The conversion of 4-nitrotoluene to 2-bromotoluene involves a multi-step synthesis where the nitro group is used to direct the incoming bromine and is subsequently removed:

1. Bromination ($Br_2$): In 4-nitrotoluene, the $-CH_3$ group is activating and ortho/para directing, while the $-NO_2$ group is deactivating and meta directing. Both groups direct the incoming electrophile ($Br^+$) to the same position: ortho to the methyl group. This forms 2-bromo-4-nitrotoluene.
2. Reduction ($Sn/HCl$): The nitro group is reduced to a primary amino group, converting 2-bromo-4-nitrotoluene to 4-amino-2-bromotoluene (or 3-bromo-4-methylaniline).
3. Diazotisation ($NaNO_2/HCl$): The primary aromatic amine reacts with nitrous acid at 273-278 K to form a diazonium salt: 3-bromo-4-methylbenzenediazonium chloride.
4. Deamination ($H_3PO_2/H_2O$): The diazonium salt is treated with a mild reducing agent like hypophosphorous acid ($H_3PO_2$) in the presence of water. This replaces the diazonium group ($-N_2^+Cl^-$) with a hydrogen atom ($-H$), yielding the final product, 2-bromotoluene.