Convert (i) 3-Methylaniline into 3-nitrotoluene. (ii) Aniline into 1,3,5 - tribromobenzene.
(i) Conversion of 3-Methylaniline into 3-nitrotoluene:
3-Methylaniline contains both –NH₂ and –CH₃ groups. To convert the –NH₂ group into –NO₂ group, diazotization reaction is used.
Step 1: 3-Methylaniline is treated with NaNO₂ and HCl at 273-278 K to form benzene diazonium chloride (diazotization).
Step 2: The diazonium salt is then treated with nitrous acid (HNO₂) or copper nitrite [Cu(NO₂)₂] to replace the diazonium group with –NO₂ group, yielding 3-nitrotoluene.
This method is used for preparing substituted aromatic compounds which cannot be prepared by direct substitution.
(ii) Conversion of Aniline into 1,3,5-tribromobenzene:
Step 1: Aniline undergoes bromination with bromine water at room temperature to give a white precipitate of 2,4,6-tribromoaniline.
Aniline + 3Br₂ (Br₂/H₂O) → 2,4,6-Tribromoaniline + 3HBr
Step 2: The –NH₂ group in 2,4,6-tribromoaniline is converted to diazonium salt by treatment with NaNO₂ and HCl at 273-278 K (diazotization).
Step 3: The diazonium salt is then treated with hypophosphorous acid (H₃PO₂) or ethanol, which replaces the diazonium group with hydrogen, yielding 1,3,5-tribromobenzene.
Explanation
These conversions demonstrate the importance of diazotization reactions in organic synthesis. The –NH₂ group in aromatic amines is highly activating, making direct substitution difficult to control. By converting –NH₂ to diazonium salts, we can replace it with various groups including –NO₂ or –H. The context explicitly mentions that aniline reacts with bromine water to give 2,4,6-tribromoaniline, and discusses the high reactivity of aromatic amines in electrophilic substitution reactions.