How will you convert benzene into
(i) p – nitrobromobenzene
(ii) m – nitrobromobenzene
The position of the substituent is different for both compounds but their chemical nature is same. So we have to consider the steps in which nitro and bromo groups are attached to the benzene ring.
Option (i) when bromine is treated with Br2 in presence anhydrous FeBr3 it undergoes electrophilic substitution to give bromobenzene.
This bromobenzene is ortho para directing due positive mesomeric effect (resonance effect) or +R effect caused by Bromine attached to the ring. This activates the ring with negative charge and allows for electrophilic substitution when treated with conc. HNO3 and conc. H2SO4 at temperature of 323 – 333 K.
Here p-nitrobromobenzene is major product as ortho product has more stearic hindrance due to Br and NO2.
Option (ii) here we need a meta-derivative of benzene, so we first need to deactivate the benzene by introduction of nitro group using conc. HNO3 and H2SO4 as follows:
Due to presence of nitro group the ring is deactivated by –R effect or negative resonance effect, and positive charge is developed at ortho and para position which makes it less active site for electrophilic substitution. So when treated with Br2 in presence of FeBr3 we get a Meta derivative of nitrobenzene.
