Electrophilic aromatic substitution reaction substituent effects

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

A nitro group behaves the same way m both reactions it attracts electrons Reaction is retarded when electrons flow from the aromatic ring to the attacking species (electrophilic aromatic substitution) Reaction is facilitated when electrons flow from the attacking species to the aromatic ring (nucleophilic aromatic substitution) By being aware of the connection between reactivity and substituent effects you will sharpen your appreciation of how chemical reactions occur... 

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. 

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 ... 

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. 

These polarizations are seen to be in the opposite direction to those in aniline (3.133), so that higher pi density remains at the Ci (junction) and C3 and C5 (meta) positions. These polarity shifts are again consistent with the well-known m-directing effect of nitro substituents in electrophilic aromatic substitution reactions, and the results are again quite independent of which starting Kekule structure is selected for the localized analysis.63... 

Hammond postulate has been used to explain the effect of substituents on the rate of benzilic acid rearrangements, mechanism of electrophillic aromatic substitution reactions and reactions involving highly reactive intermediates such as carbonium ions and carbon ions. 

Predicting the Effect of a Substituent on the Rate and Regiochemistry of an Electrophilic Aromatic Substitution Reaction... 

Table 17.1 Effect of Substituents on the Rate and Regiochemistry of Electrophilic Aromatic Substitution Reactions...

Predict the effect of these substituents on the rate and regiochemistry of electrophilic aromatic substitution reactions ... 

If the other reactant is an electrophile and a strong Lewis acid or proton acid is present, then the aromatic ring acts as the nucleophile and the reaction is one of the electrophilic aromatic substitution reactions listed in Table 17.2. Do not forget to consider the directive and rate effects of substituents on the aromatic ring. 

When a disubstituted benzene undergoes an electrophilic aromatic substitution reaction, the directing effect of both substituents has to be considered. If both substituents direct the incoming substituent to the same position, the product of the reaction is easily predicted. 

All of these effects are observed when comparing the rates of various electrophilic aromatic substitution reactions. Activating substituents increase the rate of reaction relative to benzene. The rate of reaction for the nitration of anisole, for example, was 9.7 x 10 times faster than nitration of benzene. The reaction of anisole with nitric and sulfuric acids, gave 44% of o-nitroanisole, 56% of p-nitroanisole and < 1% of m-nitro-anisole.2 9 contrasts with reactions involving deactivating substituents, where selectivity for the meta -product is usually very good. Nitration of nitrobenzene, for example, gave 1,3-dinitrobenzene in 94% yield, with only 6% of the ortho product and < 1% of the para product. ... 

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