Question

Nitrobenzene on further excessive nitration gives
A. Sym-trinitrobenzne
B. M-Dinitrobenzne
C. P-Dinitrobenzene
D. All of these

Answer 1

Hint: Nitration is a chemical process by which nitro group (s) are introduced to a chemical compound. Nitration can be done by reacting benzene with \[{\rm{HN}}{{\rm{O}}_{\rm{3}}}\] (nitric acid ) in presence of sulphuric acid (\[{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}\]).

Complete Step by Step Solution:
Let's discuss nitrobenzene first. Nitrobenzene is a chemical species possessing the chemical formula of\[{{\rm{C}}_{\rm{6}}}{{\rm{H}}_{\rm{5}}}{\rm{N}}{{\rm{O}}_{\rm{2}}}\] . It shows the insolubility in water. It is A pale yellow coloured oil possessing the odour of almond. Freezing of it gives green-yellow coloured crystals. It is manufactured in huge quantities from benzene. The structure of nitrobenzene is,

Image: Nitrobenzene

Here, we have to find out the product which is produced when excessive nitration of nitrobenzene occurs.

Here, nitrobenzene undergoes excessive nitration. When nitrobenzene undergoes a reaction with nitric acid in presence of sulphuric acid, the following reaction occurs.
\[{\rm{HN}}{{\rm{O}}_{\rm{3}}} + {{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}} \to {\rm{N}}{{\rm{O}}_{\rm{2}}}^ + + {{\rm{H}}_{\rm{2}}}{\rm{O}} + {\rm{HS}}{{\rm{O}}_{\rm{4}}}^ - \]

Although \[{\rm{HN}}{{\rm{O}}_{\rm{3}}}\] is an acid, in the above reaction, it acts as a base and \[{{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}\] acts as an acid. And we know, nitro group is an meta directing and deactivating group. Therefore, the product obtained is,

Image: Nitration of nitrobenzene

Further nitration of nitration is not possible because of the deactivating nitro group.
Hence, the option B is right.

Note: Nitration of benzene takes place in three steps, First step is, Activation of the electrophile by the acid. Second step is an attack of the activated electrophile by benzene and the last step is deprotonation to restore the aromatic ring and regeneration of the acidic catalyst.