Arrange the compounds in increasing order of their rate of nitration and give reason: Benzene , Toluene,Nitrobenzene, Hexadeuterobenzene `(C_6D_6)`.

Explanation

Increasing order of rate of nitration of given compounds is : nitrobenzene `lt` benzene = hexadeuterobenzene `lt` toluene.
The electron-attracting nitro `(-NO_2)` group decreases the electron density of the nitrobenzene ring and as a result ,its nitration occurs at a rate slower than that of benzene . On the other hand, the electron -repelling methyl `(-CH_3)` group increases the electron density of the toluene ring and as a result, its nitration proceeds at a rate faster than that of benzene. Benzene and hexadeuterobenzene `(C_6D_6)` undergo nitration at the same rate because in aromatic electrophilic substitution reaction, cleavage of C-H or C-D bond does not occur at the rate- determining step.