Give reasons for the following:(a)Reduction with iron scrap and HCl is preferred for the preparation of amines from nitro compounds. (b)Lower aliphatic amines are soluble in water. (c)Pyridine is used in the acylation reaction of amines. (d)Although amino group is o- and p-directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-nitroaniline.
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In reasoning questions, always consider electronic effects (inductive, resonance), medium conditions (acid/base), and steric effects for explanations.
(a) Step 1: Iron and HCl is a mild and selective reducing agent. Step 2: It avoids strong reducing conditions that might affect other functional groups. Step 3: Iron is cheap and environmentally friendly, forming less toxic waste. (b) Step 1: Lower aliphatic amines have nitrogen atoms with lone pairs. Step 2: These lone pairs form hydrogen bonds with water molecules. Step 3: Hydrogen bonding enhances solubility. (c)Step 1: During acylation, HCl is released. Step 2: Pyridine, being a weak base, neutralizes this HCl. Step 3: This prevents the protonation of the amine, allowing the reaction to proceed efficiently. (d)Step 1: In acidic nitration medium, the -NH\(_2\) group gets protonated to form -NH\(_3^+\). Step 2: The anilinium ion is deactivating and meta-directing. Step 3: Hence, nitration gives a significant amount of meta product despite the original directing nature of -NH\(_2\).
