Each cyclic structure shown contains a heteroatom that has lone-pair electrons. For each structure, determine whether the lone-pair electrons are delocalized and whether the structure is aromatic.
The Solution
A. Pyridine is already an aromatic compound according to Huckel's rule
system, n=2. So the lone pair in the N atom doesn't participate in the resonance with the benzene ring and so it is not delocalized.
system, n=2. So the lone pair in the N atom doesn't participate in the resonance with the benzene ring and so it is not delocalized.B. Lone pair in B do take part in the cyclic resonance with the pyrrole ring because had there been no delocalization, the system would be Antiaromatic. So, in B, the lone pair is delocalized.
C. Unlike A, the pyrylium cation is also aromatic, so the lone pair in the O is not delocalized.
[NOTE- Though one of the lone pairs in O is delocalized to produce the pyrylium cation. After the aromatic system is produced, the other lone is now out of the plane of the aromatic system and cannot take part in resonance (also because any extra pi-electron will make the system antiaromatic)].
D. One of the lone pairs is delocalized to make the ring aromatic, unlike B. The other lone pair is localized.
