Benzene radical substitution

Toluene, an aLkylben2ene, has the chemistry typical of each example of this type of compound. However, the typical aromatic ring or alkene reactions are affected by the presence of the other group as a substituent. Except for hydrogenation and oxidation, the most important reactions involve either electrophilic substitution in the aromatic ring or free-radical substitution on the methyl group. Addition reactions to the double bonds of the ring and disproportionation of two toluene molecules to yield one molecule of benzene and one molecule of xylene also occur. 

Ueno and coworkers10 have found that the facile displacement of sulfonyl group from a-alkylated allyl p-tolyl sulfones 18 by tri-n-butyltin radical in the presence of 2,2 -azobis[2-methylpropanenitrile] (AIBN) occurs smoothly in refluxing benzene (equation 11). In contrast, vinyl sulfones undergo the radical substitution reaction to give vinylstannanes in the presence of AIBN at a higher temperature11. 

In alternant hydrocarbons (p. 55), the reactivity at a given position is similar for electrophilic, nucleophilic, and free-radical substitution, because the same kind of resonance can be shown in all three types of intermediate (cf. 20,22, and 23). Attack at the position that will best delocalize a positive charge will also best delocalize a negative charge or an unpaired electron. Most results are in accord with these predictions. For example, naphthalene is attacked primarily at the 1 position by NOj, NHJ, and Ph, and always more readily than benzene. 

There are some inherent limits to the usefulness of such reactions. Radical substitutions are only moderately sensitive to substituent directing effects, so that substituted reactants usually give a mixture of products. This means that the practical utility is limited to symmetrical reactants, such as benzene, where the position of attack... 

A review considering the generation and characterization of radical ions, their reactions, formation of species with three-electron bonds, and radical cations of strained systems has been published." The redox and acidity properties of a number of substituted benzene radical cations were smdied by pulse radiolysis. ... 

The addition of silyl radicals to double bonds in benzene or substituted benzenes (Reaction 5.2) is the key step in the mechanism of homolytic aromatic substitution with silanes [8,9]. The intermediate cyclohexadienyl radical 2 has been detected by both EPR and optical techniques [21,22]. Similar cyclohex-adienyl-type intermediates have also been detected with heteroaromatics like furan and thiophene [23]. 

Vinyl ether radical-cations also react in a radical substitution fashion with an adjacent electron rich benzene ring [59]. However the reaction products from simple examples such as 33 themselves readily undergo a further anodic oxidation... 

Compounds in the two groups differ in a number of ways. The two differ chemically in that the aliphatic undergo free-radical substitution reactions and the aromatic undergo ionic substitution reactions. In this chapter you examine the basics of both ciromatic and heterocyclic ciromatic compounds, concentrating on benzene and related compounds. 

This diazotization reaction is compatible with the presence of a wide variety of substituents on the benzene ring. Arenediazonium salts are extremely important in synthetic chemistry, because the diazonio group (N=N) can be replaced by a nucleophile in a radical substitution reaction, e.g. preparation of phenol, chlorobenzene and bromobenzene. Under proper conditions, arenediazonium salts react with certain aromatic compounds to yield products of the general formula Ar-N=N-Ar, called azo compounds. In this coupling reaction, the nitrogen of the diazonium group is retained in the product. 

Homolytic (free-radical) substitution may occur in any of the 2 to 6 positions of pyridine. Thus, the reaction of pyridine with benzene-diazonium salts gives a mixture of 2-, 3-, and 4-phenylpyridine. 

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... 

Radical fragmentation of 2-nitrophenyl-azo-trityl resin was studied in the presence of various radical acceptor solvents to elucidate possible radical reaction pathways. When using benzene as solvent, only 2-nitro-bi-phenyl was formed as the product of radical substitution reaction (SNR) in 67% yield. Hydrogen-radical abstraction from the polymer backbone (e.g., from the benzylic units of polystyrene) was completely suppressed. When toluene was used as solvent, a mixture of the following products was obtained nitrobenzene, 4-methyl-2 -nitrobisphenyl, 2-methyl-2 -nitro-bisphenyl, and 3-methyl-2 -nitrobisphenyl (9 9 1 1). In the case of toluene, the nitro-aryl radicals undergo H-abstraction with radical substitution as a competing reaction pathway. These results indicate that H-abstraction... 

Radical substitution may also proceed through the cation radical stage. The monograph by Nonhebel and Walton (1974) discusses the introduction of the benzoyloxy group into the aromatic ring. Thus, the interaction of benzoyl peroxide with the benzene deriva-... 

Radical substitution reactions involving allylic tin derivatives could be accompanied by a photoinduced 1,3-rearrangement54,55. A photostationary mixture of cinnamyl(tri-phenyl)stannane with its regioisomer l-phenylprop-2-enyl(triphenyl)stannane has been shown to form in the photolysis of ( )-cinnamyl(triphenyl)stannane in benzene under aerobic conditions, or in the presence of halogenated organic compounds or radicaltrapping reagents (equation 21). 

At 400°C and without a catalyst, chlorine reacts in its atomic form and causes a free-radical substitution. Of all the bonds to carbons of the benzene ring, the bond between carbon and nitrogen is the weakest (292 kJ, 70 kcal/mol) compared to carbon-hydrogen bond (415 kJ, 99 kcal/mol) and carbon-fluorine bond (443 kJ, 106 kcal/mol). Therefore, atomic chlorine cleaves the bond between carbon and nitrogen and replaces the nitro group. The product is l-chloro-3,4-difluorobenzene (compound Q) [36. ... 

Benzene and substituted benzenes normally act as six-electron donors, although dihapto and tetrahapto complexes are also known. Dibenzenechromium was prepared eariy in this century but was not characterized until 1954. It was first synthesized via a Grignard synthesis. When PhMgBr reacts with CrClj in diethylelher solvent, a mono-hapto complex [CrPh lEt O) ] forms which rearranges. presumably by a free radical reaction to give, among other products, This ion can be reduced... 

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