Amines

🟢 Lite — Quick Review (1h–1d)

Rapid summary of amines — classification, nomenclature, basicity trends, preparation methods, and key reactions for JEE Main revision.

Amines — Key Facts for JEE Main

• Definition: Amines are organic derivatives of ammonia (NH₃) where one or more hydrogen atoms are replaced by alkyl or aryl groups.

  • Primary (1°) amine: R–NH₂ (one R group attached to N)
  • Secondary (2°) amine: R₂NH (two R groups)
  • Tertiary (3°) amine: R₃N (three R groups)
  • Quaternary ammonium salt: R₄N⁺ X⁻ (four R groups, positively charged N)

• Nomenclature:

  • Aliphatic: prefix + amine (methylamine CH₃NH₂, ethylamine C₂H₅NH₂)
  • Aromatic: aniline (C₆H₅NH₂), N,N-dimethylaniline
  • IUPAC: e.g., CH₃CH₂CH₂NH₂ = propan-1-amine; CH₃NHCH₂CH₃ = N-methylethanamine

• Physical Properties:

  • Lower members (C₁–C₃) are gases with strong ammonia-like odour; C₄–C₈ are liquids; higher members are solids.
  • Boiling points: 1° > 2° > 3° amines (due to hydrogen bonding in 1° and 2°).
  • Solubility in water: decreases with increasing molecular weight; lower members are miscible.
  • Aromatic amines are generally toxic, azo dyes can cause cancer.

• Basicity (pK_b values):

  • Aliphatic amines: pK_b ≈ 3–4 (more basic than NH₃)
  • Aniline: pK_b ≈ 9.4 (much less basic due to delocalisation of lone pair into aromatic ring)
  • Order of basicity in aqueous solution: R₂NH > RNH₂ > R₃N > ArNH₂ > NH₃
  • Electron-donating groups (–CH₃, –OCH₃) increase basicity; electron-withdrawing groups (–NO₂, –Cl) decrease it.
  • In gas phase, basicity order reverses: R₃N > R₂NH > RNH₂ (no solvation effect).

• Preparation Methods:

  • Ammonolysis of alkyl halides: R–X + NH₃ (excess) → R–NH₂ + R₂NH + R₃N (mixture; use excess NH₃ to favour primary amine).
  • Gabriel synthesis: Phthalimide + KOH → potassium salt; then R–X → N-alkyl phthalimide; hydrazinolysis → primary amine. Used to avoid polysubstitution.
  • Hofmann bromamide degradation: R–CONH₂ + Br₂ + 4NaOH → R–NH₂ (primary amine with one less carbon). Useful for aniline from benzamide.
  • Reduction of nitro compounds: R–NO₂ + [H] → R–NH₂. Catalytic hydrogenation (H₂/Ni) or metal/acid reduction (Fe/HCl for aromatic nitro).
  • Reductive amination (Manni-Bykov method): Carbonyl compound + NH₃ + H₂ (catalytic) → amine.
  • Schmidt reaction: R–COOH + HN₃ → R–NH₂ (primary amine) with loss of CO₂ and N₂.

• Chemical Reactions:

