Aniline, basicity resonance effect

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... 

As we move to A-methylaniline, we see only a modest change in pK ,. This is undoubtedly due to the electron-donating effect of the methyl group, and this would be expected to stabilized the conjugate acid, increasing observed basicity. There is a modest increase in basicity, but it is apparent that the resonance effect, as in aniline, is also paramount here, and this compound is also a weak base. However, diphenylamine (A-phenylaniline) is an extremely weak base this can be ascribed to the resonance effect allowing electron delocalization into two rings. 

Resonance Effects on Basicity Arylamines (anilines and their derivatives) are much 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... 

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 ... 

Silicon bound to a phenyl group can also influence the bond system by additional (p- -d) back donation from carbon to silicon. In agreement with this model, p-trimethylsilyl-substituted benzoic acid shows a greater acidity than expected from inductive effects. Furthermore, p-trimethylsilyl phenol exhibits a greater acidity than phenol itself, and p-trimethylsilyl aniline shows a decreased basicity as compared with that of the nonsubstituted compound. This behaviour can be described by the following resonance structures [Eqs. (4) and (5)] ... 

Another type of proximity effect arises when the placement of a substituent interferes with the orbital overlap required for resonance stabilization. This can be seen clearly in the acidity of substituted benzoic acids and in the basicity of substituted anilines. 

Phenols are more acidic than alcohols because one of the non-bonding electron pairs on oxygen is drawn into the benzene ring by resonance. This stabilizes the phenoxide ion that is formed upon ionization and thus the acidity of phenol is enhanced by the phenomenon. This same withdrawal of electrons by the benzene ring stabilizes aniline and decreases the availability of the nonbonding electron pair on nitrogen. Both effects decrease the basicity of aniline relative to alkyl amines. 

The fluorescence of m aromatic compound with acidic or basic ring substituents is usually pH dependent. Bvtih the wavelength and the emission intensity are likely to be different for the protonaied and unproto-nated forms of the compound. The data for phenol and aniline shown in Table 15-1 illustrate this effect. The changes in emission i>f compounds of this type arise from the differing number of resonance species that are assxiciated with the acidic and basic forms of the molecules. For example, aniline has several resonance forms but anilinium has only one. That is. 

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