Amines & Diazonium Salts

Amines are nitrogen-containing organic compounds classified as primary (1 degree, RNH₂), secondary (2 degree, R₂NH), and tertiary (3 degree, R₃N). Aniline () is the prototypical aromatic amine.

Preparation Methods: Reduction of nitro compounds (RNO₂ →(Sn/HCl or H₂/Pd) RNH₂) is the most direct route. Gabriel phthalimide synthesis selectively produces 1 degree aliphatic amines ONLY: phthalimide + KOH → potassium phthalimide + RX → N-alkylphthalimide →(N₂H₄ or NaOH) 1 degree amine. It cannot make 2 degree, 3 degree, or aromatic amines (ArNH₂). Hoffmann bromamide degradation: RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O. The product amine has ONE FEWER carbon than the starting amide — a key distinguishing feature.

Basicity of Amines — The Solvation Paradox: In the gas phase, basicity follows the expected electronic order: 3 degree > 2 degree > 1 degree > NH₃ (more alkyl groups = more +I effect = better electron donation to a proton). In aqueous solution, the order changes to 2 degree > 1 degree > 3 degree > NH₃ for aliphatic amines. The reversal for 3 degree amines occurs because R₃NH+ (the conjugate acid) has three bulky alkyl groups that prevent effective hydration (solvation) by water molecules. Poor solvation of the conjugate acid shifts the equilibrium away from protonation, making 3 degree amines appear weaker bases in water. Aniline is a much weaker base than aliphatic amines because its lone pair on nitrogen is delocalized into the benzene ring by resonance, reducing availability for protonation. Electron-withdrawing groups (p-NO₂) further decrease basicity; electron-donating groups (p-OCH₃) increase it.

Distinction Tests: The carbylamine test (isocyanide test) is specific for 1 degree amines: R-NH₂ + CHCl₃ + 3KOH → R-NC (isocyanide, foul smell) + 3KCl + 3H₂O. 2 degree and 3 degree amines do NOT respond. The Hinsberg test uses benzenesulfonyl chloride (C₆H₅SO₂Cl): 1 degree amine gives sulfonamide soluble in NaOH (has acidic N-H); 2 degree amine gives sulfonamide insoluble in NaOH (no N-H); 3 degree amine gives no reaction.

Diazonium Salts: Diazotization of aromatic 1 degree amines: ArNH₂ + NaNO₂ + 2HCl →(0-5 deg C) ArN₂+Cl- + NaCl + 2H₂O. Diazonium salts are extremely versatile:

Sandmeyer reaction: ArN₂+ + CuCl/HCl → ArCl; ArN₂+ + CuBr/HBr → ArBr; ArN₂+ + CuCN/KCN → ArCN (Cu₂X₂ or CuX catalyst). Gattermann reaction: Same products (ArCl, ArBr) but uses Cu powder instead of CuX. Schiemann (Balz-Schiemann) reaction: ArN₂+ + HBF₄ → ArN₂+BF₄- →(heat) ArF + BF₃ + N₂. This is the ONLY reliable route to aryl fluorides. Coupling reactions produce azo dyes: ArN₂+ + C₆H₅OH (alkaline) → p-hydroxyazobenzene (, orange-red); ArN₂+ + C₆H₅NH₂ (weakly acidic) → p-aminoazobenzene (yellow). Coupling with phenol occurs in alkaline medium (phenoxide is the activated substrate).

The key testable concept is the aqueous basicity order of aliphatic amines (2 degree > 1 degree > 3 degree) caused by the solvation effect, and that Gabriel phthalimide synthesis produces ONLY primary aliphatic amines.