  • Acylation: R–NH₂ + (R’CO)₂O → R–NH–COR’ + R’COOH. 1° and 2° amines undergo; 3° do not (no replaceable H).
  • Benzoylation: R–NH₂ + C₆H₅COCl → R–NH–COC₆H₅ (Schotten-Baumann reaction).
  • Carbylamine test (isocyanide test): 1° aliphatic amine + CHCl₃ + alcoholic KOH → foul-smelling isocyanide (R–NC). Aromatic 1° amines also give; 2° and 3° do not.
  • Hinsberg test: Reaction with benzenesulphonyl chloride (BsCl): 1° gives soluble salt; 2° gives insoluble Bs₂N–R; 3° gives no reaction. Used to distinguish 1°, 2°, 3° amines.
  • Diazotisation: 1° aromatic amine + NaNO₂ + HCl (0–5°C) → diazonium salt (Ar–N₂⁺Cl⁻). Diazonium salts are unstable above 5°C; can be used in Sandmeyer reactions.
  • Hofmann elimination: 3° amine + CH₃I (excess) → quaternary ammonium iodide; heating → alkene + tertiary amine (least substituted alkene predominantly).
  • Reaction with nitrous acid:
    • 1° aliphatic: R–NH₂ + HNO₂ → R–OH + N₂ + H₂O (effervescence). Used in detection.
    • 1° aromatic: forms diazonium salt (see diazotisation above).
    • 2° aliphatic/aromatic: R₂NH + HNO₂ → N-nitrosoamine (yellow oil).
    • 3° aliphatic: R₃N + HNO₂ → nitrite salt (no characteristic test).
  • Oxidation: Amines oxidise to various products; e.g., primary amines to carboxylates, tertiary amines to amine oxides (R₃N→O).
  • Electrophilic substitution on aromatic amines: Aniline undergoes bromination (Br₂/H₂O) without catalyst to give 2,4,6-tribromoaniline (white precipitate). N-acetylation protects the –NH₂ group before further substitution.

• Aliphatic diazo compounds: Via diazotisation of 1° aliphatic amines; very unstable; decompose to carbocations and N₂.

• Uses: Methylamine in insecticidal formulations; aniline in dye, rubber, and pharmaceutical industries; EDTA (ethylenediaminetetraacetic acid) as chelating agent; adrenaline as hormone/neurotransmitter.

⚡ JEE Main Exam Tips:

• Always check whether the amine is aliphatic or aromatic when predicting basicity.
• In Hinsberg test, 1° amine gives Bs₂N–R precipitate that dissolves in alkali; 2° amine gives insoluble Bs₂N–R that does not dissolve.
• Remember: carbylamine test is ONLY for 1° amines (both aliphatic and aromatic).
• Hofmann elimination gives the least substituted alkene (Zaitsev’s rule is not followed; it follows Hofmann’s rule due to bulky leaving group).
• Gabriel synthesis is preferred when you need a pure primary amine without polysubstitution products.

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🟡 Standard — Regular Study (2d–2mo)

Detailed study guide on amines covering classification, structure, preparation, reactions with mechanisms, and JEE Main–level problem solving.

Amines — JEE Main Study Guide

Amines are classified as derivatives of ammonia. The nitrogen atom in amines is sp³ hybridised with a trigonal pyramidal geometry and one lone pair of electrons. In aromatic amines like aniline, the lone pair is partially delocalised into the benzene ring, reducing its availability for protonation and thus decreasing basicity compared to aliphatic amines.

Basicity and Salt Formation: Basicity of amines is measured by the equilibrium constant K_b (or pK_b). A higher K_b (lower pK_b) indicates a stronger base. In aqueous solution: $$K_b = \frac{[R_3NH^+][OH^-]}{[R_3N]}$$ The pK_b of ethylamine is 3.25 (K_b ≈ 5.6×10⁻⁴), while aniline has pK_b = 9.4 (K_b ≈ 4×10⁻¹⁰). The significant difference arises because the anilinium ion (C₆H₅NH₃⁺) resonance stabilises the conjugate acid by dispersing the positive charge over the aromatic ring, whereas aliphatic ammonium ions have no resonance.

Solved Example: Arrange the following in increasing order of basicity: methylamine, aniline, dimethylamine, ammonia. Justify.

Solution: In aqueous solution, basicity follows: dimethylamine (pK_b = 3.27) > methylamine (pK_b = 3.36) > ammonia (pK_b = 4.75) > aniline (pK_b = 9.40). The order is: dimethylamine > methylamine > ammonia > aniline. Dimethylamine is most basic because it has two electron-donating methyl groups enhancing electron density on N; aniline is least because the lone pair is delocalised into the aromatic ring.

Solved Example: What happens when aniline is treated with (a) acetic anhydride, (b) benzenesulphonyl chloride, (c) nitrous acid at 0–5°C?

