20. The Variety of Organic         Compounds   Previous PageNext Page
        Amines and Other Organic Bases

Aromatic -OH compounds are more acidic than aliphatic R-OH alcohols because of the influence of the delocalized ring electrons. By the same token, aromatic amines are weaker bases than the aliphatic amines that we have just discussed. Methyl amine, , is similar in strength to ammonia; phenyl amine or aniline, , is a weak base with a of only 4.63.

The reason is outlined to the right. In terms of the resonance structures that can be drawn, the unprotonated aniline molecule resembles the phenolate ion, and the protonated anilinium ion is like the phenol molecule. All four electron pairs around the nitrogen in the anilinium ion are tied up in sigma bonds to hydrogen or carbon, and none are free for delocalization into the aromatic ring system. The loss of a proton in the anilinium molecule leaves a lone pair on the nitrogen atom that can be shared with the aromatic ring. As in the phenolate ion, the delocalized system is expanded from six atoms and six electrons to seven atoms and eight electrons. In both phenol and aniline, increased delocalization favours the unprotonated species, which are the phenolate ion and the neutral aniline molecule.

Phenol loses a proton easily to form the phenolate ion, and therefore is a reasonably stong acid. Aniline only reluctantly accepts a proton to form the anilinium ion, and hence is a weak base. When an -NH group is attached to an aliphatic radical it receives no comparable delocalization stabilization. It is less reluctant to accept a proton on its nitrogen lone pair, and hence aliphatic amines are stronger bases.

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