Electrophilic aromatic substitution activating/deactivating effects

Rate and Regioselectivity in the Nitration of (Trifluoromethyl)benzene 474 Substituent Effects in Electrophilic Aromatic Substitution Activating Substituents 476 Substituent Effects in Electrophilic Aromatic Substitution Strongly Deactivating Substituents 480 Substituent Effects in Electrophilic Aromatic Substitution Halogens 482 Multiple Substituent Effects 484 Retrosynthetic Analysis and the Synthesis of Substituted Benzenes 486 Substitution in Naphthalene 488 Substitution in Heterocyclic Aromatic Compounds 489... 

Take a minute to compare the effect a substituent has on the reactivity of a benzene ring toward electrophilic aromatic substitution with the effect the same substituent has on the p Ta of phenol. Notice that the more strongly deactivating the substituent (for example, NO2), the lower the p Tg of the phenol, and the more strongly activating the substituent (for example, OCH3), the higher the p Tg of the phenol. 

Table 12.2 summarizes orientation and rate effects in electrophilic aromatic substitution reactions for a variety of frequently encountered substituents. It is arianged in order of decreasing activating power the most strongly activating substituents are at the top, the most strongly deactivating substituents are at the bottom. The main features of the table can be summarized as follows ... 

In the discussion of electrophilic aromatic substitution (Chapter 11) equal attention was paid to the effect of substrate structure on reactivity (activation or deactivation) and on orientation. The question of orientation was important because in a typical substitution there are four or five hydrogens that could serve as leaving groups. This type of question is much less important for aromatic nucleophilic substitution, since in most cases there is only one potential leaving group in a molecule. Therefore attention is largely focused on the reactivity of one molecule compared with another and not on the comparison of the reactivity of different positions within the same molecule. 

Activating/Deactivating Effects on Electrophilic Aromatic Substitution. 190... 

Iodination of arenes2 Iodination of arenes can be effected by reaction with HgO HBF4 in the presence of iodine. The orientation conforms to that observed in electrophilic aromatic substitution except that ortho-attack is favored over para-attack in activated arenes. The method is particularly useful for meta-iodination of deactivated arenes (99% selectivity). 

In nucleophilic as in electrophilic aromatic substitution, then, a substituent group affects reactivity by its ability to attract or release electrons in nucleophilic as in electrophilic aromatic substitution, a substituent group exerts its effect chiefly at the position ortho and para to it. The kind of effect that each group exerts, however, is exactly opposite to the kind of effect it exerts in electrophilic aromatic substitution. In nucleophilic aromatic substitution electron withdrawal causes activation, and electron release causes deactivation. 

Electrophilic aromatic substitution is a situation in which it is useful to discuss TS structure in terms of a reaction intermediate. The ortho, para, and meta directing effects of aromatic substituents were among the first structure-reactivity relationships to be developed in organic chemistry. Certain functional groups activate aromatic rings toward substitution and direct the entering electrophile to the ortho and para positions, whereas others are deactivating and lead to substitution in the meta position. The bromination of methoxybenzene (anisole), benzene, and nitrobenzene can serve as examples for discussion. 

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