Mechanism, radical with aromatic compounds

Kwok, E. S. C., R. Atkinson, and J. Arey, Kinetics and Mechanisms of the Gas-Phase Reactions of the NO, Radical with Aromatic Compounds, Int. J. Chem. Kinet., 26, 511-525 (1994b). 

Figure 11 shows a typical example of the temperature-dependent behavior for the reactions of OH radical with aromatic compounds. The measured bimolecular rate constants of OH radical with nitrobenzene showed distinctly non-Arrhenius behavior below 350°C, but increased in the slightly subcritical and supercritical region. Feng a succeeded in modeling these data with a three-step reaction mechanism originally proposed by Ashton et while Ghandi etal. claimed to have developed a so-called multiple collisions model to predict the rates for the reactions of OH radical in sub- and super-critical water. 

Nitrations are highly exothermic, ie, ca 126 kj/mol (30 kcal/mol). However, the heat of reaction varies with the hydrocarbon that is nitrated. The mechanism of a nitration depends on the reactants and the operating conditions. The reactions usually are either ionic or free-radical. Ionic nitrations are commonly used for aromatics many heterocycHcs hydroxyl compounds, eg, simple alcohols, glycols, glycerol, and cellulose and amines. Nitration of paraffins, cycloparaffins, and olefins frequentiy involves a free-radical reaction. Aromatic compounds and other hydrocarbons sometimes can be nitrated by free-radical reactions, but generally such reactions are less successful. 

These results lead to a general mechanism for the reaction nitrenium ions with aromatic compounds (Fig. 13.71). Initial encounter leads to a 7i-complex (141). The latter is converted into isomeric a complexes (142-144) which, in turn, either tau-tomerize to give stable adducts (145-147) or else dissociate to give radicals. The relative rates of these processes depend on the reactivities of the nitrenium ion and the arene. With less reactive nitrenium ions the 7i-complex is relatively long lived. With more reactive nitrenium ions the n complex forms in a low steady-state... 

Due to its electrophilic properties the OH° reacts at the position with the highest electron density of the target molecule. Detailed information can be found in von Sonntag (1996), which gives a good overview of the degradation mechanism of aromatics by OH° in water. Buxton et al. (1988) showed that the reaction rate constants for hydroxyl radicals and aromatic compounds are close to the diffusion limit. 

The reaction of formamide with aromatic compounds under ultraviolet irradiation is still unexplored and only preliminary results have so far been obtained. In the cases already studied it has been found that this reaction must be sensitized with a ketonic sensitizer, usually acetone, in order to take place. The mechanism of the photoamidation of aromatic compounds certainly differs from the one of simple olefins. The detailed mechanism still awaits further experimental evidence, and in some cases involves, most probably, radical combinations and not addition of radical to unsaturated systems. Interactions of the excited sensitizer with aromatic compounds, having in some cases triplet energies similar or just a bit higher than those of the sensitizers used, must be brought into consideration. Experimentally it has been shown that the photosensitized amidation of benzene leads to benzamide (11),... 

The reaction of free radicals with aromatic amines and phenols may proceed by two routes, abstraction of H from the O—H or N—H bond and addition of the radical to an inhibitor. Free radicals and atoms, H-, HO, CH3 and C6H5, are known to add to aromatic compounds. Both abstraction and addition are observed simultaneously in alcohol oxidation (the Boozer and Hammond mechanism [74]), while hydrocarbon peroxy radicals only abstract H-atoms from inhibitors. ... 

Neta P, Madhavan V et al (1977) Rate constants and mechanism of reaction of sulfate radical anion with aromatic compounds. J Am Chem Soc 99(1) 163-164... 

The mechanism of reactions of the nitrate radical with aromatic and substituted aromatic compounds has been studied in detail using the photolysis of CAN [43, 44]. The photochemical reaction of CAN (Equation 4.4) with toluene derivatives in acetonitrile was found to produce side-chain nitroxidation products in high yields ... 

It is commonly accepted that reactions of CH with unsaturated hydrocarbons proceed without any entrance barrier via the formation of an initial intermediate which decomposes via hydrogen elimination. These reactions are exothermic and show several exit pathways due to the existence of isomers of final products. However there is debate on the exact nature of the reaction mechanism. As for the reactions of CH with acetylene and ethylene, the most likely mechanism seems to be the addition of the CH radical on the carbon multiple bonds to form a 3-carbon-atom cycle. It is believed that this cyclic intermediate isomerises to give linear intermediates which decompose to give the final products. The insertion of the CH radical in the C-H bond of the molecule is not thought to be a favourable entrance chaimel. For the reactions of CH with aromatic compounds a similar mechanism is expected. With respect to anthracene the observed behaviour of the rate coefficient is usually indicative of a reaction which... 

WHa(A -C5H6)2 only reacts with aromatic compounds CeHgR to produce WHCCeHiRlC/t -CsHsla in the presence of conjugated dienes. This suggests that hydrogen abstraction by the diene is an important feature of the mechanism. This, and the observation that for R H only the p-isomer of the product is obtained, can be accommodated by the sequence shown in Scheme 3. A free-radical mechanism seems unlikely as one would then expect m- and p-compounds. ... 

Various other observations of Krapcho and Bothner-By are accommodated by the radical-anion reduction mechanism. Thus, the position of the initial equilibrium [Eq. (3g)] would be expected to be determined by the reduction potential of the metal and the oxidation potential of the aromatic compound. In spite of small differences in their reduction potentials, lithium, sodium, potassium and calcium afford sufficiently high concentrations of the radical-anion so that all four metals can effect Birch reductions. The few compounds for which comparative data are available are reduced in nearly identical yields by the four metals. However, lithium ion can coordinate strongly with the radical-anion, unlike sodium and potassium ions, and consequently equilibrium (3g) for lithium is shifted considerably... 

Aromatic compounds can also be arylated by aryllead tricarboxylates. Best yields ( 70-85%) are obtained when the substrate contains alkyl groups an electrophilic mechanism is likely. Phenols are phenylated ortho to the OH group (and enols are a phenylated) by triphenylbismuth dichloride or by certain other Bi(V) reagents. O-Phenylation is a possible side reaction. As with the aryllead tricarboxylate reactions, a free-radical mechanism is unlikely. ... 

The photolysis of chlorinated aromatic compounds occurs by several processes which follow predictable routes 13). They frequently undergo photochemical loss of chlorine by dissociation of the excited molecule to free radicals or, alternatively, through a nucleophilic displacement reaction with a solvent or substrate molecule. Either mechanism is plausible, and the operation of one or the other may be influenced by the reaction medium and the presence of other reagents. 

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