7-bromo-1,3,5-cycloheptatriene exists as ionic species in aqueous solution while 5-bromo-1,3-cyclopentadiene doesn't ionise even in presence of `Ag^(+)(aq)`, Explain.

Explanation

7-Bromo-1,3,5-cycloheptatriene, on ionization, gives tropylium ion. Since , tropylium ion contains `6 pi`-electrons which are completely delocalized, 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 ionization, will give 1,3-cyclopentadienyl cation which contains `4pi`-electrons and hence in antiaromatic. Being antiaromatic, it is highly unstable and hence is not formed even in the presence of `Ag^+` ion which otherwise facilitates ionization.