Amines

Amines

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

• Amines are organic derivatives of ammonia (NH₃) in which one or more N–H hydrogens are replaced by alkyl (R–) or aryl (Ar–) groups.
• Classification by degree: primary (1°, R–NH₂), secondary (2°, R₂NH), tertiary (3°, R₃N) based on how many hydrocarbon groups sit on nitrogen. A quaternary ammonium salt (R₄N⁺X⁻) has four groups and a positive N.
• Basicity is governed by nitrogen’s lone pair. Gas phase order: 3° > 2° > 1° > NH₃. Aqueous order (aliphatic): 2° > 1° > 3° > NH₃ due to combined inductive, steric and solvation effects. Aniline (pK_b ≈ 9.4) is far weaker than aliphatic amines because its lone pair is delocalised into the benzene ring.
• Must-remember reactions for CUET: Carbylamine test (1° amines only, gives foul-smelling isocyanide with CHCl₃ + alc. KOH), Hofmann bromamide reaction (R–CONH₂ + Br₂ + 4 NaOH → R–NH₂), and diazotisation of ArNH₂ at 0–5 °C with NaNO₂/HCl to form ArN₂⁺Cl⁻, which couples with phenols to form azo dyes.
• High-yield CUET pointers: Identify 1°/2°/3° using the Hinsberg test; remember aniline is a weaker base but a strong ortho/para director in EAS; Gabriel synthesis gives pure primary amines.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Nomenclature and Structure

IUPAC naming uses the suffix -amine (methanamine, benzenamine/aniline) or the prefix amino- for substituents (e.g., p-aminobenzoic acid). The nitrogen in amines is sp³ hybridised with one lone pair, giving a pyramidal geometry. In aniline, resonance pulls the lone pair into the ring, giving the N–C(aryl) bond partial double-bond character and making the N nearly planar — this single structural fact explains aniline’s reduced basicity and its behaviour in electrophilic aromatic substitution.

Classification

• 1° amine: R–NH₂ (one alkyl/aryl group, e.g., methylamine, aniline)
• 2° amine: R₂NH (e.g., dimethylamine)
• 3° amine: R₃N (e.g., trimethylamine)
• Quaternary ammonium salt: R₄N⁺X⁻ (e.g., tetramethylammonium chloride) — permanently charged, no N–H bond.

Basicity Trends

The base dissociation constant of an amine in water is:

K_b = [RNH₃⁺][OH⁻] / [RNH₂], with pK_b = pK_w − pK_a = 14 − pK_a(RNH₃⁺).

For aliphatic amines, alkyl groups are +I (electron-donating), which raises electron density on N and increases basicity — but bulky 3° amines are poorly solvated by water, so the aqueous order becomes 2° > 1° > 3° > NH₃. Aromatic amines are much weaker because the lone pair delocalises into the ring (resonance), lowering availability for protonation.

Preparation Methods

Method	Equation / Reagent	Product
Reduction of nitro compounds	ArNO₂ + 6[H] (Sn/HCl or Fe/HCl) → ArNH₂ + 2H₂O	1° arylamine
Hofmann bromamide	R–CONH₂ + Br₂ + 4 NaOH → R–NH₂ + Na₂CO₃ + 2 NaBr + 2 H₂O	1° amine (one C less)
Reduction of nitriles	R–C≡N + 4[H] (LiAlH₄ or H₂/Ni) → R–CH₂–NH₂	1° amine
Ammonolysis	R–X + NH₃ → R–NH₂ + HX (gives mixture of 1°, 2°, 3°)	mixed amines
Gabriel synthesis	Phthalimide + KOH → potassium phthalimide + R–X → hydrolysis → R–NH₂	pure 1° amine
Reductive amination	R–CHO + NH₃ + H₂/Ni → R–CH₂–NH₂	1° amine

Characteristic Reactions and Tests

• Carbylamine (isocyanide) test: Only 1° amines (aliphatic and aromatic) give a foul-smelling isocyanide (R–NC) when warmed with CHCl₃ + alcoholic KOH. CUET often asks which class gives a positive test.
• Hinsberg test: Uses benzenesulphonyl chloride (PhSO₂Cl). 1° amines form soluble sulfonamides that dissolve in KOH; 2° amines give insoluble sulfonamides; 3° amines do not react.
• Reaction with HNO₂: 1° aliphatic → alcohol + N₂; 1° aromatic → diazonium salt (at 0–5 °C); 2° → N-nitrosamine (yellow oil); 3° → no useful reaction.
• Acylation: Acid chlorides or anhydrides convert amines to amides, a useful protecting strategy.

