Hybridization effect, basicity pyridine

Pyridine, like benzene, is an aromatic system with six jt electrons (see Section 11.3). The ring is planar, and the lone pair is held in an sp orbital. The increased s character of this orbital, compared with the sp orbital in piperidine, means that the lone pair electrons are held closer to the nitrogen and, consequently, are less available for protonation. This hybridization effect explains the lower basicity of pyridine compared with piperidine. Pyrrole is also aromatic, but there is a significant difference, in that both of the lone pair electrons are contributing to the six-jr-electron system. As part of the delocalized Jt electron system, the lone pairs are consequently not available for bonding to... 

Nicotine has two nitrogen atoms, one as a cyclic tertiary amine and one in a pyridine ring. The basicities are easily distinguished, in that a pyridine system is much less basic than a simple amine. This is essentially a hybridization effect, the nitrogen lone pair in pyridine being held in an sp orbital. This means the lone pair electrons are held closer to the nitrogen, and are consequently less available for protonation than in an sp -hybridized aliphatic amine. Hence, as mentioned above, pyridine has p Ta approximately 5. It follows that pA"a 8.1 is more appropriate for the pyrrolidine nitrogen. 

Aromatic amines are generally less basic than aliphatic amines.This is true both when the nitrogen atom is part of the aromatic system (as in j pyridine, a hybridization effect), and 1 when the nitrogen atom is bonded to the aromatic ring (as in aniline,... 

Hybridization effects Increasing the percent s-character in Pyridine is iess basic than piperidine, the orbitai with the lone pair decreases basicity. 

Correlations between the basicity of an amine and the hybridization of the nitrogen atom have been developed. Early work revealed that the basicity is diminished by increasing the s-character in the nitrogen lone pair . Thus, the PA of piperidine (sp nitrogen) is 23 kJmol greater than that of pyridine (sp nitrogen). Similarly, the PA of aziridine is lower than that of dimethylamine by 21 klmol because of ring strain, which tends to increase the proportion of s-character in the exocycUc bonds. However, care must be taken to allow for polarizability effects in such comparisons . ... 

Proton transfer from water or other acid to pyridine does not involve the electrons of the aromatic sextet. Why, then, is pyridine a considerably weaker base than aliphatic amines The answer is that the unshared pair of electrons on the pyridine nitrogen lies in a relatively electronegative sp hybrid orbital, whereas in aliphatic amines, the unshared pair lies in an sp hybrid orbital. This effect decreases markedly the basicity of the electron pair on an spi hybridized nitrogen compared with that on an sp hybridized nitrogen. 

The basicity of heterocyclic amines varies over a wide range and reflects both the hybridization of the orbital of nitrogen containing the lone pair electrons and the effects of delocalization. Pyridine is a substantially weaker base than alkylamines such as piperidine. The electron pair of pyridine occupies an sp2-hybridized orbital and lies closer to the nucleus than the electron pair in the sp -hybridized orbital of alkylamines. As a result, pyridine is a weaker base (larger than an alkylamine. 

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