Substituent Effects in Electrophilic Substitutions

Only one product can form when an electrophilic substitution occurs on benzene, but what would happen if we were to carry out a reaction on an aromatic ring that already has a substituent A substituent already present on the ring has two effects  

Substituents affect the orientation of the reaction. The three possible disubstituted products—ortho, meta, and para—are usually not formed in equal amounts. Instead, the nature of the substituent already present on the benzene ring determines the position of the second substitution. An -OH group directs substitution toward the ortho and para positions, for instance, while a carbonyl group such as -CHO directs substitution primarily toward the meta position. Table 9.2 lists experimental results for the nitration of substituted benzenes. 

Meta-directing deactivators Ortho- and para-directing deactivators  

FIGURE 9.16 Classification of substituent effects in electrophilic aromatic substitution. 

Compare the electrostatic potential maps of benzaldehyde (deactivated), chlorobenzene (weakly deactivated), and phenol (activated) with that of benzene. The ring is more positive (yellow) when an electron-withdrawing group such as -CHO or -Cl is present and more negative (red) when an electron-donating group such as -OH is present. 

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The more extensive problem of correlating substituent effects in electrophilic substitution by a two-parameter equation has been examined by Brown and his co-workers. In order to define a new set of substituent constants. Brown chose as a model reaction the solvolysis of substituted dimethylphenylcarbinyl chlorides in 90% aq. acetone. In the case ofp-substituted compounds, the transition state, represented by the following resonance structures, is stabilized by direct resonance interaction between the substituent and the site of reaction. 

Data also exist for the base-catalyzed protiodesUylation of XCglltSiMes in aqueous DMSO . This reaction involves the base-catalyzed cleavage of the silicon compound to give an aromatic anion Ar , and the usual substituent effects in electrophilic substitution are reversed electron-attracting groups increase the reaction rate and electron-withdrawing groups decrease it. Thus the m- and p-nitro compounds are about 10 times more reactive than the parent (X = H) compound. 

Substituent effects in electrophilic substitution of polycyclic compounds are similar to those for benzene, but all the reactions go faster, because there is less loss of resonance energy in the RDS. 

There were two schools of thought concerning attempts to extend Hammett s treatment of substituent effects to electrophilic substitutions. It was felt by some that the effects of substituents in electrophilic aromatic substitutions were particularly susceptible to the specific demands of the reagent, and that the variability of the polarizibility effects, or direct resonance interactions, would render impossible any attempted correlation using a two-parameter equation. - o This view was not universally accepted, for Pearson, Baxter and Martin suggested that, by choosing a different model reaction, in which the direct resonance effects of substituents participated, an equation, formally similar to Hammett s equation, might be devised to correlate the rates of electrophilic aromatic and electrophilic side chain reactions. We shall now consider attempts which have been made to do this. 

SUBSTITUENT EFFECTS IN ELECTROPHILIC AROMATIC SUBSTITUTION ACTIVATING SUBSTITUENTS... 

Substituent Effects in Electrophilic Aromatic Substitution Activating Substituents... 

These relative rate data per position are experimentally detennined and are known as partial rate factors. They offer a convenient way to express substituent effects in electrophilic aromatic substitution reactions. 

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. 

Substituents on an aromatic ring may show activating and directing effects in electrophilic substitution. The same situation is encountered in nucleophilic photosubstitution. 

It is subsequently interesting to enquire the degree of additivity of effects in electrophilic substitution and carbonium ion reactions of substituted five-membered rings, using aj, for 4-substituents and at for 5-substituents. As reasoned previously, the p values for such correlations compared with those for the equivalent reactions in benzene indicate the status of the o+ values for the heteroatoms. If the p values are the same... 

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