# basicity of pyridine and pyrrole

1226 CHAPTER 25 • THE CHEMISTRY OF THE AROMATIC HETEROCYCLES B. Acidity of Pyrrole and Indole Pyrrole and indole are weak acids. 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. Aromaticity and Basicity 2.1 Pyrrole 2.2 Imidazole 2.3 Pyridine 2.4 Pyrimidine 2.5 Purine 3. In this example, we cannot use either the steric factor or inductive factor to explain their basicity. With p K a values of about 17.5, pyrrole and indole are about as acidic as alcohols and about 15–17 pK a units more acidic than primary and secondary amines (Sec. 8 but pyridine (where N is part of the ring's double bond) has a p K a 5. 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. 1 4. 0) and strong acidity (pK a = 15) for a 2º-amine. 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. Adding air acid to pyrrole could prevent delocalization and could destroy the a romaticity. Electrophilic Aromatic Substitution 5. The basicity of heterocyclic rings such as pyridine is more basic than the pyrrole due to the more resonance of pi-electron and non-bonding electrons. Therefore, this would result in protonation being unfavourable, and protonated pyrrole becomes unstable. Oxidation-Reduction 6. H-bond Formation … 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. 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. As a result, pyrrole is a much weaker base than pyridine ($\mathrm{p}K_\mathrm{a} = -3.8$). In pyrrole, the electron pair is part of the aromatic system. Pyrrolidine is technically tetrahydropyrrole. Π-excessive and Π-deficient Heterocycles 4. So, basicity order ===> imidazole > pyridine> pyrazole > pyrrole imidazole (pkb-7.0) : Moderately strong base. As a result, pyrrole is a very weak base. Pyrrole, C 4 H 4 N H (in which N contributes a lone pair) has a p K a − 3. The corresponding values for the saturated amine pyrrolidine are: basicity 11.2 and acidity 32. However, aqueous solution, aromatic heterocyclic amines such as pyrrole … Basicity of Pyridine vs Pyrrole The lower the pKb value of a compound, the stronger a base it is. 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. 2. pyrrole (pkb-13.6) : Non basic (or can refer as a very weakly basic in nature). 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