What are five-membered rings with one heteroatom?
Five-membered rings with one heteroatom are pentagonal heterocycles that have one atom in the ring other than carbon.
It is one of the most relevant groups of heterocyclic compounds, and the three most notable are: pyrrole, furan and thiophene.
These heterocycles can be considered as aromatic, to a greater or lesser degree, according to their physicochemical properties and their resonance energies.
All of them present an excess of π electrons, since they have 6 electrons distributed in the 5 atoms of the ring.
Thus, they are π-excessive heterocyclic compounds. Therefore, due to this, their chemistry presents certain parallels to benzene aromatic compounds, and nucleophilic compounds such as aniline or phenol.
Another suitable compound for comparison would be a diene conjugate. Both types of compounds are nucleophilic and therefore react preferentially with electrophiles.
The chemistry of each of the three compounds mentioned above (pyrrole, furan and thiophene) will be discussed separately. However, there are many similarities, and common reactions in all three.
From the point of view of the valence bond, these compounds can be considered as resonance hybrids of various structures. Thus, the result of the delocalization of the unshared electron pair of the heteroatom can be explained graphically. That is, the degree of negative charge presented by the carbons of the ring.
Structure I is the main contributor because there is no load separation. Structures II and III may be more predominant than IV and V, due to the smaller load separation between them.
For benzene it is possible to write two resonant structures.
However, for these heterocycles there is only one structure without charge separation.
This limitation is reflected in the experimental and calculated heats of combustion. Thus, it shows that the stabilization energies of the heterocycles correspond to approximately half that of benzene, which for pyrrole, thiophene and furan are: 87.69, 66.51, and 121.10 kJ·mol−1, respectively.
Moreover, as the electronegativities of the heteroatoms are in the order O > N > S, the II—V resonance structures are of minor importance in the case of furan compared to pyrrole and thiophene. Oxygen is more reluctant to give up its electron pair, and therefore furan is less aromatic than the other two.
Therefore, the aromaticity of these heterocycles will depend on the two electrons that the heteroatom contributes to the π-system.