Amines are organic nitrogen compounds derived from ammonia ($NH_{3}$) by replacing one, two, or three hydrogen atoms with alkyl or aryl groups. They are classified as primary (1°, $RNH_{2}$), secondary (2°, $R_{2}NH$), or tertiary (3°, $R_{3}N$) based on the number of carbon-containing substituents on nitrogen. A quaternary ammonium salt (R_{4}N^{+}$$X^{-}) has all four positions on N occupied by carbon groups and carries a permanent positive charge — it is not a base. Aniline (Nc1ccccc1) is the prototypical aromatic amine and is the benchmark for comparing aromatic versus aliphatic amine basicity.

Preparation Methods: The four principal routes to amines each have distinct scope and limitations. Reduction of nitro compounds ($RNO_{2}$ or $ArNO_{2}$) using Sn/HCl or $H_{2}$/Pd/C gives primary amines directly and is the most widely used industrial route for aniline production. Gabriel phthalimide synthesis is a highly selective method producing only primary aliphatic amines: phthalimide is deprotonated by KOH to form potassium phthalimide, which undergoes SN2 reaction with a primary alkyl halide, followed by hydrazinolysis ($N_{2}H_{4}$) to release the free amine. Crucially, Gabriel synthesis cannot produce aromatic amines (ArN$H_{2}$) because aryl halides (ArX) do not undergo SN2 reactions, nor can it produce secondary or tertiary amines. Hoffmann bromamide degradation converts an amide (RCON$H_{2}$) into a primary amine with one fewer carbon (RCON$H_{2}$ + $Br_{2}$ + 4NaOH → $RNH_{2}$ + $Na_{2}CO_{3}$ + 2NaBr + 2$H_{2}$O); the carbonyl carbon is expelled as sodium carbonate. Ammonolysis of alkyl halides with excess $NH_{3}$ gives a mixture of 1°, 2°, 3°, and quaternary salts and is not selective.

Basicity — The Solvation Paradox: In the gas phase, electron-donating alkyl groups raise the electron density on nitrogen, making proton acceptance easier. Consequently, the gas-phase basicity order follows the inductive sequence: 3° > 2° > 1° > $NH_{3}$. In aqueous solution, a competing factor — the solvation stability of the conjugate acid — changes the picture entirely. When an amine accepts a proton ($H^{+}$), the resulting conjugate acid ($R_{3}NH^{+}$, $R_{2}NH_{2}^{+}$, or $RNH_{3}^{+}$) must be stabilised by hydrogen bonding with water molecules. The conjugate acid of a tertiary amine ($R_{3}NH^{+}$) has only one N–H bond and three bulky alkyl groups surrounding the nitrogen, physically hindering water molecules from forming a stable solvation shell. This poor solvation of $R_{3}NH^{+}$ destabilises the protonated form and shifts the equilibrium toward the unprotonated amine, reducing the observed basicity. As a result, the aqueous basicity order becomes: 2° > 1° > 3° > $NH_{3}$. Aniline is far weaker than any aliphatic amine (pKb ≈ 9.38 vs ≈ 3.3–4.5) because the lone pair on nitrogen delocalises into the benzene π system via resonance, reducing its availability for protonation. Electron-withdrawing groups at the para position (e.g., –$NO_{2}$) further weaken aniline's basicity, while electron-donating groups (e.g., –$OCH_{3}$) modestly increase it.

Identification Tests: The carbylamine test uses $CHCl_{3}$ and KOH to produce an isocyanide (R–NC) with a characteristically foul odour; this reaction is exclusive to primary amines (1°). Secondary and tertiary amines give no isocyanide. The Hinsberg test uses benzenesulfonyl chloride ($C_{6}H_{5}SO_{2}Cl$) with NaOH: a 1° amine forms a sulfonamide that retains an N–H (acidic proton) and is therefore soluble in NaOH; a 2° amine forms a sulfonamide with no N–H and is insoluble in NaOH; a 3° amine does not react with the sulfonyl chloride at all.

Diazonium Salt Chemistry: Primary aromatic amines undergo diazotization with Na$NO_{2}$ and HCl at strictly controlled 0–5 °C to form diazonium salts (ArN_{2}^{+}$$Cl^{-}). Temperature must be kept low because diazonium salts decompose rapidly above 5 °C, releasing $N_{2}$ and forming phenol. Diazonium salts are among the most versatile intermediates in aromatic chemistry. The Sandmeyer reaction converts $ArN_{2}^{+}$ into ArCl (CuCl/HCl), ArBr (CuBr/HBr), or ArCN (CuCN/KCN) using cuprous ($Cu^{+}$) catalysts. The Gattermann reaction achieves the same ArCl and ArBr products using metallic copper powder (Cu°) rather than a copper salt. The Schiemann (Balz-Schiemann) reaction is the only reliable route to aryl fluorides: ArN_{2}^{+}$$Cl^{-} + $HBF_{4}$ → ArN_{2}^{+}$$BF_{4}^{-} → ArF + $BF_{3}$ + $N_{2}$ on heating. Azo coupling reactions form intensely coloured azo dyes (–N=N– chromophore): coupling with phenol in alkaline medium (phenoxide as the activated nucleophile) gives p-hydroxyazobenzene (orange-red), and coupling with aniline in weakly acidic medium gives p-aminoazobenzene (yellow). The medium requirement is a frequently tested NEET trap — alkaline for phenol, weakly acidic for aniline.

NEET Importance: This chapter consistently contributes 2–3 questions per year. The aqueous basicity order, Gabriel synthesis limitations, Hoffmann carbon-count reduction, Sandmeyer vs Schiemann distinction, and azo coupling medium conditions are the five highest-yield areas.