91Ó°ÊÓ

The aromatic compounds are those that meet the following criteria:

1. The structure must be cyclic have a continuous ring of overlapping p orbitals, usually in a planar conformation.
2. Delocalization of the pi electrons over the ring must lower the electronic energy.
3. It should have (4n+2) pi electrons, where n is an integer.

The antiaromatic compounds are those that meet the following criteria:

1. The structure must be cyclic have a continuous ring of overlapping p orbitals, usually in a planar conformation.
2. Delocalization of the pi-electron over the ring must lower the electronic energy.
3. It should have 4n pi electrons, where n is an integer.

A cyclic compound that does not have a continuous, overlapping of p orbitals cannot be aromatic or antiaromatic. It is said to be nonaromatic or aliphatic

(a) Cyclononatetraene cation is cyclic having continuous overlapping of p orbitals and it has 4n pi electrons. Hence, it is antiaromatic.

(b) Cyclononatetraene anion is cyclic having continuous overlapping of p orbital and it has 4n+2 pi electrons. Hence, it is aromatic.

(c) [16] annulene dianion is cyclic having continuous overlapping of p orbital and it has 4n+2 pi electrons. Hence, it is aromatic.

(d) [18] annulene dianion is cyclic having continuous overlapping of p orbital and it has 4n pi electrons. Hence, it is antiaromatic.

(e) The given compound is cyclic but does not have a continuous, overlapping ring of p orbitals. Hence, it is non aromatic.

(f) [20] annulene dication is cyclic having continuous overlapping of p orbital and it has 4n+2 pi electrons. Hence, it is aromatic.