Basicity of Arylamines

As noted previously, arylamines are generally less basic than alkylamines. Anilinium ion has piCg = 4.63, for instance, whereas methylammonium ion has picia = 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, arylamines are stabilized relative to alkylamines because of their five resonance forms  

FIGURE 18.2 Arylamines have a larger positive AC° for protonation and are therefore less basic than alkylamines, primarily because of resonance stabilization of the ground state. Electrostatic potential maps show that lone-pair electron density is delocalized in the amine but the charge is localized in the corresponding ammonium ion. 

Substituted arylamines can be either more basic or less basic than aniline, depending on the substituent. Electron-donating substituents, such as -CH3 and -OCH3, which increase the reactivity of an aromatic ring toward electrophilic substitution (Section 9.8), also increase the basicity of the corresponding arylamine. Electron-withdrawing substituents, such as -Cl, -NO2, and -CN, which decrease ring reactivity toward electrophilic substitution, also decrease arylamine basicity. Table 18.2 considers only p-substituted anilines, but similar trends are observed for ortho and meta derivatives. 

Rank the following compounds in order of increasing basicity  

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Another fallacy to be refuted by PA data was the widespread belief that the low basicity of arylamines compared to alkylamines simply reflects delocalization of the lone pair of electrons on the nitrogen atom within the aromatic ring. Thus, the low basicity of aniline (pKa = 4.58) compared to ammonia (pKa = 9.27) is often attributed to such a conjugative... 

Multiple substitution by strongly electron-withdrawing groups diminishes the basicity of arylamines still more. Aniline is 3800 times as strong a base as p-nitroaniline and 10 times more basic than 2,4-dinitroaniline. 

The chemistry of amines is dominated by the lone-pair electrons on nitrogen, which makes amines both basic and nucleophilic. The basicity of arylamines is generally lower than that of alkylamines because the nitrogen lone-pair electrons are delocalized by interaction with the aromatic v system. Electron-withdrawing substituents on the aromatic ring further weaken the basicity of a substituted aniline, while electron-donating substituents increase basicity. Alkylamines are sufficiently basic that they exist almost entirely in their protonated form at the physiological pH of 7.3 inside cells. 

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