Benzenes radical attack

It is estimated that thiophene reacts with phenyl radicals approximately three times as fast as benzene. Intramolecular radical attack on furan and thiophene rings occurs when oxime derivatives of type (112) are treated with persulfate (8UCS(Pt)984). It has been found that intramolecular homolytic alkylation occurs with equal facility at the 2- and 3-positions of the thiophene nucleus whereas intermolecular homolytic substitution occurs mainly at position 2. 

In an unusual example of displacement of fluonne by hydroxyl, hydroxyl radicals attack fluorinated benzenes Hexafluorobenzene is the least reactive The hydroxyl radical generates the pentafluorocyclohexadienonyl radical from it [13] (equation 13) These unstable species are detected spectroscopically Their disap-... 

Separate experiments in which tert.-butoxy radicals were produced thermally in benzene from di-tert.-butyl peroxyoxalate failed to reveal any direct reaction of these radicals with amine II. Even at higher temperatures (A/ 150°C, dichlorobenzene, +00+ decomposition), the +0 radicals attacked neither amine II nor nitroxide I. The earlier described experiments of ketone photooxidation showed additionally that amine II displays no specially marked reactivity towards peroxy radicals. 

A very early indication that C-nitroso-compounds are particularly susceptible to radical attack was the methyl affinity value of 105 determined for nitrosobenzene (Heilman el al., 1957). This figure reflects the reactivity of nitrosobenzene, relative to that of benzene, towards attack by methyl radicals. It was apparently several years before this reactivity was linked with nitroxide formation... 

Reduction of phenyldiazonium chloride in acetonitrile containing a high concentration of an aromatic substrate, which can act as a free-radical trap, leads to phenylation of the substrate in 14 - 33% yields together with 50 - 50% of benzene formed by phenyl radical attack on the acetonitrile [132], Intramolecular capture of the phenyl radical, in an electrochemical equivalent of the Pschorr reaction, is much more successful and phenanthrene derivatives can be prepared in 90 - 96% yield [133],... 

Free radical attack at the pyridine ring is noted for its low selectivity and substituents have little effect. Arylation takes place at all three positions, but halogen atoms preferentially attack the a-, and alkyl radicals the a- and y-positions. Metals such as sodium and zinc transfer a single electron to pyridine to form anion radicals. These can dimerize by reaction at the a- or y-position to yield dipyridyls by loss of hydride ion. Thus, reduction of pyridine by chemical and catalytic means is easier than reduction of benzene. 

Were reversal of Equation 9.99 to compete with the subsequent removal of a hydrogen atom from 40, the rate of the hydrogen removal step would enter the overall rate expression the reaction would then show an isotope effect. (See Section 7.4, p. 385.) In some instances, for example, when the benzoyl radical attacks benzene, the initial addition is apparently reversible,175 and an isotope effect is found.176... 

Study of isomer distribution in substitution of benzene rings already carrying one substituent presents some potential pitfalls. Inspection of product ratios for ortho, meta, and para substitution, as in investigation of electrophilic substitution (Section 7.4, p. 392), might be expected to give misleading results because of the side reactions that occur in radical substitution. The isomeric substituted cyclo-hexadienyl radicals first formed by radical attack partition between the simple substitution route and other pathways (Equation 9.102). In order for the... 

Table 9.12 compares partial rate factors for substitution by phenyl radical with those for electrophilic bromination. Selectivity is clearly much lower for the radical substitution furthermore, for attacking phenyl radical, nearly all positions in the substituted benzenes are more reactive than in benzene itself, a finding that reflects the tendency for most substituents to stabilize a radical, and thus to lower transition state energy for formation of the cyclohexadienyl intermediate, when compared with hydrogen. The strong polar effects, which cause the familiar pattern of activation and deactivation in the electrophilic substitutions, are absent. One factor that presumably contributes to the low selectivity in radical attack is an early transition state in the addition step, which is exothermic by roughly 20 kcal mole-1.178... 

Because hydroxyl radicals have indiscriminate reactivity, they can react with almost all types of organic and inorganic compounds. Most aromatic compounds undergo radical attack on the aromatic ring in a manner similar to that of benzene systems. The products and the rate constants for hydroxyl radical attack on aromatic compounds are listed in Table 5.11. The data were obtained from the pulse radiolysis studies (Buxton et al., 1988). 

The reaction mechanism for the reaction of substituted benzenes with hydroxyl radical appears to be hydroxylation. Hydroxyl radical attack on... 

The dione was a secondary product. These three products show a preferential attack of the hydroxyl radical on the benzene ring at positions 5 and 8, as is expected from literature results (see Fig. 2 in the case where the OFF radical attack is initiated at position 5). The amount of 5-hydroxy quinoline was diminished by a factor of ca. 2 for the photocatalytic degradation. Although 8-hydroxyquinoline was not entirely extracted from the Ti02 surface, it was clear that a lower amount was formed by the heterogeneous... 

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). 

Assuming the usual initial distribution of the three reactive species, it is obvious that OH radical plays the major role in the decomposition of these solutes. Toluene is also significantly attacked by hydrogen atoms. These data may help to explain the effects shown in Table 4. Benzene decomposition was most severely affected by the changing water quality. This may be due to almost complete reliance on OH radical attack, and the presence of a sixfold higher DOC concentration competing for OH in wastewater. 

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]. 

Table 7.1 Partial rate factors for radical attack on benzene rings /is the rate of attack at the site designated relative to the rate of attack at one of the carbon atoms of benzene itself...

At low temperatures, radical attack involves addition to the ring, but above about 600 K the process is reversed, with the exception of O atom attack [114] so that phenol is an important product for benzene. 

Photocatalytic degradation of 4-chlorophenol in TiOz aqueous suspensions produces 4-chlorocatechol, an ortho hydroxylated product, as the main intermediate. This result disagrees with data reported by other researchers, who proposed the formation of a para-hydroxylated product, hydroquinone, as the major intermediate. Results also indicated that further oxidation of 4-chlorocatechol yields hydroxy hydroquinone, which can readily be oxidized and mineralized to carbon dioxide. Complete dechlorination and mineralization of 4-chlorophenol can be achieved. In contrast, direct photolysis of 4-chlorophenol produces hydroquinone and p-benzoquinorie as the main reaction products. The photocatalytic oxidation reaction, initially mediated by TiO2, is generated by an electrophilic reaction of the hydroxyl radical attacking the benzene ring. 

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