Question

The following hydrocarbon has an unusually large dipole moment. Explain how a large dipole moment might arise

(a)Cyclopnonatetraene cation

(b) Cyclononatetraene anion

(c) [16] annulene dianion

(d) [18] annulene dianion

(e)

(f) [20] annulene dication

Short answer

(a)

(b)

(c)

(d)

(e)

(f) [20] annulene dication

Step-by-step solution

Criteria for aromatic and antiaromatic compounds

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

Classification of the aromatic, antiaromatic and nonaromatic molecule

(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.