Benzenoid radical

Hitchcock PB, Lappert MF, Protchenko AV. The first crystalline alkali metal salt of a benzenoid radical anion without a stabilizing substituent and of a related dimer X-ray strucmres of the toluene radical anion and of the benzene radical anion dimer potassium-crown ether salts. Am Chem Soc. 2001 123 189-190. 

Ha.logena.tlon, One review provides detailed discussion of direct and indirect methods for both mono- and polyhalogenation (20). As with nitration, halogenation under acidic conditions favors reaction in the benzenoid ring, whereas reaction at the 3-position takes place in the neutral molecule. Radical reactions in the pyridine ring can be important under more vigorous conditions. 

The production of aryl radicals from peroxides normally provides a cleaner method of arylation than the methods based on the decomposition of azo and diazo compounds, and, in the case of benzenoid compounds, better yields of arylated products are obtained. The... 

A distinction between these four possibilities can be made on the basis of the kinetic isotope effect. There is no isotope effect in the arylation of deuterated or tritiated benzenoid compounds with dibenzoyl peroxide, thereby ruling out mechanisms in which a C5— bond is broken in the rate-determining step of the substitution. Paths (ii) and (iii,b) are therefore eliminated. In path (i) the first reaction, Eq. (6), is almost certain to be rate-determining, for the union of tw o radicals, Eq. (7), is a process of very low activation energy, while the abstraction in which a C—H bond is broken would require activation. More significant evidence against this path is that dimers, Arz, should result from it, yet they are never isolated. For instance, no 4,4 -dinitrobiphenyl is formed during the phenylation of... 

As a result of most of the early work on the homolytic arylation of monosubstituted benzenoid compounds it was concluded that the attacking radical was directed to the ortho and para positions in the benzene ring regardless of the nature of the substituent. Similarly,... 

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. 

Masnovi et al., 1986). As applied to benzenoid donors ArH, the formation of the pertinent ion-radical pair by charge-transfer excitation is summarized in Scheme 9. All the experimental observations with various benzenoid donors and tetranitromethane indeed coincide with the formulation in Scheme 9. Thus the exposure of ArH to tetranitromethane (TNM) as in Fig. 13 leads immediately to the EDA complex in (53) (Sankararaman et al.,... 

The simultaneous annihilation of the aromatic cation radical by its triad partners N02 and pyridine, as presented in Scheme 12, is most apparent in the nature of the aromatic product (i.e., stoichiometry) from mesitylene, among all the other aromatic (benzenoid) donors. Thus, the direct incorporation of N02 and pyridine into the aromatic nucleus is shown by the concomitant ring nitration and pyridination of mesitylene (Kim et al., 1993),... 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

The typical reactions of alkyl groups attached to benzenoid rings involve benzyl-type radical intermediates. An azine ring can stabilize a methyl radical just as can a phenyl ring, and thus most alkylpyridines and azines show these reactions. 

Electron transfer to and from the exocyclic double bond of diphenylmethylenes 94c306 and 96c307 has been effected by electrochemical techniques. In each case reduction gives a stable anion radical while oxidation leads to a quasistable cation radical that can be further oxidized to a very short-lived dication. The spectroelectrochemical results306,307 show that naphthalene 96c" is more stable than the benzenoid 94" as excitation requires... 

Rule A-56. Radicals for Non-benzenoid Ring Systems (Alternative in part to Rule A-55.1)... 

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