8 Structures of Heteroaromatic Compounds
Other fi ve-membered aromatic heterocycles have exactly comparable structures to that of pyrrole: thiophene (resonance
energy ~122 kJ mol^1 ) is the ‘most aromatic’ of the trio and furan (~68 kJ mol^1 ) the ‘least aromatic’ and, indeed, furan
does behave as a diene, rather than an aromatic molecule, in some cases. Note that in these two heterocycles, the heter-
oatom has two different types of lone pair – one involved in the aromatic sextet and the other, in the plane of the ring, in
an sp^2 hybrid orbital, and NOT involved in the aromatic -system. The electron drift away from the heteroatoms in these
two heterocycles is less than that in pyrrole and as a result both have dipole moments directed towards the heteroatom.
Structure of indoles
When a benzene ring is fused to a pyrrole, as in the 10-electron indole, strong polarisation is seen only in the hetero-
cyclic ring as implied by the resonance contributors.
Structures of azoles (illustrated using imidazole)
Finally, we consider the azoles: fi ve-membered heterocycles with a nitrogen and another heteroatom located either
adjacent to the nitrogen (1,2-azoles) or in a 1,3-relationship (1,3-azoles). We can understand their structures by con-
sidering one typical example – imidazole.
The two nitrogen atoms are completely different – one of them is like the nitrogen in pyrrole, but the other (the imine
nitrogen) is like the nitrogen in pyridine. The former donates a pair of electrons into the aromatic -system; the latter
donates just one electron into the aromatic system. The former carries an N-hydrogen, the latter does not. The former
does not have a pair of electrons in the plane of the ring, the latter does.