7-bromo-1, 3, 5-cycloheptatriene exists as an ion

hydrocarbons
cbse

#1

7-bromo-1, 3, 5-cycloheptatriene exists as an ion whereas 5-bromo-1, 3-cyclopentadiene does not form an ion even in presence of ${{Ag}^{+}}$. Explain


#2

7-bromo-1, 3, 5-cycloheptatriene, on ionisation, gives tropylium ion. Since, tropylium ion contains 6 $\pi$-electrons which are completely delocalised, therefore, according to Huckel rule, it is aromatic and hence stable. Being highly stable, it is easily formed.

In contrast, 5-bromo-1,3-cyclopentadiene, on ionisation, will give 1, 3-cyclopentadienyl cation which contains 4$\pi$-electrons and hence, is anti-aromatic. Being anti-aromatic, it is highly unstable and hence is not formed even in the presence of ${{Ag}^{+}}$ ion which otherwise facilitates ionisation.