Class 12 Chemistry Notes Chapter 4 (Amines) – Chemistry-II Book

Alright class, let's begin our detailed study of Chapter 4: Amines. This is a very important chapter, not just for your board exams but also for various competitive government exams where chemistry is a component. Amines are organic derivatives of ammonia and play crucial roles in biological systems and industrial chemistry. Pay close attention to the concepts, especially basicity comparisons and the reactions of diazonium salts.

Chapter 4: Amines - Detailed Notes for Exam Preparation

1. Introduction

• Definition: Amines are considered derivatives of ammonia (NH₃) obtained by replacing one, two, or all three hydrogen atoms with alkyl (R) or aryl (Ar) groups.
• Structure: Nitrogen atom in amines is sp³ hybridized (like in NH₃), resulting in a pyramidal geometry. It has one lone pair of electrons, which is responsible for its basic nature and nucleophilicity. Bond angle (C-N-E, where E is H or C) is slightly less than the tetrahedral angle (109.5°) due to lone pair-bond pair repulsion (e.g., ~108° in trimethylamine).

2. Classification

Based on the number of hydrogen atoms replaced in ammonia:

• Primary (1°) Amine: One H atom replaced by R/Ar group (R-NH₂, Ar-NH₂). Example: CH₃NH₂ (Methanamine), C₆H₅NH₂ (Aniline).
• Secondary (2°) Amine: Two H atoms replaced by R/Ar groups (R₂NH, Ar₂NH, R-NH-Ar). Groups can be same or different. Example: (CH₃)₂NH (N-Methylmethanamine), C₆H₅NHCH₃ (N-Methylaniline).
• Tertiary (3°) Amine: All three H atoms replaced by R/Ar groups (R₃N, Ar₃N, R₂NAr, etc.). Groups can be same or different. Example: (CH₃)₃N (N,N-Dimethylmethanamine), (C₆H₅)₃N (Triphenylamine).
• Quaternary Ammonium Salts: When the nitrogen atom forms four bonds with alkyl/aryl groups and carries a positive charge, it forms a salt with an anion (R₄N⁺ X⁻). Example: [(CH₃)₄N]⁺Cl⁻ (Tetramethylammonium chloride).

3. Nomenclature

• Common Names:
  • Aliphatic amines: Named by prefixing alkyl group(s) to 'amine' (e.g., Ethylamine, Diethylamine, Triethylamine). For mixed amines, groups are named alphabetically.
  • Aryl amines: The simplest one, C₆H₅NH₂, is universally known as Aniline. Others are often named as derivatives of aniline (e.g., p-Toluidine, N-Methylaniline).
• IUPAC Names:
  • Aliphatic amines: Named as 'Alkanamines'. The '-e' of the parent alkane is replaced by '-amine'. The position of the -NH₂ group is indicated by the lowest possible number. (e.g., CH₃CH₂NH₂ is Ethanamine, CH₃CH(NH₂)CH₃ is Propan-2-amine).
  • Secondary/Tertiary amines: If identical groups are attached, prefix 'di-' or 'tri-' is used before the alkyl group name (e.g., (CH₃CH₂)₂NH is N-Ethylethanamine). If different groups are attached, the largest alkyl group is chosen as the parent alkane (alkan-amine). Other smaller alkyl groups are treated as substituents on the nitrogen atom and designated by the prefix 'N-' (e.g., CH₃NHCH₂CH₃ is N-Methylethanamine, (CH₃)₂(NCH₂CH₂CH₃) is N,N-Dimethylpropan-1-amine).
  • Aryl amines: Aniline is the accepted IUPAC name for C₆H₅NH₂. Substituted anilines are named accordingly (e.g., 4-Bromoaniline, N,N-Dimethylaniline). Sometimes, the -NH₂ group is treated as a substituent ('amino-') if another functional group of higher priority is present (e.g., H₂N-CH₂COOH is 2-Aminoethanoic acid).

