Overview — Amines, Diazonium Salts and Nitro Compounds

Nitrogen functional groups appear in dyes, drugs, fertilizers, alkaloids and proteins. Historically, urea was the first organic compound synthesised in the laboratory. This lesson focuses on amines (classification, naming, preparation, basicity, reactions, Hinsberg test), arenediazonium salts as synthetic intermediates (Sandmeyer, coupling), and nitro compounds (prep, reduction, directing effects).

Section 1: Amines — Classification, Structure & Naming (28.1–28.2.1)

Amines are ammonia derivatives: one, two or three H atoms replaced by alkyl/aryl groups.

1°: RNH₂ · 2°: R₂NH · 3°: R₃N. Four alkyl groups on N give a quaternary ammonium salt R₄N⁺X⁻.

Nitrogen is approximately sp³ hybridised; geometry is trigonal pyramidal (lone pair at one tetrahedral vertex). Bond angles near 109.5°. Aromatic amines have –NH₂ on the ring; the parent is aniline (benzenamine).

IUPAC: longest chain + -amine (methanamine, ethanamine, propan-1-amine). For 2°/3°, use the prefix N- for substituents on nitrogen (N-methylmethanamine, N-ethyl-N-methylpropan-1-amine). Aromatic derivatives: 2-chloroaniline, 4-nitroaniline, 2-methylaniline (o-toluidine).

Section 2: Preparation of Amines (28.2.2)

(i) From alkyl halides: RX + NH₃ → RNH₂ + NH₄X. Primary product can react further to 2°, 3° and R₄N⁺X⁻. Use excess ammonia to favour the primary amine (e.g. C₂H₅Br + excess NH₃ → ethanamine).

(ii) Reduction: nitriles (H₂/Pt or Na/C₂H₅OH) give primary amines with one more carbon than the parent alkyl of RCN (CH₃CH₂CN → CH₃CH₂CH₂NH₂). Amides + LiAlH₄ → amines with the same carbon count (CH₃CONH₂ → CH₃CH₂NH₂). Nitro compounds + Sn/HCl or H₂/Ni → amines (nitrobenzene → aniline).

(iii) Hofmann bromamide reaction: RCONH₂ + Br₂ + 4KOH → RNH₂ + K₂CO₃ + 2KBr + 2H₂O. The amine has one carbon less than the amide (propanamide → ethanamine). High-yield exam reaction for chain shortening.

Section 3: Properties & Reactions of Amines (28.2.4–28.2.5)

Physical: lower aliphatic amines are gases (smell of ammonia); higher are liquids. H-bonding raises boiling points vs hydrocarbons. Lower members water-soluble; solubility falls as alkyl size grows. All soluble in common organic solvents.

Basicity: lone pair on N accepts H⁺. Alkyl groups are electron-releasing → aliphatic amines stronger bases than ammonia. Expected order 1° < 2° < 3° from +I alone, but aqueous order is typically R₂NH > RNH₂ > R₃N because tertiary amines suffer steric hindrance and poorer solvation of the cation. Aromatic amines < NH₃ < aliphatic amines — the ring withdraws electron density (resonance) so aniline is a weaker base.

Alkylation: RNH₂ + RX successively → 2°, 3°, then quaternary salt.

Acylation: RNH₂ + R′COCl → RNHCOR′ (amide). Aniline + CH₃COCl → acetanilide — used to protect –NH₂ before ring nitration.

Carbylamine reaction: 1° amine + CHCl₃ + alcoholic KOH (heat) → isocyanide RNC (offensive odour). Classic test for primary amines only.

Nitrous acid: aromatic 1° amines at 273–278 K → stable enough diazonium salts (diazotisation). Aliphatic 1° amines give unstable diazonium ions that lose N₂ → alcohols (ethanamine → ethanol + N₂).

Schiff bases: primary amines + aldehydes/ketones → imines (C=N).

Ring substitution (aniline): –NH₂ is strongly activating and o/p-directing (resonance puts e⁻ density at o/p). Br₂(aq) → 2,4,6-tribromoaniline (all three positions). Direct nitration is messy (oxidation); acetylate first → nitrate → hydrolyse to p-nitroaniline. Sulphonation path: anilinium salt rearranges on heating → sulphanilic acid.

