Organic chemistry
Pyridine, indole, furan, thiophene
Aromatic heterocycles replace one or more carbon atoms of a benzene-like ring by heteroatoms, changing basicity, electron distribution and reactivity.
IntuitionIntuition: a ring with a personality
Replace one ring carbon of benzene by nitrogen and the ring no longer shares its electron cloud evenly. In pyridine the nitrogen lone pair stays outside the π system and is available for bonding to protons, while in furan one oxygen lone pair contributes to the aromatic sextet and is much less basic.
SchoolSchool level: names, formulas and typical examples
Definition:
A ring compound whose ring contains atoms other than carbon, commonly nitrogen, oxygen or sulfur. Aromatic heterocycles satisfy a conjugated, cyclic π system that gives unusual stability.
Pyridine is a six-membered C₅H₅N ring that acts like an aza-benzene. Indole is benzene fused to pyrrole and is the scaffold of tryptophan, serotonin and many dyes. Furan and thiophene are five-membered oxygen- and sulfur-containing rings that are electron rich relative to benzene.
| Ring | Formula | Main feature |
|---|---|---|
| Pyridine | C₅H₅N | Basic N lone pair; coordinate donor |
| Indole | C₈H₇N | Benzene fused to pyrrole; electron rich |
| Furan | C₄H₄O | O lone pair contributes to aromaticity |
| Thiophene | C₄H₄S | More benzene-like than furan |
UndergraduateUniversity: electron distribution and electrophilic substitution
In pyridine, electronegative nitrogen withdraws π electron density, making the ring less reactive toward electrophilic aromatic substitution than benzene and most reactive at the 3-position. In furan, thiophene, pyrrole and indole the heteroatom donates electron density into the ring; these rings are electron rich and substitution patterns differ by position and heteroatom.
Example: Explain relative basicity
Pyridine is a much stronger base than pyrrole. Why does the nitrogen lone pair accept a proton more readily in pyridine?
Solution
In pyridine the N lone pair occupies an in-plane sp² orbital that does not take part in the aromatic sextet, so protonation does not destroy aromaticity. In pyrrole the lone pair is part of the π sextet; protonation would remove aromatic stabilization.
References
- Heterocyclic Chemistry (5th ed.) · John A. Joule, Keith Mills, 2010