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Question

Which of the following compound(s) are stable in their ionic structure?

A
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B
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C
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D
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Solution

The correct options are
B
D
Order of stability:
Aromatic > Non-aromatic > Antiaromatic
Option (a):
It is planar, has conjugation and the number of π electrons in ring system are 4, which follow (4n) π e rule, thus making it antiaromatic and the least stable.
Option (c) :
The ring that has negative charge on carbon is planar , has conjugation has 6π e i.e. (4n+2)π e , making it aromatic.
The ring that has positive charge on carbon is planar, has conjuagtion and 4π e i.e. it has (4n) π e . So, it is anti-aromatic and thus formation of charges on that ring is highly unlikely due to its unstability.

So, ionic form of that compound is unstable due to 1 anti-aromatic ring.

In option (b) it forms a tropylium cation which is planar , has conjugation and has 6π electron. So, ionic form is highly stable due to aromaticity.
Structure in option (d) forms a two ring system which are both planar, have conjugation and have 6 and 2 π electrons respectively, thus both are obeying the (4n+2) π e Huckel's rule. Hence, both rings are becoming aromatic on charge separation.
Hence, ionic form is stable.
So, structures in option (b) and (d) are stable in their ionic form.

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