Amines (Class 12 & JEE)

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### Introduction to Amines - **Definition:** Organic compounds derived from ammonia (NH₃) by replacing one or more hydrogen atoms with alkyl or aryl groups. - **Classification:** - **Primary (1°):** One H replaced (R-NH₂, Ar-NH₂) - **Secondary (2°):** Two H replaced (R₂NH, Ar₂NH, R-NH-Ar) - **Tertiary (3°):** Three H replaced (R₃N, Ar₃N, R₂N-Ar, R-N(Ar)₂) - **Structure:** Pyramidal geometry, sp³ hybridized N atom, a lone pair of electrons on N. ### Nomenclature - **Common Name:** Alkylamines (e.g., Methylamine, Dimethylamine, Trimethylamine) - **IUPAC Name:** Alkanamines (e.g., Methanamine, Ethanamine) or N-substituted alkanamines for secondary/tertiary (e.g., N-Methylethanamine, N,N-Dimethylethanamine). - **Aryl Amines:** Aniline (Benzenamine) and its derivatives. ### Preparation of Amines #### 1. Reduction of Nitro Compounds - **Reaction:** R-NO₂ / Ar-NO₂ $\xrightarrow{\text{Sn/HCl or Fe/HCl or H₂/Pd}}$ R-NH₂ / Ar-NH₂ - **Example:** Nitrobenzene $\xrightarrow{\text{Sn/HCl}}$ Aniline #### 2. Ammonolysis of Alkyl Halides - **Reaction:** R-X $\xrightarrow{\text{NH₃}}$ R-NH₂ $\xrightarrow{\text{R-X}}$ R₂NH $\xrightarrow{\text{R-X}}$ R₃N $\xrightarrow{\text{R-X}}$ R₄N⁺X⁻ (Quaternary Ammonium Salt) - **Note:** This method often yields a mixture of 1°, 2°, 3° amines and quaternary salts. Excess NH₃ favors 1° amine. #### 3. Reduction of Nitriles - **Reaction:** R-C≡N $\xrightarrow{\text{LiAlH₄ or H₂/Ni}}$ R-CH₂-NH₂ (Primary amine with one more carbon) - **Example:** CH₃-C≡N $\xrightarrow{\text{LiAlH₄}}$ CH₃-CH₂-NH₂ (Ethanamine) #### 4. Reduction of Amides - **Reaction:** R-CO-NH₂ $\xrightarrow{\text{LiAlH₄}}$ R-CH₂-NH₂ (Primary amine) - **Example:** CH₃-CO-NH₂ $\xrightarrow{\text{LiAlH₄}}$ CH₃-CH₂-NH₂ #### 5. Gabriel Phthalimide Synthesis - **Purpose:** Exclusively for preparing **primary aliphatic amines**. - **Mechanism:** 1. Phthalimide reacts with alcoholic KOH to form potassium phthalimide. 2. Potassium phthalimide reacts with alkyl halide (R-X) to form N-alkylphthalimide. 3. Hydrolysis (acidic or basic) or hydrazinolysis (with N₂H₄) of N-alkylphthalimide yields primary amine R-NH₂ and phthalic acid/salt or phthalhydrazide respectively. - **Limitation:** Cannot be used for aromatic primary amines (aryl halides do not undergo nucleophilic substitution with phthalimide anion). #### 6. Hoffmann Bromamide Degradation Reaction - **Reaction:** R-CO-NH₂ $\xrightarrow{\text{Br₂/KOH}}$ R-NH₂ + K₂CO₃ + 2KBr + 2H₂O - **Key Feature:** Amine formed has one carbon atom less than the parent amide. - **Example:** Ethanamide $\xrightarrow{\text{Br₂/KOH}}$ Methanamine ### Physical Properties - **State:** Lower aliphatic amines are gases (fishy odor). Higher ones are liquids or solids. Aniline is a colorless liquid. - **Boiling Point:** 1° > 2° > 3° (due to