Electrophilic aromatic substitution ortho, para-directing substituents

The situation is more complicated if there is more than one substituent on the benzene ring. However, it is usually possible to predict the major products that are formed in an electrophilic aromatic substitution reaction. When the substituents direct to the same position, the prediction is straightforward. For example, consider the case of 2-nitrotoluene. The methyl group directs to the positions ortho and para to itself—that is, to positions 4 and 6. The nitro group directs to positions meta to itself—that is, also to positions 4 and 6. When the reaction is run, the products are found to be almost entirely 2,4-dinitrotoluene and 2,6-dinitrotoluene, as expected ... 

When a benzene ring with an ortho-para-directing substituent undergoes an electrophilic aromatic substitution reaction, what percentage of the product is the ortho isomer and what percentage is the para isomer ... 

Why IS there such a marked difference between methyl and trifluoromethyl substituents m their influence on electrophilic aromatic substitution s Methyl is activating and ortho para directing trifluoromethyl is deactivating and meta directing The first point to remember is that the regioselectivity of substitution is set once the cyclohexadienyl cation intermediate is formed If we can explain why... 

Returning to Table 12 2 notice that halogen substituents direct an incoming electrophile to the ortho and para positions but deactivate the ring toward substitution Nitration of chlorobenzene is a typical example of electrophilic aromatic substitution m a halobenzene... 

Sometimes the orientation of two substituents m an aromatic compound precludes Its straightforward synthesis m Chloroethylbenzene for example has two ortho para directing groups m a meta relationship and so can t be prepared either from chloroben zene or ethylbenzene In cases such as this we couple electrophilic aromatic substitution with functional group manipulation to produce the desired compound... 

Electrophilic aromatic substitution (Section 12 14) Halo gen substituents are slightly deactivating and ortho para directing Br... 

Anticipating the products of electrophilic aromatic substitution can be more difficult when two or more substituents compete for control. For example, both methyl and methoxy groups are ortho para directors, and compete for control in electrophilic substitution of 2-methylanisole. The reaction product depends on which substituent has the stronger directing influence. 

The hydroxyl group is a strongly activating, ortho- and para-directing substituent in electrophilic aromatic substitution reactions (Section 16.4). As a result, phenols are highly reactive substrates for electrophilic halogenation, nitration, sulfonation, and lTiedel-Crafts reactions. 

As a result of these substituent-induced polarizations, the complementary conjugative interactions at each ring site become somewhat imbalanced (so that, e.g., the donor-acceptor interaction from C3—C4 to C5—C(, is 23.1 kcal mol-1, but that in the opposite direction is only 16.4 kcal mol-1). From the polarization pattern in (3.133) one can recognize that excess pi density is accumulated at the ortho (C2, C6) and para (C4) positions, and thus that the reactivity of these sites should increase with respect to electrophilic attack. This is in accord with the well-known o, /(-directing effect of amino substitution in electrophilic aromatic substitution reactions. Although the localized NBO analysis has been carried out for the specific Kckule structure of aniline shown in Fig. 3.40, it is easy to verify that exactly the same physical conclusions are drawn if one starts from the alternative Kekule structure. 

Table 8.1 Directing effects of substituents in electrophilic aromatic substitution Electron-releasing groups ortho and para directors...

Electrophilic aromatic substitution in biphenyl is best understood by considering one ring as the functional group and the other as a substituent. An aryl substituent is ortho, para-directing. Nitration of biphenyl gives a mixture of o-nitrobiphenyl and p-nitrobiphenyl. 

In nitration, an electrophilic aromatic substitution, the electrophile is a nitronium cation. It attacks the benzene ring in the place of the highest electron density. In o-fluorotoluene, there are several positions of high electron density ortho and para with respect to the methyl group, and ortho and para with respect to fluorine. Both substituents, methyl and fluorine, direct the entering electrophile to the activated positions. Whose influence is stronger, that of the methyl group, or that of fluorine ... 

Draw resonance forms for the sigma complexes resulting from electrophilic attack on substituted aromatic rings. Explain which substituents are activating and which are deactivating, and show why they are ortho, para-directing, or meta-allowing. Problems 17-47,48, 54, 57, 50, and 64... 

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 effort makes olefins and aromatics more electron deficient. Fluorine therefore is activating only fur 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-directing fluorme substituent is also activating in electrophilic aromatic substitutions [72 ]... 

Ortho, para director (Section 18.7) A substituent on a benzene ring that directs a new group to the ortho and para positions during electrophilic aromatic substitution. 

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