CUET Question Patterns

Expect MCQs on (i) identification of amine class from a structure, (ii) selecting the correct reagent for a given transformation, (iii) basicity ordering with reasoning, and (iv) the carbylamine/Hinsberg distinction. Numerical questions may ask you to compute pK_b from a given pK_a of the conjugate acid.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Diazonium Salts in Depth

Aromatic 1° amines react with NaNO₂ + HCl at 0–5 °C to give diazonium salts (ArN₂⁺Cl⁻). These are electrophilic at the terminal nitrogen and undergo two reaction classes:

Substitution (with loss of N₂): ArN₂⁺ + CuCl (HCl) → ArCl (Sandmeyer); ArN₂⁺ + CuBr → ArBr; ArN₂⁺ + CuCN → ArCN; ArN₂⁺ + H₃O⁺ + heat → ArOH; ArN₂⁺ + H₃PO₂ → ArH. These reactions are valuable for installing –Cl, –Br, –CN, –OH on an aromatic ring at positions dictated by existing groups.

Coupling (azo dye formation): ArN₂⁺ attacks electron-rich aromatic rings of phenols (pH 9–10) or naphthols/tertiary aromatic amines to form coloured azo compounds (Ar–N=N–Ar’). Coupling occurs at the para position (or ortho if para is blocked) and is the industrial basis of dyes such as methyl orange and congo red.

Ortho/Para Direction and Aniline’s Reactivity

The –NH₂ group in aniline is a strong activator and an o/p-director in EAS because nitrogen donates its lone pair into the ring. However, in strongly acidic conditions (HNO₃/H₂SO₄), aniline gets protonated to PhNH₃⁺, which is deactivating and meta-directing — this is why direct nitration of aniline gives a messy product and chemists protect the amine by acetylation (forming acetanilide) before nitrating.

Hofmann Bromamide — Mechanism in Brief

The amide nitrogen is deprotonated, then brominated to give an N-bromoamide. Base abstracts the N–H proton to form a nitrenoid/negative nitrogen, which undergoes a 1,2-shift of the R group from C to N with expulsion of Br⁻ (akin to a Lossen-type rearrangement). The resulting isocyanate (R–N=C=O) hydrolyses to the carbamate and finally to the 1° amine R–NH₂ with loss of CO₂. CUET may ask why this gives a 1° amine with one fewer carbon than the starting amide.

Common Mistakes and Traps

• Quaternary ≠ tertiary: A 3° amine (R₃N) has no N–H; a quaternary ammonium ion (R₄N⁺) carries a permanent positive charge and cannot act as a base in the usual sense.
• Basicity vs. nucleophilicity: Both depend on the lone pair, but nucleophilicity also depends on sterics — so 3° amines are poor nucleophiles but reasonable bases in gas phase.
• Carbylamine test fails for 2° and 3° amines — a frequent MCQ trap.
• Hofmann vs. Mendius reduction: Mendius reduces nitriles (R–CN → R–CH₂–NH₂); Hofmann degrades amides (R–CONH₂ → R–NH₂). Do not interchange them.
• Aniline’s pK_b ≈ 9.4 vs. methylamine’s pK_b ≈ 3.36 — a difference of six orders of magnitude, all due to resonance delocalisation.

Practice Prompts

1. An organic compound A (C₇H₇NO₂) on reduction with Sn/HCl gives B (C₇H₉N), which on treatment with NaNO₂/HCl at 0–5 °C gives C. Adding C to an alkaline solution of β-naphthol produces a brilliant red dye. Identify A, B, C and write all three reactions. (Answer: A = p-nitrotoluene; B = p-toluidine; C = p-methylbenzenediazonium chloride.)
2. Account for the following in ≤120 words: “Methylamine is a stronger base than aniline in water, but aniline is a stronger nucleophile than 2,6-dimethylaniline in benzene.” Tie each statement to inductive/resonance effects and steric hindrance respectively.

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