Organic Compounds Containing Nitrogen — Top 20 Questions

Both statements are TRUE. Kjeldahl method is NOT applicable for pyridine (and other compounds where N is in a ring like nitro compounds, azo compounds) because the nitrogen in the ring structure cannot be easily converted to NH₃/ammonium sulphate. Kjeldahl works only for amino-type nitrogen.

C₂H₅NH₂ + HNO₂ → [C₂H₅N₂⁺Cl⁻] → C₂H₅OH + N₂ + HCl. Each mole of ethanamine produces 1 mol HCl. Moles of HCl = moles of NaOH used = 0.2 mol. Moles of ethanamine = 0.2 mol. X = 0.2 × 45 = 9 g.

Gabriel synthesis: Phthalimide + KOH → potassium phthalimide → + C₆H₅CH₂Cl → N-benzylphthalimide. N-benzylphthalimide structure: phthalimide ring (benzene ring: 3π, C=O: 2π bonds in ring) + benzyl group (benzene ring: 3π). Wait: phthalimide has: 1 benzene (3π) + 2 C=O (2π) + N-CH₂-C₆H₅ benzene (3π). Total π bonds = 3+2+3 = 8.

Step (i): C₆H₅NO₂ + Sn/HCl → C₆H₅NH₂ (aniline). Step (ii): C₆H₅NH₂ + NaNO₂/HCl → C₆H₅N₂⁺Cl⁻ (diazonium salt). Step (iii): Sandmeyer with Cu₂Cl₂ → C₆H₅Cl (chlorobenzene, M=112.5). Step (iv): 2C₆H₅Cl + 2Na → C₆H₅-C₆H₅ + 2NaCl (Wurtz-Fittig). Product: biphenyl (C₁₂H₁₀), M = 154 g/mol.

Moles of aniline = 9.3/93 = 0.1 mol. Reaction: C₆H₅NH₂ → C₆H₅N₂⁺Cl⁻ → coupling with phenol → 4-(phenylazo)phenol (orange azo dye, C₁₂H₁₀N₂O, M=198 ≈ 197). Moles of dye = 0.1 mol. Mass = 0.1 × 197 = 19.7 ≈ 20 g.

Benzyl chloride + NH₃ (excess) → C₆H₅CH₂NH₂ (benzylamine, primary amine). Primary amines with Hinsberg's reagent: R-NH₂ + C₆H₅SO₂Cl → R-NHSO₂C₆H₅ (N-substituted sulfonamide, soluble in NaOH — clear solution). This distinguishes 1° from 2° (insoluble precipitate) and 3° (no reaction).

In aqueous solution, basicity of aliphatic amines: (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃. While in gas phase, more methyl groups = more basic (inductive effect). In water, solvation of ammonium ions matters: (CH₃)₃NH⁺ has poor solvation (3 methyl groups block water access) → less basic than (CH₃)₂NH₂⁺.

Aromatic diazonium salts are stable (can be stored at 0−5°C for reactions) because the N₂⁺ group is in conjugation with the benzene ring — the positive charge is delocalised into the ring through resonance. Aliphatic diazonium salts are extremely unstable and immediately lose N₂.

C₆H₅NH₂ + (CH₃CO)₂O → C₆H₅NHCOCH₃ (acetanilide) + CH₃COOH. This is acylation of aniline — the amino group acts as nucleophile, attacks the carbonyl of acetic anhydride. Acetanilide is an important intermediate (used in dye industry, pain relief history).

Carbylamine test: R-NH₂ + CHCl₃ + 3KOH → R-NC (isocyanide, foul smell) + 3KCl + 3H₂O. This test is specific for primary amines only (both aliphatic and aromatic). Secondary and tertiary amines do NOT give positive carbylamine test.

−NH₂ group is a powerful ortho-para director and activating group. Aniline + Br₂/H₂O → 2,4,6-tribromoaniline immediately as white precipitate (like phenol + Br₂). The reaction occurs without catalyst due to strong activation by NH₂ group. Used as a test for aniline.

Reduction of nitrobenzene to aniline: Sn + HCl (or Fe + HCl) in acidic medium is the most common lab method. Also: H₂/Ni (catalytic hydrogenation) works. Zn/NaOH gives different products (azoxybenzene, azobenzene depending on conditions). LiAlH₄ and NaBH₄ can also reduce nitro groups.

Structure: CH₃−CH₂−CH(NH₂)−COOH. COOH at C1, NH₂ at C2, CH₂ at C3, CH₃ at C4. IUPAC: 2-aminobutanoic acid. This is also called 2-amino butyric acid or homoalanine.

Diazonium salts are unstable — they decompose above 5°C releasing N₂ gas and forming phenol. Maintaining 0−5°C keeps them stable for coupling reactions. In industry, diazonium salts are used immediately without isolation. The low temperature is essential for successful diazotisation.

Coupling: C₆H₅N₂⁺Cl⁻ + β-naphthol (in NaOH) → para-hydroxy azobenzene dye. The product 1-(phenylazo)-2-naphthol is an intensely coloured orange-red azo dye. Azo dyes (−N=N−) are the most important class of synthetic dyes used in textiles.

In aniline, the lone pair on N is in conjugation with the benzene ring π system → delocalised (resonance) → less available for accepting H⁺ → weaker base. In cyclohexylamine (aliphatic), lone pair on N is fully available → stronger base. Kb: cyclohexylamine (4.5×10⁻⁴) >> aniline (3.8×10⁻¹⁰).

Hofmann degradation: RCONH₂ + Br₂ + 4NaOH → RNH₂ + Na₂CO₃ + 2NaBr + 2H₂O. Product is a primary amine with ONE LESS CARBON than the starting amide (CO group is lost as CO₂). Example: CH₃CONH₂ → CH₃NH₂. Useful for preparing primary amines.

Gabriel synthesis: phthalimide + KOH → potassium phthalimide → + RX (SN2) → N-alkylphthalimide → + H₂NNH₂ (hydrazine) → primary aliphatic amine (RNH₂) + phthalhydrazide. Gives pure primary amine without 2° or 3° amine contamination — a key advantage over ammonolysis of alkyl halides.

Hinsberg's test: Primary amines → sulfonamide soluble in NaOH. Secondary amines → sulfonamide insoluble in NaOH (precipitate). Tertiary amines → no reaction (no N−H to react with sulfonyl chloride). The different outcomes allow identification of all three classes.

Sodium nitroprusside test: with primary amines → no reaction. With secondary amines → red colour (due to formation of coloured nitroso derivative). With H₂S → violet/purple colour. With acetone → red colour (Legal's test for ketones). The red colour distinguishes secondary amines.