Solution: (a) Acetylation: Aniline + (CH₃CO)₂O → acetanilide (N-phenylacetamide). This protects the amino group; acetylation reduces basicity and activates the ring toward electrophilic substitution at the para position. (b) Hinsberg test: Aniline (1° aromatic amine) reacts with benzenesulphonyl chloride to give N-phenylbenzenesulphonamide, which is soluble in KOH (forms salt). So it dissolves — confirming a 1° amine. (c) Diazotisation: Aniline + NaNO₂/HCl (0–5°C) → benzenediazonium chloride (C₆H₅–N₂⁺Cl⁻). This is a key intermediate in converting –NH₂ to –Cl, –Br, –CN, –OH, –SO₃H via Sandmeyer reactions.

Preparation Comparison: Gabriel synthesis is specific for primary amines and avoids polysubstitution. It uses phthalimide (imide), which has only one N–H (pK_a ≈ 8.5) that can be deprotonated by KOH. The resulting anion is a good nucleophile for SN2 with alkyl halides. Subsequent hydrazinolysis (NH₂NH₂) releases the primary amine. This method fails for aryl amines (aryl halides cannot undergo SN2).

Hofmann bromamide degradation is used when you want to synthesise a primary amine with one carbon less than the starting amide. The amide is treated with bromine in aqueous NaOH. The reaction proceeds via intermediate formation of isocyanate (Curtius rearrangement analogue).

Reaction Mechanisms:

• Acylation: Nucleophilic attack of amine on carbonyl carbon of acid chloride/anhydride, followed by elimination of Cl⁻/acetate.
• Carbylamine formation: Base-catalysed dehydrohalogenation of dichlorocarbene (from CHCl₃ + KOH) which then attacks the amine nitrogen.
• Diazotisation: N-nitrosation of –NH₂ by HNO₂, followed by loss of water to form diazonium ion.

Important Points for JEE:

• When a mixture of amines is obtained from ammonolysis, primary amine can be separated by reacting with phthalic anhydride (Gabriel synthesis approach).
• Aromatic amines are much weaker bases — treat them as if the lone pair is partially “used up” in resonance.
• Always specify temperature for diazotisation (0–5°C); at higher temperatures, diazonium salt decomposes to phenol (replacement of –N₂⁺ by –OH).
• Quaternary ammonium hydroxides (R₄N⁺OH⁻) are strong bases; Hofmann elimination of such hydroxides gives alkenes.

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🔴 Extended — Deep Study (3mo+)

Comprehensive, advanced-level notes on amines covering detailed reaction mechanisms, stereochemistry, asymmetric synthesis, spectroscopy, and JEE Advanced–level problems.

Amines — Comprehensive JEE Notes

Molecular Orbital Perspective on Amine Basicity: In aliphatic amines, the nitrogen lone pair resides in an sp³ hybrid orbital. In aniline, the nitrogen is also sp² hybridised (or nearly sp²) with the lone pair occupying a p orbital that overlaps with the aromatic π system. This creates a five-electron, six-atom conjugated system (anilinium ion has 6 π electrons delocalised). The delocalisation energy (resonance energy) of aniline is approximately 30–40 kJ/mol lower than expected, making it less basic than cyclohexylamine (which has no resonance).

Detailed Mechanism of Hofmann Elimination: The Hofmann elimination involves formation of a quaternary ammonium salt (E2-like anti-elimination) when treated with methyl iodide, followed by silver oxide (Ag₂O) and heat:

1. R₃N + CH₃I → R₃N⁺CH₃ I⁻ (complete alkylation, even if 3°)
2. R₃N⁺CH₃ I⁻ + Ag₂O → R₃N⁺CH₃ OH⁻ (metathesis)
3. R₃N⁺CH₃ OH⁻ (heat) → R₂C=CR₂ + R₂N + CH₃OH (elimination)

The elimination follows anti-periplanar geometry (similar to E2). The β-hydrogen must be anti to the leaving group (NMe₃). Because the bulky leaving group prefers the equatorial position, the β-hydrogen on the less substituted carbon is preferentially eliminated, giving the less substituted alkene (Hofmann product) against Zaitsev’s rule.

