Question

Arrange the following compounds in the decreasing order of their rates of electrophilic aromatic substitution.

• A. I > II > III
• B. III > II > I
• C. III > I > II
• D. I > III > II

Answer 1

The correct option is B III > II > I

More the eletron density on the ring, the more the reactivity of the compound towards electrophilic substitution.
There is no extra stabilisation of the arenium ion in benzene (I), but extra stabilisation is present in biphenyl and fluorene. So, benzene (because of the low electron density) will be the least reactive among all the compounds. In the case of biphenyl (II), benzene is present as an electron-donating group and shows the +R effect. However, there will be less conjugation between both the phenyl rings due to the free rotation of the C-C bond. Its reactivity is still more than that of benzene and less than that of fluorene.
In the case of fluorene (III), the +R of benzene and +H up to some extent are present. In fluorene, two benzene rings are fused to give a five-membered ring. Due to the fixed benzene rings, full conjugation between both the phenyl rings is present.
Thus, the decreasing order of the ratesof electrophilic aromatic substitution is: III > II > I
Hence, option (b) is the correct answer.