Amines aromaticity, resonance effects

Resonance effects are also important in aromatic amines. m-Nitroaniline is a weaker base than aniline, a fact that can be accounted for by the —7 effect of the nitro group. But p-nitroaniline is weaker still, though the —I effect should be less because of the greater distance. We can explain this result by taking into account the canonical form A. Because A contributes to the resonance hybrid, " the electron density of the unshared pair is lower in p-nitroaniline than in m-nitroaniline, where a canonical form such as Ais impossible. The basicity is lower in the para compound for two reasons, both... 

Resonance effects are also important in aromatic amines. m-Nitroaniline is a weaker base than aniline, a fact that can be accounted for by the —I effect of the nitro group, But... 

We have seen (Sec. 23.3) that aromatic amines are weaker bases than aliphatic amines, since resonance stabilizes the free amine to a greater extent than it does the ion. Here we have exactly the opposite situation, phenols being stronger acids than their aliphatic counterparts, the alcohols, because resonance stabilizes the ion to a greater extent than it docs the free phenol. (Actually, of course, resonance with the ring exerts the same effect in both cases it stabilizes -and thus weakens— the base amine or phenoxide ion.)... 

As a substituent on an aromatic ring, the amine salt group has an unusually uniform effect on the chemical shifts of the ortho, meta and para hydrogens. The series of aniline salts presented indicate the minimal effect that the type of acid involved, the solvent employed and the degree of amine substitution have on the chemical shift of the phenyl protons. The aromatic resonance for all of these aniline salts appears as a relatively sharp peak in the narrow chemical shift range from 7.50-7.55 ppm. 

Resonance Effects on Basicity Aromatic amines (anilines and their derivatives) are weaker bases than simple aliphatic amines (Table 19-3). This reduced basicity is due to resonance delocalization of the nonbonding electrons in the free amine. Figure 19-5 shows how stabilization of the reactant (the free amine) makes the amine less basic. In aniline, overlap between the aromatic ring and the orbital containing nitrogen s lone pair... 

This should not be confused with the lower pK value that results upon halogenation of phenol (see Table 2.1), for this again represents inductive pull. Nonetheless, resonance effects can extend themselves to other important examples as is noted by a comparison of the basicity of cyclohexylamine and aniline. The former has a pK typical of an organic amine (Table 2.1), but the latter is considerably more acidic. That is because at any given time the electron density of the amino function of aniline is much less than that of cyclohexylamine. The nonbonding electrons are smeared into the aromatic ring via resonance ... 

The addition product, C QHgNa, called naphthalenesodium or sodium naphthalene complex, may be regarded as a resonance hybrid. The ether is more than just a solvent that promotes the reaction. StabiUty of the complex depends on the presence of the ether, and sodium can be Hberated by evaporating the ether or by dilution using an indifferent solvent, such as ethyl ether. A number of ether-type solvents are effective in complex preparation, such as methyl ethyl ether, ethylene glycol dimethyl ether, dioxane, and THF. Trimethyl amine also promotes complex formation. This reaction proceeds with all alkah metals. Other aromatic compounds, eg, diphenyl, anthracene, and phenanthrene, also form sodium complexes (16,20). 

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