intermolecular H-bonding). Amines have higher B.P. than hydrocarbons of comparable molar mass, but lower than alcohols/carboxylic acids. - **Solubility:** Lower amines are soluble in water due to H-bonding with water. Solubility decreases with increasing molar mass (hydrophobic alkyl group increases). ### Chemical Reactions of Amines #### 1. Basicity of Amines - **Reason:** Presence of a lone pair of electrons on the N atom, which can be donated. - **Factors Affecting Basicity:** - **Electron-donating groups (+I effect or +R effect):** Increase electron density on N, increasing basicity. - **Electron-withdrawing groups (-I effect or -R effect):** Decrease electron density on N, decreasing basicity. - **Steric Hindrance:** Bulky groups hinder protonation, decreasing basicity. - **Solvation Effects (in aqueous medium):** Stability of the conjugate acid (alkylammonium ion) due to H-bonding with water. - **Order of Basicity (in aqueous solution):** - **Aliphatic Amines:** - **Methylamines:** (CH₃)₂NH (2°) > CH₃NH₂ (1°) > (CH₃)₃N (3°) > NH₃ - **Ethylamines:** (C₂H₅)₂NH (2°) > (C₂H₅)₃N (3°) > C₂H₅NH₂ (1°) > NH₃ - **Aromatic Amines:** Aniline is less basic than aliphatic amines and ammonia. - **Reason:** Lone pair on N in aniline is delocalized (involved in resonance with benzene ring), making it less available for protonation. - **Effect of Substituents on Aniline Basicity:** - **Electron-donating groups (+I/+R):** Increase basicity (e.g., -CH₃, -OCH₃). *p*-toluidine > aniline. - **Electron-withdrawing groups (-I/-R):** Decrease basicity (e.g., -NO₂, -CN, -COOH, -SO₃H, -X). *p*-nitroaniline 2° > 1° > NH₃ (only +I effect considered, no solvation). #### 2. Alkylation (Hofmann's Exhaustive Alkylation) - **Reaction:** R-NH₂ $\xrightarrow{\text{excess R-X}}$ R₄N⁺X⁻ (Quaternary ammonium salt) #### 3. Acylation - **Reaction:** Amines react with acid chlorides, anhydrides, or esters to form amides. - R-NH₂ + R'-COCl $\rightarrow$ R-NH-COR' + HCl - Ar-NH₂ + (CH₃CO)₂O $\rightarrow$ Ar-NH-COCH₃ + CH₃COOH (Acetylation) - **Note:** 3° amines do not undergo acylation due to the absence of a replaceable H atom on N. #### 4. Carbylamine Reaction (Isocyanide Test) - **Purpose:** Test for **primary amines (aliphatic and aromatic)**. - **Reaction:** R-NH₂ / Ar-NH₂ + CHCl₃ + 3KOH (alc.) $\xrightarrow{\text{heat}}$ R-NC / Ar-NC (Isocyanide, foul-smelling) + 3KCl + 3H₂O - **Note:** Secondary and tertiary amines do not give this test. #### 5. Reaction with Nitrous Acid (HNO₂) - **Preparation of HNO₂:** NaNO₂ + HCl (cold, 0-5°C) - **Primary Aliphatic Amine:** R-NH₂ $\xrightarrow{\text{HNO₂}}$ R-OH + N₂↑ + H₂O (forms highly unstable diazonium salt, which decomposes to alcohol) - **Primary Aromatic Amine:** Ar-NH₂ $\xrightarrow{\text{HNO₂, 0-5°C}}$ Ar-N₂⁺Cl⁻ (Arenediazonium salt, stable at low temp) + NaCl + 2H₂O - **Secondary Amine:** R₂NH $\xrightarrow{\text{HNO₂}}$ R₂N-N=O (N-nitrosamine, yellow oily compound) - **Tertiary Amine:** - Aliphatic: R₃N + HNO₂ $\rightarrow$ (R₃NH)⁺NO₂⁻ (salt) - Aromatic: Reacts via electrophilic substitution at para position (e.g., N,N-dimethylaniline with HNO₂ gives *p*-nitroso-N,N-dimethylaniline) #### 6. Hinsberg's Test (Reaction with Benzenesulphonyl Chloride) - **Reagent:** C₆H₅SO₂Cl (Benzenesulphonyl chloride) - **Purpose:** To distinguish between 1°, 2°, and 3° amines. - **Primary Amine:** R-NH₂ + C₆H₅SO₂Cl $\rightarrow$ C₆H₅SO₂-NH-R (N-alkylbenzenesulphonamide) - This product is **acidic** (due to H attached to N) and dissolves in NaOH. - **Secondary Amine:** R₂NH + C₆H₅SO₂Cl $\rightarrow$ C₆H₅SO₂-NR₂ (N,N-dialkylbenzenesulphonamide) - This product has **no acidic H** and is insoluble in NaOH. - **Tertiary Amine:** R₃N + C₆H₅SO₂Cl $\rightarrow$ No reaction (as no H attached to N). It remains insoluble in NaOH. #### 7. Electrophilic Substitution (for Aromatic Amines e.g. Aniline) - **Amino group (-NH₂):** Strongly activating and ortho-para directing. - **Bromination:** Aniline $\xrightarrow{\text{Br₂/H₂O}}$ 2,4,6-Tribromoaniline (white precipitate) - To get monosubstituted product (e.g., *p*-bromoaniline), first protect -NH₂ group by acetylation to form acetanilide, then brominate, then hydrolyze. - **Nitration:** Direct nitration of aniline leads to oxidation products and a mixture of ortho, meta, para products (due to anilinium ion formation in acidic medium). - To get *p*-nitroaniline, protect -NH₂ group by acetylation, then nitrate, then hydrolyze. - **Sulphonation:** Aniline reacts with conc. H₂SO₄ to form anilinium hydrogen sulphate, which on heating gives sulphanilic acid (zwitterionic form). ### Diazonium Salts (Ar-N₂⁺X⁻) #### 1. Preparation - From primary aromatic amines by reaction with NaNO₂/HCl at 0-5°C (Diazotisation). #### 2. Chemical Reactions - **Replacement by Halogens/CN (Sandmeyer Reaction):** Ar-N₂⁺Cl⁻ $\xrightarrow{\text{CuCl/HCl or CuBr/HBr or CuCN/KCN}}$ Ar-Cl / Ar-Br / Ar-CN - **Replacement by Halogens (Gattermann Reaction):** Ar-N₂⁺Cl⁻ $\xrightarrow{\text{Cu/HCl or Cu/HBr}}$ Ar-Cl / Ar-Br - **Replacement by I:** Ar-N₂⁺Cl⁻ $\xrightarrow{\text{KI}}$ Ar-I - **Replacement by F (Balz-Schiemann Reaction):** Ar-N₂⁺Cl⁻ $\xrightarrow{\text{HBF₄}}$ Ar-N₂⁺BF₄⁻ $\xrightarrow{\text{heat}}$ Ar-F - **Replacement by -OH:** Ar-N₂⁺Cl⁻ $\xrightarrow{\text{H₂O/warm}}$ Ar-OH - **Replacement by -H:** Ar-N₂⁺Cl⁻ $\xrightarrow{\text{H₃PO₂ or CH₃CH₂OH}}$ Ar-H - **Replacement by -NO₂:** Ar-N₂⁺Cl⁻ $\xrightarrow{\text{HBF₄, NaNO₂/Cu}}$ Ar-NO₂ - **Coupling Reactions:** Benzene diazonium chloride reacts with electron-rich aromatic compounds (like phenols and anilines) to form colored azo dyes. - Ar-N₂⁺Cl⁻ + Ar'-OH $\xrightarrow{\text{NaOH}}$ Ar-N=N-Ar'-OH (Orange dye) - Ar-N₂⁺Cl⁻ + Ar'-NH₂ $\xrightarrow{\text{H⁺}}$ Ar-N=N-Ar'-NH₂ (Yellow dye)