Radical reactions, aromatic compounds

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. 

Davis, D. D., W. Bollinger, and S. Hscher. A kinetics study of the reaction of the OH free radical with aromatic compounds. I. Absolute rate constants for reactions with benzene and toluene at 300"K. J. Phys. Chem. 79 239-294, 1975. 

Free-Radical Chemistry Kinetic and Spectroscopic Studies of Reactions of NO, and S04 Radicals with Aromatic Compounds, Faraday Discuss., 100, 129-153 (1995). 

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). 

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. 

Neta P, Dorfman LM. Pulse radiolysis studies. XIII. Rate constants for the reaction of hydroxyl radicals with aromatic compounds in aqueous solutions. Adv Chem Ser 81. Washington, DC American Chemical Society, 1968 222-230. 

Reaction of monocyclic aromatics with O3 and NO3 radicals is generally very slow and unimportant. Atmospheric degradation of aromatics is initiated by OH radical attack. The kinetics of the reaction of OH radicals with aromatic compounds are well established [65]. As seen from Table 2 the fife-time of aromatics with respect to reaction with OH is typically a few days or less. Reaction proceeds by addition to the ring and H-atom abstraction from either the substituent groups or possibly from the ring C - H sites. In all cases the addition channel is dominant. For benzene, toluene, 0, m, p-xylene, and ethyl benzene the H-atom abstraction pathway accounts for 5-10% of the overall reaction [15], the remaining 90-95% proceeds via addition. For toluene ka/(ka + h,) = 0.07 [15,65]. 

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. 

UnsaCuraCed Aromatic Compounds. Although they do not ordinarily do so in free-radical polymerization, aromatic compounds with allyl or vinyl side chains should be capable, under certain conditions, of undergoing reaction not only in the side chains but also at the ring. This has... 

Pulse Radiolysis Studies. XIII. Rate Constants for the Reaction of Hydroxyl Radicals with Aromatic Compounds in Aqueous Solutions... 

Table I. Absolute Rate Constants for the Reactions of Hydroxyl Radical with Aromatic Compounds...

This investigation demonstrates that progress in the understanding of the complete mechanism of reactions involved in the attack of radicals on aromatic compounds is much more difficult owing to the variety of products formed in homolytic reactions. 

Rate coefficients for the reactions of OH radicals with aromatic compounds... 

The Arrhenius parameters of the reactions of hydroxyl radicals with aromatic compounds, listed in Table 2, are based on the rate coefficients (cm molecules" s ) of the reference compounds taken from recent evaluations of OH radical reactions / (2,3-dimethylbutane) = 6.2 x 10" [1], independent of temperature / (diethyl ether) = 7.3 x 10" exp(158K/T), 242-440 K [3] ... 

In particular, the concepts of potential control and the large driving force for chemical change available at electrodes generated two types of investigation. The first type concerned the realization that the first step in many electrode reactions is a simple, reversible one-electron (le ) transfer to/from the organic molecules and that stable intermediates -for example, anion radicals and cation radicals of aromatic compounds and transition... 

Davis, D. D., Bollinger, W., Fischer, S., A Kinetics Study of the Reaction of the OH Free Radical with Aromatic Compounds I. Absolute Rate Constants for Reaction with Benzene and Toluene at 300 K, J Phys Chem. (1975) 19> 293. 

Vernin, G., Jauffred, R., Richard, C., Dou, H.J.M., and Metzger, J., Reaction of thiazoyl-2-yl and benzothiazol-2-yl radicals with aromatic compounds,/. Chem. Soc., Perkin Trans. 2,1145,1972. Riou, C., Poile, J. C., Vemin, G., and Metzger, J., Les reactions de transposition photochimique en serie h terocycHque. IV. Photoisom rization des phenyl-m thylthiazole et isothiazoles isomers. Tetrahedron, 30, 879,1974. 

Z.B. Alfassi, R.H. Schuler, Reaction of azide radicals with aromatic compounds. Azide as a selective oxidant. Journal of Physical Chemistry 89 (1985) 3359—3363. 

