Pyridine template

Pyridine: the two Kekulé structures, and the charge-separated forms with N⁻ and the positive charge on C2, C4 and C6.

What the template shows

• Kekulé forms, and dipolar forms that put δ+ on C2, C4 and C6
• Resonance contributors
• Kekulé forms
• Six π electrons, aromatic; the N lone pair stays in the ring plane
• Dipolar forms
• Minor forms: N⁻ and C⁺ at C2, C4 and C6
• The N lone pair is in an sp² orbital in the ring plane, not part of the π system, so pyridine is a base (pKaH 5.2).
• Nitrogen pulls π electrons toward itself: C2, C4 and C6 are electron-poor.
• Nucleophiles attack C2 and C4 (Chichibabin reaction, SNAr of 2- and 4-chloropyridine); electrophilic substitution is slow and goes at C3.
• The dipolar forms are minor (a carbon cation without an octet), but they explain this reactivity.
• Contributors are not in equilibrium: the real species is one hybrid of them · every part of this figure is editable

Drawn with 36 atoms, 10 arrows, 3 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020 Textbook