Substitution of Benzene Rings

An electron-accepting group, such as NO, and an electron-donating gronp, snch as OH, situated ortho or para to each other tend to cancel each other out and provide a spectrum not very different from the monosubstitnted benzene ring spectrum (Table 5.8). 

Two electron-accepting groups or two electron-donating groups para to each other produce a spectrum little different from the spectrum of the monosubstituted compound. 

Source Jaffe, H.H. and Orchin, M., Theory and Applications of Ultraviolet Spectroscopy, John Wiley Sons, New York, 1962. Reprinted with the permission of the late Professor Milton Orchin. 

To become an expert in the interpretation of UV spectra requires reading of more detailed texts, as well as practice. It can be seen that the absorption maxima can be calculated with reasonable accuracy if the structure is known, usually within 5 nm or so. A complete study of this subject is beyond the scope of this book. More detailed treatments can be found in the texts by Creswell and Runquist, Pavia, Lampman and Kriz, or Silverstein, Bassler, and Morrill (5th edition or earlier) listed in the bibliography. 

Substituent Orientation Shift for Each Substituent, y, in EtOH (nm) 

Benzene is a strong absorber of UV radiation and particularly in the gas phase shows considerable hne structure in its spectrum (Fig. 5.11). Substitution on the benzene ring causes a shift in the absorption wavelengths. It is not uncommon for at least two bands to be observed, and frequently several more. The observed wavelengths of the absorption maxima of some substituted benzene rings are given in Table 5.8. These are experimental data and may be insufficient to completely identify unknown compounds. If the benzene ring is disubstimted, then calculations are necessary to predict the absorption maximum. 

Source From H.H. Jaffe and M. Orchin. Used with permission of Professor Milton Orchin. 

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A series of four fluorene-phenylene vinylene copolymers 320-323 clearly demonstrates the effect of the exact position of CN groups in the vinylene fragment on the emission of the materials (Scheme 2.48) [408], Substitution of benzene rings in copolymers 320 and 321 by thiophene results in red-shifted PL and EL, where copolymers 322 and 323 exhibit pure red emission with chromaticity values very close to the standard red (CIE x = 0.66, y = 0.34), although no PLQY values were reported. The ITO/PEDOT/322/Ca/Al device showed a very... 

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... 

So far we ve been concerned only with electrophilic substitution of benzene Two impor tant questions arise when we turn to substitution on rings that already bear at least one substituent... 

Nucleophilic Substitutions of Benzene Derivatives. Benzene itself does not normally react with nucleophiles such as haUde ions, cyanide, hydroxide, or alkoxides (7). However, aromatic rings containing one or more electron-withdrawing groups, usually halogen, react with nucleophiles to give substitution products. An example of this type of reaction is the industrial conversion of chlorobenzene to phenol with sodium hydroxide at 400°C (8). 

Substitution of thiophene rings for the benzene rings and a light modification in the carbon skeleton leads to a marked... 

Although many of the aromatic compounds based on benzene have pleasant odors, they are usually toxic, and some are carcinogenic. Volatile aromatic hydrocarbons are highly flammable and burn with a luminous, sooty flame. The effects of molecular size (in simple arenes as well as in substituted aromatics) and of molecular symmetry (e.g., xylene isomers) are noticeable in physical properties [48, p. 212 49, p. 375 50, p. 41]. Since the hybrid bonds of benzene rings are as stable as the single bonds in alkanes, aromatic compounds can participate in chemical reactions without disrupting the ring structure. 

Certain groups attached to an aromatic ring can donate electrons into its delocalized molecular orbitals. Examples of these electron-donating substituents include —NH2 and —OH. Electrophilic substitution of benzene is much faster when an electron-donating substituent is present. For example, the nitration of phenol, C6H5OH, proceeds so quickly that it requires no catalyst. Moreover, when the products are analyzed, the only products are found to be 2-nitrophenol (ortho-nitrophenol, 37) and 4-nitrophenol (pnra-mtrophcnol, 38 . 

We have seen how an —OH group can accelerate a reaction. Are there substituents that can slow down the electrophilic substitution of benzene One way to reduce the electron density in the benzene ring and to make it less attractive to elec-... 

Aromatic rings are much less reactive than their double-bond character would suggest they commonly undergo substitution rather than addition. Electrophilic substitution of benzene with electron-donating substituents is accelerated and takes place at the ortho and para positions preferentially. Electrophilic substitution of benzene with electron-withdrawing substituents takes place at a reduced rate and primarily at the meta positions. 

Hill described the Pd(OAc)2-oxidative cyclization of bisindolylmaleimides (e.g., 49) to indolo[2,3-a]pyrrolo[3,4-c]carbazoles (e.g., 50) [69], which is the core ring system in numerous natural products, many of which have potent protein kinase activity [70]. Other workers employed this Pd-induced reaction to prepare additional examples of this ring system [71, 72]. Ohkubo found that PdClj/DMF was necessary to prevent acid-induced decomposition of benzene-ring substituted benzyloxy analogues of 49, and the yields of cyclized products under these conditions are 85-100% [71]. 

Substituting the benzene ring with a double bond, Pd-catalyzed intramolecular alkoxylation of alkyne 122 also proceeded via an alkenyl palladium complex to form furan 123 instead of a benzofurans [99, 100]. In addition, 3-hydroxyalkylbenzo[fc]furans was prepared by Bishop et al via a Pd-catalyzed heteroannulation of silyl-protected alkynols with 2-iodophenol in a fashion akin to the Larock indole synthesis, [101]. 

Explain how does the -Off group attached to a carbon of benzene ring aetlvate It towards eleetrophllle substitution ... 

A functional group is a set of atoms that occurs in a wide range of compounds and confers upon them a common kind of reactivity (see Table 3.2). Phenols are generally represented by Ar-OH, in which Ar- represents an aromatic skeleton, composed of benzene rings or substituted benzene rings. Enols are molecules in which the -OH group is linked to an atom that is also engaged in a double bond. 

Since benzenesulfonyl peroxide was used as an initiator in polymerization reactions, it was thought that a free radical aromatic substitution of benzene by the benzenesulfonoxy radical takes place. A detailed study by Dannley and Knipple reveals that attachment of the sulfonoxy group derived from a bis(arylsulfonyl) peroxide to the aromatic ring occurs by electrophilic aromatic substitution (equation 5) °. 

Polystyrene itself is not used for endoprosthetic purposes and its application is accounted for only because of easy substitutions in benzene rings. The method was subsequently modified for heparinization of silicone and natural rubber, polyethylene, polypropylene, polyethylene terephthalate), and other polymers. Styrene was first grafted onto the polymers by y-radiation and then the above-described reaction was performed in the second step. All the polymers synthesized in this way contained sufficiently large amounts of immobilized heparin (2.8—15.7 ng/cm2) and displayed good thromboresistance when tested in vitro — recalcified blood was not clotted for several hours. 

A peculiar complex is formed by if coordination of Os(II) ammine complex to one of the double bonds of benzene rings, rather than rf coordination, and the coordinated benzene rings show interesting reactivity [82]. For example, Os(II) coordinates regioselectively to the 2,3-double bond of anisole to form the complex 333, and hence localization of the remaining 7r-electrons occurs. As a result, at 20 °C an electrophile attacks easily at C(4) due to electron-donation of the methoxy group. The 4H-cationic intermediate 334 is stabilized by backdonation from the metal, and the monosubstitution product 334 is formed without deprotonation. The / ara-substituted anisole 335 is... 

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