Direct nitration of aniline yields tarry oxidation products alongside nitro derivatives. In strongly acidic conditions, aniline forms the anilinium ion, which is meta-directing. Acetylation of the -NH₂ group can control nitration, favoring the formation of the p-nitro derivative as the major product.
Explain the challenges in direct nitration of aniline and how the formation of tarry oxidation products can be controlled.
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The direct nitration of aniline poses challenges due to the amino group’s activating nature, leading to poly-substitution and the formation of tarry oxidation products. The amino group facilitates the attack of nitronium ions, resulting in multiple substitutions on the aromatic ring. To control the tarry oxidation products, aniline is often first acetylated to form N-acetylaniline, reducing its reactivity. Additionally, maintaining low reaction temperatures and using a mixture of concentrated sulfuric acid and nitric acid helps control the extent of nitration. These measures minimize unwanted poly-substitution, allowing for the selective introduction of nitro groups on the aromatic ring.