocyclobutenone (138) (Scheme 21). The reactions of benzaldehyde, acetophenone, and benzophenone with OH, 0 and SC>4 have been studied by pulse radiolysis in aqueous solution.299 The addition of OH to the carbonyl moiety of benzaldehyde predominates over addition to the aromatic ring, whereas ring addition is predominant in the case of acetophenone. Disproportionation of the exocyclic OH adduct is proposed to explain the formation of benzoic acid, which is a major product in the reaction of benzaldehyde and OH or SO4T Rate constants for each reaction have been calculated. 

Aromatic hydroxylation is most commonly used for the detection of OH. However, the primary OH adducts must be oxidized to yield the final product(s). Disproportionation reactions produce these compounds usually only in very low yields. For this reason, an oxidant is required. Although oxygen may serve as an oxidant, the yields are not quantitative because of side reactions (Chap. 8). The addition of a one-electron oxidant, for example Fe(CN)63, may overcome this problem (Volkert and Schulte-Frohlinde 1968 Bhatia and Schuler 1974 Madha-van and Schuler 1980 Buxton et al. 1986), but in certain cases an even stronger oxidant such as IrCl62- maybe required (Fang et al. 1996). 

The OH-adducts of phenols behave differently as compared to those of other aromatic compounds. The parent compound, phenol, has been investigated in quite some detail (Land and Ebert 1967 Raghavan and Steenken 1980 Roder et al. 1999 Mvula et al. 2001). Its ortho- andpara- OH adducts undergo rapid H+/OH -catalyzed water elimination thereby yielding the (thermodynamically... 

In studies on the OH-induced aromatic hydroxylation, the oxidation of hy-droxycyclohexadienyl radicals by Fe(CN)63 has often been used for the determination of the yield of a given precursor radical (Volkert et al. 1967 Volkert and Schulte-Frohlinde 1968 Klein et al. 1975). Other oxidants such as Cu2+, Ag+, Fe3+ or Cr3+ give lower yields, and complications are apparent, since, for example, the oxidation potential of Ag+ (0.8 V) is higher than that of Fe(CN)63 (0.36 V Bhatia and Schuler 1974). The substituent has a strong influence on the rate of oxidation (Table 6.2), and quantitative oxidation to the corresponding phenol [reaction (11)] is only observed with electron-donating substituents (Buxton et al. 1986). Even the terephthalate ion OH-adduct requires the stronger oxidant... 

No doubt, heterocyclizations of 311 to 316 and 311 to 333 are the most complicated transformations, based on nucleophilic substitution of hydrogen, known at present. In spite of moderate yields, the method permits one to obtain in a single preparation step compounds that are otherwise very hard to obtain. Another feature is that for the first time an oxidant serves here not only for aromatization of oH-adducts but also for modification of both reactants before each next stage. This tremendously expands the synthetic possibilities of such reactions that obviously need further extensive development. 

Several computational studies have been conducted on (Reaction lb) to examine the relative importance of the initial OH addition to the ipso, ortho, meta, and para sites on the aromatic ring. A summary of the computational approaches used by different authors and the reaction energies calculated for the OH-adduct formation for the case of toluene appear in Table 14.3. The data indicate that... 

In a broad sense, pulse radiolysis can be also regarded as a labeling technique. It causes, by its oxidative or reductive attack, certain changes (i.e. OH-adducts of aromatic and heterocyclic rings or radical intermediates of cystine) which label definite places in protein molecule. This technique is useful for studying the role of functional groups of enzymes... 

The experimental methodology in radiation-induced oxidation of benzene systems involved the measurement of rate constants and the transient absorption spectra by pulse radiolysis and the determination of yields of hydroxylated products on oxidation of the hydroxycyclo-hexadienyl radicals under steady-state conditions. The two commonly used oxidants — K3Fe(CN) "and IrCl " — convert quantitatively the OH adducts to the corresponding phenolic products. Thus, the pulse radiolysis technique in combination with product analysis using analytical techniques such as UV-VIS spectroscopy, HPLC, GC-MS, etc. under steady state conditions has provided valuable information in the understanding of the oxidation reaction mechanism of aromatics in... 

The higher stability of addition products on olefinic carbon atoms C8 and C7 was explained based on loss of aromaticity because addition to the ring breaks the cyclic delocalization leading to the loss of accounting to around 30 kcal/mol of destabilization of the system. Further excellent linear correlation between the relative stabilities of the OH adducts (after accounting for the aromatic stabilization in olefinic adducts) and the maximum spin density values was also obtained. 

The [Fe(l,10-phen)3] ion (1,10-phen = 1,10-phenanthroline) reacts with [OH] to form an Fe(II) complex with a coordinated ligand-radical consisting of an OH adduct to the aromatic ligand system (A = 460 nm = 6700 M cm ) the intermediate decays via disproportionation at the ligand-radical site". Reaction of [Fefbipy),] " and [Fe(bipy)3] (bipy = 2,2 -bipyridine) with [OH] and H yields radical addition to the aromatic ligand the decay kinetics of the intermediates are complex". 

The [Ru(bipy)3] and [Ru(bipy)3] ions react with [OH] to yield the corresponding OH adducts to the ligand aromatic ring system. The [Ru(bipy)3] ion is reduced by the deprotonated form of a-hydroxyalkyl radicals, but engages only in nucleophilic addition with the protonated forms. ... 

The reaction of [OH] with [Co(NH3)jNC5H5] yields an OH adduct to the aromatic ring (A = 325 nm = 1.7 X 10 M cm ) , which can undergo disproportionation in competition with first-order intramolecular electron transfer to produce Co (k = 2.3 X 10 s ) and second-order bimolecular reaction to yield Co(III) products with modified ligands a similar competition affects the deprotonated ligand radical (k = 11 s )-, ... 

The products arising from the OH radical addition pathways are still not totally understood. The initially formed OH-aromatic adduct is expected to react under atmospheric conditions mainly with 02, again via two reaction pathways. For example, for the toluene-OH adduct,... 

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