Account for the following: (i) pKb of aniline is more than that of methylamine. (ii) Ethylamine is soluble in water whereas aniline is not. (iii) Methylamine in water reacts with ferric chloride to precipitate hydrated ferric oxide. (iv) Although amino group is o- and p- directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-nitroaniline. (v) Aniline does not undergo Friedel-Crafts reaction. (vi) Diazonium salts of aromatic amines are more stable than those of aliphatic amines. (vii) Gabriel phthalimide synthesis is preferred for synthesising primary amines.
In aniline, the -NH₂ group is attached directly to the benzene ring. The lone pair on nitrogen is in conjugation with the benzene ring through resonance, making it less available for protonation. This makes aniline less basic than methylamine, hence higher pKb value.
In aniline, the lone pair on nitrogen is involved in resonance with the benzene ring, reducing its ability to form hydrogen bonds with water. Ethylamine, being aliphatic, can form stronger hydrogen bonds with water, making it soluble whereas aniline is not.
Methylamine is basic and produces OH⁻ ions in aqueous solution. These OH⁻ ions react with Fe³⁺ from FeCl₃ to form hydrated ferric oxide (Fe₂O₃·xH₂O) as a precipitate.
During nitration in acidic medium (HNO₃ + H₂SO₄), the amino group gets protonated to form -NH₃⁺ ion, which is meta-directing and deactivating. This leads to formation of substantial amount of m-nitroaniline. To get ortho/para products, the -NH₂ group needs to be protected by acetylation first.
Aniline forms a salt with AlCl₃ (the Lewis acid catalyst used in Friedel-Crafts reaction). Due to this salt formation, nitrogen acquires a positive charge and acts as a strong deactivating group, preventing further reaction.
In aromatic amines, the diazonium ion is stabilized by resonance with the benzene ring. The positive charge can be delocalized over the aromatic ring, providing extra stability compared to aliphatic diazonium salts which lack such resonance stabilization.
Gabriel phthalimide synthesis is preferred because it yields pure primary amines without contamination by secondary or tertiary amines, which is a common problem with other methods of amine synthesis.
Explanation
The answers are derived from the provided context which explains resonance in aniline, its effect on basicity and reactivity, Friedel-Crafts reaction inhibition, and electrophilic substitution reactions. Parts (i) and (v) are directly stated in the context. Parts (ii), (iii), (iv) use inferences from the chemical principles mentioned. Parts (vi) and (vii) are not explicitly covered in the context but follow from the resonance concepts discussed.