As is broadly true for aromatic compounds, the a- or benzylic position of alkyl substituents exhibits special reactivity. This includes susceptibility to radical reactions, because of the. stabilization provided the radical intermediates. In indole derivatives, the reactivity of a-substituents towards nucleophilic substitution is greatly enhanced by participation of the indole nitrogen. This effect is strongest at C3, but is also present at C2 and to some extent in the carbocyclic ring. The effect is enhanced by N-deprotonation. 

Alkanes can be simultaneously chlorinated and chlorosulfonated. This commercially useful reaction has been appHed to polyethylene (201—203). Aromatics can be chlorinated on the ring, and in the presence of a free-radical initiator alkylaromatic compounds can be chlorinated selectively in the side chain. King chlorination can be selective. A patent shows chlorination of 2,5-di- to 2,4,5-trichlorophenoxyacetic acid free of the toxic... 

A chlorohydrin has been defined (1) as a compound containing both chloio and hydroxyl radicals, and chlorohydrins have been described as compounds having the chloro and the hydroxyl groups on adjacent carbon atoms (2). Common usage of the term appHes to aUphatic compounds and does not include aromatic compounds. Chlorohydrins are most easily prepared by the reaction of an alkene with chlorine and water, though other methods of preparation ate possible. The principal use of chlorohydrins has been as intermediates in the production of various oxitane compounds through dehydrochlorination. 

In this paper the electtode anodic reactions of a number of dihydropyridine (DHP) derivatives, quantum-chemical calculations of reactions between DHP cation-radicals and electrochemiluminescers anion-radicals (aromatic compounds) and DHP indirect ECL assay were investigated. The actuality of this work and its analytical value follow from the fact that objects of investigation - DHP derivatives - have pronounced importance due to its phaiTnacology properties as high effective hypertensive medical product. 

The major organic reactions of BrCl consist of electrophilic brominations of aromatic compounds. Many aromatic compounds do not react in aqueous solution unless the reaction involves activated aromatic compounds (an example being phenol). Bromine chloride undergoes free-radical reactions more readily than bromine. 

This last result bears also on the mode of conversion of the adduct to the final substitution product. As written in Eq. (10), a hydrogen atom is eliminated from the adduct, but it is more likely that it is abstracted from the adduct by a second radical. In dilute solutions of the radical-producing species, this second radical may be the adduct itself, as in Eq. (12) but when more concentrated solutions of dibenzoyl peroxide are employed, the hydrogen atom is removed by a benzoyloxy radical, for in the arylation of deuterated aromatic compounds the deuterium lost from the aromatic nucleus appears as deuterated benzoic acid, Eq. (13).The over-all reaction for the phenylation of benzene by dibenzoyl peroxide may therefore be written as in Eq, (14). 

In many cases, however, the ortho isomer is the predominant product, and it is the meta para ratio which is close to the statistical value, in reactions both on benzenoid compounds and on pyri-dine. " There has been no satisfactory explanation of this feature of the reaction. One theory, which lacks verification, is that the radical first forms a complex with the aromatic compound at the position of greatest electron density that this is invariably cither the substituent or the position ortho to the substituent, depending on whether the substituent is electron-attracting or -releasing and that when the preliminary complex collapses to the tr-complex, the new bond is most likely to be formed at the ortho position.For heterocyclic compounds such as pyridine it is possible that the phenyl radical complexes with the nitrogen atom and that a simple electronic reorganization forms the tj-complex at the 2-position. 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

On the basis of the reaction of alkyl radicals with a number of polycyclic aromatics, Szwarc and Binks calculated the relative selectivities of several radicals methyl, 1 (by definition) ethyl, 1.0 n-propyl, 1.0 trichloromethyl, 1.8. The relative reactivities of the three alkyl radicals toward aromatics therefore appears to be the same. On the other hand, quinoline (the only heterocyclic compound so far examined in reactions with alkyl radicals other than methyl) shows a steady increase in its reactivity toward methyl, ethyl, and n-propyl radicals. This would suggest that the nucleophilic character of the alkyl radicals increases in the order Me < Et < n-Pr, and that the selectivity of the radical as defined by Szwarc is not necessarily a measure of its polar character. 

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