Resonance stabilization aromatic amines

Figure 23.1. Molecular structure and position of equilibrium. Resonance-stabilized aromatic amine is weaker base than ammonia. (Plots aligned with each other for easy comparison.)...

In comparison, neither ammonia or the ammonium ion is stabilized by resonance. Thus, aromatic amines are less basic than aliphatic amines. An additional factor that determines basicity is the substituents in the amine. Summarizing,we may say that any substituent that stabilizes the positive charge of the cunmonium ion increases the amine s basicity, and anything that destabilizes it. 

As noted previously, arylamines are generally less basic than alkylamines. Anilinium ion has pKa = 4.63, for instance, whereas methylammonium ion has pfCa = 10.64. Arylamines are less basic than alkylamines because the nitrogen lone-pair electrons are delocalized by interaction with the aromatic ring tt electron system and are less available for bonding to H+. In resonance terms, aryl-amines are stabilized relative to alkylamines because of their five resonance forms. 

Prlmaiy aliphatic amines form highly imstable allqrldlazonlum salts (refer to Section 13.6). Primaiy aromatic amines form arenedlazonlum salts which are stable for a short time in solution at low temperatures (273-278 K). The stability of arenedlazonlum Ion Is explained on the basis of resonance. 

Compcired to the aliphatic amines, the aromatic amines have lower values. This lower value indicates that the product of the protonation of aromatic amines is less stable. The decrease in stability is due to a loss in resonance stabilization of the protonated form. 

The lone pair of electrons on the nitrogen atom makes the amines Lewis bases. As Lewis bases, they may behave as nucleophiles. Because aromatic amines are resonance stabilized, they re weaker nucleophiles than alkyl amines. 

Alter the position of the amine/amine-generating group(s) in the aromatic ring(s) so as to distort the planarity of the compound and reduce the force of conjugation and thus the resonance stability of the electrophilic nitrenium ion. 

With the exception of 2,5-diaminopyrrole, which exists predominantly in the 2,5-bisiminopyrrolidine form, the C-aminopyrroles possess the structure of normal aromatic amines and this is generally reflected in their chemical properties. The C-aminopyrroles are, however, less basic than one might expect for an aromatic amine and it is evident that 2-aminopyrroles do not form the pyrrolylammonium ions, but are protonated at the 5-position, giving rise to the resonance-stabilized cations, e.g. (475) (68TL4605, 76S51). 

The low basicity of aromatic amines is thus due to the fact that the amine is stabilized by resonance to a greater extent than is the ion. 

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.)... 

Imidazole and its derivatives form an interesting and important class of heterocyclic aromatic amines. Imidazole is approximately 100 times more basic than pyridine. Protonation of imidazole yields an ion that is stabilized by the electron delocalization represented in the resonance structnres shown ... 

Fig. 1. Role of Resonance Stabilization in Contributing to Carcinogenic Activity of Aromatic Amines.

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