Orbital hybridization aniline

CaveU and Chapman made the interesting observation that a difference exists between the orbital involved in the quatemization of aromatic nitrogen heterocycles and aromatic amines, which appears not to have been considered by later workers. The lone pair which exists in an sp orbital of the aniline nitrogen must conjugate, as shown by so many properties, with the aromatic ring and on protonation or quatemization sp hybridization occurs with a presumed loss of mesomerism, whereas in pyridine the nitrogen atom remains sp hybridized in the base whether it is protonated or quaternized. Similarly, in a saturated compound, the nitrogen atom is sp hybridized in the base and salt forms. 

The pATa of the aminium ions is generally lower than those of the corresponding amine. The greater acidity of the aminium ion can be attributed to the greater amoimt of s character in the lone-pair orbital of the amine radical, (CH3)2N , which is sp -hybridized compared, with the more basic lone pair orbital in (CH3)2NH, which is sp -hybridized. In contrast, the pA a of the anilinium radical is much higher than that of the protonated aniline. The basicity of the anilinium radical is comparable with that of (CH3)2NH+ , reflecting the sp -hybrid character of the N-H bonds in the two species. The low basicity of the parent aniline results from... 

The structure of aniline reflects a compromise between two modes of binding the nitrogen lone pair (Figure 22.3). The electrons are more strongly attracted to nitrogen when they are in an orbital with some s character— an -hybridized orbital, for exam-... 

---