Section 4: Distinguishing Amines — Hinsberg (28.2.7)

Shake amine with benzenesulphonyl chloride and excess KOH, then acidify.

1°: forms N-alkylbenzenesulphonamide with acidic N–H → dissolves in KOH as potassium salt (clear solution) → acidification precipitates free sulphonamide.

2°: forms N,N-dialkylsulphonamide with no acidic H → insoluble precipitate; acidification does not dissolve it.

3°: no reaction with PhSO₂Cl; if amine is insoluble it remains separate, then dissolves on acidification as ammonium-type salt.

Uses of amines: solvents, drug intermediates, long-chain quaternary salts as detergents, aniline derivatives in dyes and developers, hair dyes (1,4-diaminobenzene), herbicides, and as sources of diazonium salts for synthesis.

Section 5: Diazonium Salts (28.3)

Arenediazonium salts are unstable above ~5–10 °C and explosive when dry — handle cold and usually in situ. The –N₂⁺ group is a superb leaving group and can be replaced by many substituents.

Sandmeyer: ArN₂⁺ + CuCl / CuBr / CuCN → ArCl / ArBr / ArCN (with appropriate HX/HCN conditions).

Iodide: ArN₂⁺ + KI → ArI (no copper needed).

Fluoride: treat with HBF₄, isolate ArN₂⁺BF₄⁻, heat carefully → ArF.

Hydroxyl: aqueous conditions with copper salts → phenol.

Hydrogen: H₃PO₂ (hypophosphorous acid) → ArH (deamination).

Coupling: ArN₂⁺ + phenol or tertiary aryl amine (mild alkaline/acid as required) → azo compounds (dyes), e.g. p-hydroxyazobenzene from phenol.

Section 6: Nitro Compounds (28.4)

Hydrocarbons with –NO₂ replacing H. Aliphatic nitroalkanes are 1°, 2° or 3° by the carbon bearing NO₂. IUPAC: nitro- prefix (nitromethane, 2-nitropropane, nitrobenzene, 1,3-dinitrobenzene).

Preparation: (i) RX + AgNO₂ (ethanolic) → RNO₂ (+ some R–O–N=O nitrite). (ii) Vapour-phase nitration of alkanes (~680 K) — mixture of products. (iii) Aromatic: conc. HNO₃ + conc. H₂SO₄ → nitrobenzene (classic electrophilic nitration).

Physical: polar → higher b.p. than alkanes; nitrobenzene yellow liquid (bitter-almond odour); many aromatics yellow solids; denser than water; insoluble in water, soluble in organic solvents.

Reduction: RNO₂ → RNH₂ with H₂/Ni, Sn/HCl or Fe/HCl, LiAlH₄. Nitrobenzene → aniline. Medium matters for partial reduction (e.g. Zn/NH₄Cl → phenylhydroxylamine; alkaline Zn → azobenzene — higher detail).

Hydrolysis: 1° nitroalkanes + dilute acid → carboxylic acid + NH₂OH; 2° → ketones.

Thermal decomposition: nitroalkanes can explode on heating (large gas volume) — explosives and rocket-fuel context.

Ring chemistry: –NO₂ is electron-withdrawing → deactivating and meta-directing. Halogenation, further nitration and sulphonation of nitrobenzene give mainly meta products.

Uses: solvents, intermediates for explosives, detergents, medicines and amines; fuel for small engines/rockets (gas expansion on decomposition).

Exam Connections and Chapter Summary

High-yield checklist: (1) 1°/2°/3°/4° classification and IUPAC with N- prefixes; (2) prep — excess NH₃, reductions, Hofmann (−1 C); (3) basicity order aliphatic > NH₃ > aromatic and 2° peak in water; (4) carbylamine for 1° only; (5) Hinsberg solubility logic; (6) diazotisation conditions and Sandmeyer/coupling map; (7) aliphatic vs aromatic HNO₂ products; (8) protect aniline for nitration; (9) RNO₂ prep, reduction to amine, –NO₂ meta; (10) multi-step: nitrobenzene → aniline → diazonium → target.

Builds on L27 (amides, carbonyls for Schiff bases) and aromatic substitution from earlier organic lessons. Next: biomolecules (L29). Practice writing full equations for Hofmann, carbylamine, Sandmeyer and Hinsberg — they dominate exam papers.