Basicity, alkylamines arylamines

In addition to being more basic than arylamines, alkylamines are also more nucleophilic. All the reactions in Table 22.4 take place faster with alkylamines than with arylanines. 

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... 

In addition to being more basic than arylamines, alkylamines are also more nucleophilic. 

Predict the basicity of alkylamines, arylamines and heterocyclic amines. 

Alkylamines (RNH2) are more basic than arylamines (C6H5NH2), which have a delocalized lone pair from the N atom (25.1 OB). 

Intriguingly, the scope of amines is complementary in the former case, strongly basic alkylamines are applicable whereas the latter conditions accommodate less basic arylamines and amides. 

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. 

Strongly basic amines (alkylamines, pK 3—6) react with syn,syn-l 5-dimethylpentadienyliron tricarbonyl to give cw,tra/M -dienylamines, but weakly basic amines (arylamines, pKi> ca. 10) react with inversion to give /ran5,tra/tr-dienylamines. The best explanation of this difference is that product control is kinetic for the strong bases, thermodynamic for the weak ones. ... 

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. 

Much of the resonance stabilization is lost on protonation, however, so the energy difference between protonated and nonprotonated forms is higher for arvlamines than it is for alkylamines. As a result, arylamines are less basic. Figure 24.3 illustrates the difference. 

Nonaromatic heterocyclic compounds, piperidine, for example, are similar in basicity to alkylamines. When nitrogen is part of an aromatic ring, however, its basicity decreases markedly. Pyridine, for example, resembles arylamines in being almost 1 million times less basic than piperidine. 

The amination of aryl halides and triflates catalyzed by palladium complexes is suitable for use in complex synthetic problems. Many substrates will produce high yields of mixed arylamines with one of the existing catalyst systems. Nevertheless, there are many combinations of substrates for which the amination chemistry may be substantially improved. For the most part, these reactions involve nitrogen centers, such as those in pyrroles, indoles, amides, imidazoles and other heterocyclic groups that are less basic than those in standard alkylamines. Although mild reaction conditions have been developed for many substrates, the harsh conditions used in many of the applications indicate that continued studies on developing mild condi-... 

Arylamines are less basic than alkylamines because the electron pair on N is deiocalized. 

Arylamines have a larger positive AG° for protonation and are therefore less basic than alkylamines, primarily because of resonance stabilization of their ground state. Electrostatic potential maps show that lone-pair electron density is delocalized in aniline compared to cyclohexylamine but that the corresponding ammonium ions localize charge in the same way. 

---