Para ring positions

Figure 7.27 Quinoid resonance forms activating the para ring position.

Injthe similar case of bromine in the para ring position it is known that only one-half is removed by silver. It has been suggested that the bromine is replaced by another radical in the following fashion ... 

Arylnitrenium ions are likewise capable of adding to n nucleophiles. With substituted aromatics (e.g., toluene) there exists the possibility of three reactive sites on the nitrenium ion (the nitrogen, ortho- and para-ring positions), along with up to three possible sites on the arene (ortho, para, and meta in the case of a monosub-stituted trap). Thus in a typical case there is the possibility of nine distinct regio-isomers. Obviously, any synthetic utility of such chemistry relies on the ability of the reagents to react in a selective manner. 

The negative charge on the alkoxide anion, RO, cannot be delocalized, but on PhO the - charge is delocalized to the ortho and para ring positions as indicated by the starred sites in the resonance hybrid. 

Fluorescein is bound at the bottom of the cleft that harbors bihverdin IX, in the wild-type BBP structure (Fig. 8.2). Its xanthenolone moiety is located close to the center of the j8-barrel, while the carboxyphenyl group is oriented towards the entrance of the pocket. The para ring position (with respect to the central carbon atom of the triphenylmethane dye), which carried the linker group during the selection experiments for this anticalin [40], is accessible from the solvent via a narrow charmel. An area of 454 A", corresponding to 91% of the solvent-accessible surface of fluorescein, became buried in the complex. 

Note that diethylbenzene has three isomers in orto-, meta-, and para- ring positions. Up to six substitutions at the benzene ring might be theoretically possible. Propylene present in ethylene as impurity may also produce cumene and higher alkylates as byproducts. Using an excess of benzene can improve the yield by converting the polyalkylbenzene into ethylbenzene, as for example ... 

Some phenols are not sufficiently acidic to behave much like carboxylic acids. This is especially true of those substituted with electron-withdrawing groups at the ortho and para ring positions. These phenols, however, can usually be eliminated... 

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