# basicity of pyridine and pyrrole

23.5D). Pyridine is a weaker base than saturated amines of similar structure because its electron pair is in an sp 2-hybridized orbital, and the electron pair is more tightly held by the atom. The corresponding values for the saturated amine pyrrolidine are: basicity 11.2 and acidity 32. Pyrrolidine is technically tetrahydropyrrole. So, basicity order ===> imidazole > pyridine> pyrazole > pyrrole 1 4. pyrazole (pkb-11.5) : The inductive effect is larger in pyrazole because the two N atoms are closer Hence, the pyrazole is a weaker base compare to imidazole. However, aqueous solution, aromatic heterocyclic amines such as pyrrole … Pyrrole C4H4NH (in which N contributes a lone pair) has a pKa - 3.8, but Pyridine (where N is part of the ring's double bond) has a pKa 5.14. Therefore, this would result in protonation being unfavourable, and protonated pyrrole becomes unstable. In this example, we cannot use either the steric factor or inductive factor to explain their basicity. 0) and strong acidity (pK a = 15) for a 2º-amine. Pyrrolidine: Pyridine: Looking at the structure of both compounds, we can see that the N atom in pyrrolidine is sp3, while that of pyridine is sp2. Oxidation-Reduction 6. As a result, pyrrole is a much weaker base than pyridine ($\mathrm{p}K_\mathrm{a} = -3.8$). 1226 CHAPTER 25 • THE CHEMISTRY OF THE AROMATIC HETEROCYCLES B. Acidity of Pyrrole and Indole Pyrrole and indole are weak acids. DNA and RNA Bases 7. In pyrrole, the electron pair is part of the aromatic system. H-bond Formation … As a result, pyrrole is a very weak base. Adding air acid to pyrrole could prevent delocalization and could destroy the a romaticity. Pyrrole, C 4 H 4 N H (in which N contributes a lone pair) has a p K a − 3. 8 but pyridine (where N is part of the ring's double bond) has a p K a 5. Electrophilic Aromatic Substitution 5. Once protonated, pyrrole loses its aromaticity due to the loss of lone pair to proton ($\ce{H+}$). Although pyrrole is an amine, it is not basic.The unshared pair of electrons, which is normally responsible for the usual basicity of a mines, is delocalized in an "aromatic sextet", and is not available for bonding to a proton. Π-excessive and Π-deficient Heterocycles 4. 2. Additional evidence for the aromatic character of pyrrole is found in its exceptionally weak basicity (pK a ca. In this case, pyridine is the stronger base. Electron pair availability indicates the strength of basicity. Aromaticity and Basicity 2.1 Pyrrole 2.2 Imidazole 2.3 Pyridine 2.4 Pyrimidine 2.5 Purine 3. imidazole (pkb-7.0) : Moderately strong base. Tautomers 8. Since pyridine has a lower pKb value, it is a stronger base than pyrrole. Basicity of Pyridine vs Pyrrole The lower the pKb value of a compound, the stronger a base it is. pyrrole (pkb-13.6) : Non basic (or can refer as a very weakly basic in nature). 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