Stereochemistry of Amines: Aliphatic amines are pyramidal at nitrogen (if three different substituents, the nitrogen is a stereogenic centre). However, amines undergo rapid inversion at room temperature (inversion barrier ~25 kJ/mol, faster than NMR timescale), making them achiral. However, quaternary ammonium salts with four different substituents are permanently chiral (no inversion). Example: (CH₃)(C₂H₅)(C₃H₇)(C₄H₉)N⁺I⁻ is chiral.

Sandmeyer Reactions: Benzenediazonium salts treated with cuprous salts undergo replacement of –N₂⁺:

• CuCl: → chlorobenzene
• CuBr: → bromobenzene
• CuCN: → benzonitrile
• CuNO₂: → nitrobenzene
• CuH (hypophosphite): → benzene (reductive deamination) These reactions proceed via radical intermediates (single electron transfer from Cu⁺ to diazonium).

Curtius and Schmidt Rearrangements:

• Curtius: Acid chloride + NaN₃ → acyl azide; heating in dry solvent → isocyanate + N₂; hydrolysis → primary amine + CO₂.
• Schmidt: Carboxylic acid + hydrazoic acid (HN₃) in presence of acid → primary amine with loss of CO₂ and N₂. More reactive than Curtius.

spectroscopic Properties:

• IR: N–H stretching in primary amines: two bands (symmetric and asymmetric) around 3300–3500 cm⁻¹. Secondary amines: one band. Tertiary amines: no N–H stretch. Aromatic amines show C=N aromatic ring stretches at 1600 cm⁻¹.
• ¹H NMR: –NH₂ protons appear at δ 0.5–5 ppm (broad, exchangeable with D₂O). α-CH₂/CH₃ next to N are deshielded: δ 2.5–3.5 ppm.
• Mass Spec: α-cleavage gives iminium ions (R₂N⁺=CH₂). McLafferty rearrangement occurs for amines with γ-H.

Advanced Problem: An unknown amine (C₄H₁₁N) reacts with benzenesulphonyl chloride to give an insoluble product that does not dissolve in KOH. With nitrous acid, it gives a yellow oil. With excess CH₃I followed by Ag₂O and heat, it gives only one alkene. Identify the amine.

Solution: The product insoluble in KOH after Hinsberg test indicates a secondary amine (R₂NH). Yellow oil with HNO₂ is characteristic of N-nitrosoamine from secondary amine. The alkene product with only one type suggests a symmetric secondary amine: diethylamine (C₂H₅)₂NH. Reaction with excess CH₃I gives (C₂H₅)₂N⁺CH₃ I⁻; Ag₂O/heat elimination gives ethene (CH₂=CH₂) only — consistent with β-hydrogens on identical ethyl groups. So the amine is diethylamine.

Distinction Between Aliphatic and Aromatic Amines in Reactions:

Reagent	1° Aliphatic	2° Aliphatic	3° Aliphatic	1° Aromatic
Hinsberg (BsCl)	Dissolves in alkali	Insoluble, no dissolution	No reaction	Dissolves in alkali
Carbylamine	Foul smell	No reaction	No reaction	Foul smell
HNO₂ (0–5°C)	N₂ gas evolution	Yellow oil (nitrosoamine)	Salt (no characteristic)	Diazonium salt
Acylation	Yes	Yes	No	Yes (acetanilide)

⚡ JEE Advanced Pro Tip: When a question involves separation of amine mixtures, recall that 1° aliphatic amines can be separated via Gabriel synthesis; aromatic amines can be separated by converting to acetanilide (soluble in organic solvent) vs non-acylated (water soluble). Always look for characteristic colour changes — e.g., N-nitrosoamines are yellow oils with a characteristic smell.