Basicity of Pyridine

Basicity of Pyridine explains its nitrogen lone pair, resonance effects, and comparison with pyrrole and aliphatic amines.

• The basicity of pyridine refers to its ability to accept a proton (H⁺), which depends on the availability of its lone pair of electrons on the nitrogen atom.

Structure and Basicity

Pyridine is an aromatic heterocycle with the formula C₅H₅N. It’s similar to benzene, but one CH group is replaced by a nitrogen atom.

Here’s how its structure influences its basicity:

1. Lone Pair on Nitrogen:

   • The nitrogen in pyridine is sp² hybridized.
   • Its lone pair of electrons is in an sp² orbital perpendicular to the π system of the aromatic ring.
   • This lone pair does not participate in aromaticity, so it is available to accept protons (H⁺) — making pyridine a Lewis base.

2. Basicity Compared to Other Compounds:

   • Pyridine is less basic than aliphatic amines (like methylamine) because the nitrogen is more electronegative due to its inclusion in an aromatic ring.
   • It is more basic than pyrrole, where the nitrogen lone pair is part of the aromatic sextet and thus not available for protonation.

3. pKa of the Conjugate Acid:

   • The pKa of the conjugate acid of pyridine (pyridinium ion) is about 2, indicating it is a weak base.

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