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Heteroaromatic Compounds

Heteroaromatic compounds may contain several heteroatoms in several of their rings. The limit is, as always, your imagination.

YouTube video: pyridine and pyrrole▶ Watch the video on YouTube

Hückel put forward rules for aromaticity and antiaromaticity:

A system of π bonds is aromatic when:

  1. conjugated
  2. cyclic
  3. planar
  4. holding 4n + 2 electrons

If it holds 4n electrons instead, the π system is antiaromatic.

All the following molecules comply with the previous conditions of aromaticity.

In order to count the 4n + 2 π electrons, the heteroatoms may or may not bring their lone pairs into play.

Structure and numbering of pyridine, with its lone pairsStructure and numbering of pyrimidine, with its lone pairsStructure and numbering of pyrazine, with its lone pairsStructure and numbering of quinoline, with its lone pairs
PyridinePyrimidinePyrazineQuinoline
Structure and numbering of isoquinoline, with its lone pairsStructure and numbering of pyrrole, with its lone pairsStructure and numbering of indole, with its lone pairsStructure and numbering of purine, with its lone pairs
IsoquinolinePyrroleIndolePurine
Structure and numbering of imidazole, with its lone pairsStructure and numbering of pyrazole, with its lone pairsStructure and numbering of furan, with its lone pairsStructure and numbering of thiophene, with its lone pairs
ImidazolePyrazoleFuranThiophene

Comparison of the basicity of pyridine and pyrrole

The electron lone-pairs of the heteroatoms may or may not be shared in the 4n + 2 electron pool, one of the conditions for aromaticity. It all depends on the structure.

What can one deduce from the following basicity data?

Pyridine takes up a proton with its lone pair and forms the pyridinium ion

pKb = 8.8

Pyridine is definitely basic, isn't it?

This fact indicates that the lone pair of nitrogen does not contribute to the aromaticity. The six electrons in the six p orbitals comply by themselves with Hückel's 4n + 2 (n = 1) rule.

Pyrrole is protonated only at a carbon, with loss of aromaticity, because the nitrogen lone pair is part of the aromatic sextet

pKb > 18

Pyrrole is not basic at all.

This is a clear indication that the lone pair of nitrogen does contribute to the aromaticity, is heavily delocalized and thus cannot react with an acid. Hückel's 4n + 2 (n = 1) rule is fulfilled by the four electrons provided by the four carbons and the two electrons shared by the nitrogen.

The electronic structure of both rings allows us to understand their different basicity properties:

Orbitals of pyridine: the nitrogen lone pair lies in an sp2 orbital, in the plane of the ring, and is not part of the aromatic sextet
Orbitals of pyrrole: the nitrogen lone pair occupies the p orbital and is part of the aromatic sextet

Reactivity of pyridine and pyrrole towards SEAr

Electrophilic aromatic substitution (SEAr) is studied later in this unit. Click here to see what SEAr is about.

Concerning reactivity, are pyridine and pyrrole more or less reactive than benzene towards SEAr? What is your best guess?

Resonance forms of pyridine: the nitrogen attracts the negative charge and leaves positive charges on carbons 2, 4 and 6

Pyridine's nitrogen withdraws electron density from its ring.

Pyridine is less reactive than benzene towards SEAr.

Resonance forms of pyrrole: the nitrogen lone pair is delocalized and puts negative charge on the ring carbons

Pyrrole's nitrogen cannot withdraw electron density from the ring; if anything, it can donate it.

Pyrrole is more reactive than benzene towards SEAr.