123.Aromaticity(16) – Heterocyclic aromatic systems(2)-Furan,pyrrole and thiophene.

Furan, Pyrrole and Thiophene.

All the above structures are similar to each other , except for the fact that pyrrole has a lone pair and one N-H bond instead of two lone pair of electrons(as in furan and thiophene). All the atoms in the above structure are sp² hybridized. The distribution of electrons in these structures is as follows-

(Electrons in sp² hybridized orbitals ⇒ form σ bonds
Electrons in unhybridized p-orbitals- perpendicular to σ framework ⇒ involved in aromatic sextet.)

Furan ,Thiophene and Pyrrole are all aromatic. Why?

1. All atoms in these structures are sp² hybridized.So , all the four carbon atoms and the heteroatom are in the same plane. ∴ All the three structures are planar.
2. They all have an aromatic sextet i.e 6π(4n+2 π , where n=1) electrons. These electrons are in the unhybridized p-orbital of each atom- perpendicular to the plane of the ring. The double bonds and the lone pair involved in the sextet are in conjugation as shown below-
3. All structures are cyclic.

Pyrrole is isoelectronic with cyclopentadienyl ion (121.Aromaticity(14) – Homocyclic aromatic systems.). However, pyrrole is electrically neutral. The presence of nitrogen leads to loss of radial symmetry in the molecule.

Also,the resonance structures of pyrrole are not equivalent as in cyclopentadienyl ion, due to the presence of the heteroatom. The first resonance structure has no charge. The next structures have a positive charge on N and the negative charge and double bonds alternate with each other as shown below-

Similar resonating structures can be written for furan and thiophene. The only difference is that the N-H will be replaced by O and S respectively. Also, the electronegativity of S and O is more than N. Thus, the resonating structures with positive charge on hetero atom(S/O) will make smaller contribution to the overall structure of these compounds.

Unlike pyridine, pyrrole is an extremely weak base as the lone pair of electrons are required to maintain the aromatic sextet.

Order of Aromaticity

Benzene is more aromatic than thiophene , pyrrole and oxygen because all the π electrons are totally involved in forming the aromatic sextet. Whereas in other molecules, the heteroatoms being more electronegative than carbon, they pull the electron cloud towards themselves.Thus, there is an uneven charge distribution.

More electronegativity of the hetero atom less – is the aromaticity of the compound.

Oxygen is the most electronegative and so it is the least aromatic. Sulphur is less electronegative .Also the 3s and 3p electrons are the valence electrons in sulphur atom as opposed to 2s and 2p in oxygen and nitrogen. Thus , the above trend depends on electronegativities of the heteroatom in the structure.

In the next post we will continue talking about some more heterocyclic compounds.Till then,

Be a perpetual student of life and keep learning …

Good day !