Substituent Effects in Electrophilic Aromatic Substitution Activating Substituents

12 SUBSTITUENT EFFECTS IN ELECTROPHILIC AROMATIC SUBSTITUTION ACTIVATING SUBSTITUENTS 

Our analysis of substituent effects has so far centered on two groups methyl and triflu-oromethyl. We have seen that a methyl substituent is activating and ortho, para-directing. A trifluoromethyl group is strongly deactivating and meta-directing. What about other substituents  

Halogen substituents are shghtly deactivating but are ortho, para-directing. 

Some of the most powerful activating substituents are those in which an oxygen atom is attached directly to the ring. These substituents include the hydroxyl group as well as alkoxy and acyloxy groups. All are ortho, para directors. 

Hydroxyl, alkoxy, and acyloxy groups activate the ring to such an extent that bromina-tion occurs rapidly even in the absence of a catalyst. 

12 Substituent Effects in Electrophilic Aromatic Substitution Activating Substituents 

Very strongly activating —NHj (amino) Ortho, para-directing 

Comparative energy diagrams for nitronium ion attachment to (a) benzene and at the (b) ortho, (c) meta, and id) para positions of (trifluoromethyl)-benzene. Eact (ortho) Eact (para) act (meta) Eact (benzene). 

All the ring positions of (trifluoromethyl)benzene are deactivated compared with benzene. The meta position is simply deactivated less than the ortho and para positions. The partial rate factors for nitration of (trifluoromethyl)benzene are 

Chapter 12 Reactions of Arenes Electrophilic and Nucleophilic Aromatic Substitution 

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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... 

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 ... 

A Summary of Substituent Effects in Aromatic Substitution A summary of the activating and directing effects of substituents in electrophilic aromatic substitution is shown in Table 16.2. 

The effect of monofluorination on alkene or aromatic reactivity toward electrophiles is more difficult to predict Although a-fluonne stabilizes a carbocation relative to hydrogen, its opposing inductive effect makes olefins and aromatics more electron deficient. Fluorine therefore is activating only for electrophilic reactions with very late transition states where its resonance stabilization is maximized The faster rate of addition of trifluoroacetic acid and sulfuric acid to 2-fluoropropene vs propene is an example [775,116], but cases of such enhanced fluoroalkene reactivity in solution are quite rare [127] By contrast, there are many examples where the ortho-para-dueeting fluorine substituent is also activating in electrophilic aromatic substitutions [128]... 

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. 

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