Benzenes, substituted, from

The above definition implies that the reactivity of an aromatic compound depends upon the reaction which is used to measure it, for the rate of reaction of an aromatic compound relative to that for benzene varies from reaction to reaction (table 7.1). However, whilst a compoimd s reactivity can be given no unique value, different substitution reactions do generally set aromatic compoimds in the same sequence of relative reactivities. 

The 4-nitromethy (benzene arises from ipso attack of NOW at the isopropyl-substituted ring carbon. Unlike methyl, the isopropyl group is eliminated rapidly as propene. Can we say that the other products, 3 and 4, arise by direct substitution Evidently not, because nitration at 0° gives two other products, 5 and 6, which must be formed by ipso attack at the inelhyl-heanng carbon. At low temperatures, intermediate ion 7 is attacked by the weakly nucleophilic ethanoate ion to give 5 and 6. Both of these adducts solvolyze rapidly in 78% sulfuric acid to give 3 only ... 

The concept of electronic delocalization has germinated in the pre-electron period to Kekule s structural theory and its application to benzene as a prototype of a family of compounds so-called aromatics . Kekule had to address two major properties of benzene revealed from substitution experiments. The first was the empirical equivalence of all positions of benzene, what is called today the Dfjh symmetry of both geometry and electronic structure, and second the persistence of the aromatic essence in chemical reactions, what we recognize today as aromatic stability . Thus, Kekule postulated that there is a Ce nucleus and the four valences of the carbons are distributed to give two oscillating structures, which when cast in our contemporary molecular drawings look like part a in Scheme 2.39-44 One of the many alternative hypotheses on the nature of... 

Arenes usually undergo electrophilic substitution, and are inert to nucleophilic attack. However, nucleophile attack on arenes occurs by complex formation. Fast nucleophilic substitution with carbanions with pKa values >22 has been extensively studied [44]. The nucleophiles attack the coordinated benzene ring from the exo side, and the intermediate i/2-cvclohexadienyl anion complex 171 is generated. Three further transformations of this intermediate are possible. When Cr(0) is oxidized with iodine, decomplexation of 171 and elimination of hydride occur to give the substituted benzene 172. Protonation with strong acids, such as trifluoroacetic acid, followed by oxidation of Cr(0) gives rise to the substituted 1,3-cyclohexadiene 173. The 5,6-trans-disubstituted 1,3-cyclohexadiene 174 is formed by the reaction of an electrophile. 

Poijarvi-Virta and co-workers synthesized di- and trinuclear zinc complexes from benzene substituted in the 1,3 or 1,3,5 positions by 1,5,9-triazacyclododecane ligands via methyloxy linkers [46] as shown in Fig. 4.23. 

A plausible mechanism was reported for the catalytic formation of benzene resulting from phosphorus-carbon bond cleavage which occurs during propylene hydroformylation catalyzed by triphenylphos-phine-substituted rhodium carbonyls under higher H2 partial pressures (Scheme 37). " ... 

Mutants of Pseudomonas putida were found to exhibit an arene dioxygenase activity, which has been exploited in whole-cell reactions for the regio- and enantioselective preparation of cw-dihydrodiols starting from benzene, substituted benzenes, and polycyclic or heteroaromatic compounds [48], The products are invaluable precursors for natural product synthesis, as exemplified in Scheme 8 [49],... 

FIGURE 14 (PLATE 3). Localization domains of mono-X-substituted benzenes C6H5X (from left to right top X = H, OH, F, bottom X = Cl, Br, I). The ELF value defining the boundary isosurface, t (r) = 0.659 corresponds to the critical point of index 1 on the separatrix between adjacent V(C, C) basins of benzene. Colour code magenta = core, orange = monosynaptic, blue = protonated disynaptic, green = disynaptic. Adapted from Reference 220 with permission... 

Traditionally, phenol is produced from benzene, cresols from toluene and xylenols from xylenes. There are three isomers of xylenes, namely, / -xylene, o-xylene, and m-xylenes. Accordingly, there are six isomers of xylenols which have been mentioned earher vide 1.1. Some of the properties of various xylenols have also been mentioned. Commercially, xylenols have assumed great significance. In some cases, xylenols can be used as substitutes of cresols. However, some of the individual xylenol isomers have importance in organic chemical synthesis. 

Explain why the oscillator strengths of charge-transfer transitions in EDA complexes are low and why the absorption maximum of the charge-transfer band of tetracyanoethylene complexes with methylated benzenes is red shifted upon increasing methyl substitution (from vmax = 2.6 pm 1 for benzene to = 1.9 pm 1 for hexamethylbenzene), 343... 

Benzene formed from photolysis of the 1 1 complex is a cage-escape product from 3(Jul-CHO+ /Ph ). Benzene formed from the photolysis of the 2 1 complex is an in-cage product from 3((Jul-CHO)2 /Ph ). The formation of 2 1 complexes of amino-substituted ketones and iodonium salts has been suggested to account for the high photosensitivity of polymeric Mannich bases with iodonium salts [102]. Formation of 2 1 donor iodonium cation complexes has been rationalized by consideration of the crystal structures of diphenyliodonium halides, which crystallize as dimers with square planar iodine atoms with two bridging halide counterions [102,108]. 

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