4. Methods of Preparation

• 1. Reduction of Nitro Compounds: Very important method, especially for aromatic amines.
  • Reagents: H₂/Pd, Pt, or Ni; Sn/HCl; Fe/HCl (preferred because FeCl₂ formed gets hydrolysed to release HCl, requiring less acid).
  • R-NO₂ + 6[H] → R-NH₂ + 2H₂O
  • Ar-NO₂ + 6[H] → Ar-NH₂ + 2H₂O (e.g., Nitrobenzene → Aniline)
• 2. Ammonolysis of Alkyl Halides (Hoffmann's Ammonolysis):
  • Nucleophilic substitution reaction of alkyl halide (R-X) with ethanolic solution of ammonia.
  • NH₃ + R-X → RNH₃⁺X⁻ → RNH₂ (1° amine) + HX
  • Limitation: The primary amine formed is also nucleophilic and reacts further with R-X to form 2°, 3° amines and finally quaternary ammonium salt. This yields a mixture of products.
  • RNH₂ + R-X → R₂NH (2°) → R₃N (3°) → R₄N⁺X⁻ (Quaternary salt)
  • To get primarily 1° amine, use a large excess of ammonia.
• 3. Reduction of Nitriles (Cyanides):
  • Nitriles on reduction give primary amines.
  • Reagents: H₂/Ni, Pt or Pd; LiAlH₄; Na(Hg)/C₂H₅OH (Mendius reduction).
  • R-C≡N + 4[H] → R-CH₂-NH₂ (Ascent of series - adds one carbon)
• 4. Reduction of Amides:
  • Amides on reduction give amines with the same number of carbon atoms.
  • Reagent: Strong reducing agent like LiAlH₄ followed by hydrolysis.
  • R-CONH₂ + 4[H] --(LiAlH₄/ether)--> R-CH₂-NH₂ + H₂O
• 5. Gabriel Phthalimide Synthesis:
  • Used exclusively for the preparation of pure primary aliphatic amines. Aromatic primary amines cannot be prepared because aryl halides do not undergo nucleophilic substitution easily with the anion formed by phthalimide.
  • Steps:
    • Phthalimide reacts with ethanolic KOH to form potassium phthalimide.
    • Potassium phthalimide reacts with alkyl halide (R-X) to form N-Alkylphthalimide.
    • N-Alkylphthalimide on hydrolysis with aqueous alkali (NaOH) or acid gives the primary amine (R-NH₂) and phthalic acid salt/acid.
• 6. Hoffmann Bromamide Degradation Reaction:
  • Used for converting a primary amide into a primary amine with one carbon atom less than the parent amide (Descent of series or step-down reaction).
  • Reagents: Amide reacts with Br₂ in aqueous or ethanolic solution of NaOH or KOH.
  • R-CONH₂ + Br₂ + 4NaOH → R-NH₂ + Na₂CO₃ + 2NaBr + 2H₂O
  • Key intermediate: Isocyanate (R-N=C=O).

5. Physical Properties

• State & Odour: Lower aliphatic amines are gases with fishy odour. Higher amines (3+ carbons) are liquids, and still higher ones are solids. Aniline and other arylamines are usually colourless liquids but get coloured on storage due to atmospheric oxidation.
• Solubility: Lower aliphatic amines (1°, 2°) can form hydrogen bonds with water molecules, hence are soluble. Solubility decreases with increase in molar mass (hydrophobic alkyl part increases). Tertiary amines are less soluble than 1° and 2° amines of comparable mass due to inability to donate H for H-bonding (though they can accept). Aromatic amines are generally insoluble in water due to the large hydrophobic aryl group.
• Boiling Points:
  • Amines are polar and can form intermolecular H-bonds (except 3° amines among themselves).
  • Boiling points are higher than non-polar compounds (like alkanes) of comparable molar mass.
  • Order of boiling points for isomeric amines: 1° > 2° > 3° (due to extent of H-bonding).
  • Boiling points of amines are lower than alcohols or carboxylic acids of comparable molar mass because N-H bond is less polar than O-H bond (due to lower electronegativity of N than O), leading to weaker H-bonds.

6. Chemical Reactions

• A. Basic Character of Amines:
  • Amines act as Lewis bases due to the presence of a lone pair of electrons on the nitrogen atom. They are also Brønsted-Lowry bases as they can accept a proton.
  • RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻ ; Kb = [RNH₃⁺][OH⁻] / [RNH₂]
  • Higher Kb or lower pKb value indicates stronger basicity (pKb = -log Kb).
  • Comparison of Basicity:
    • Aliphatic Amines vs Ammonia: Aliphatic amines are generally stronger bases than ammonia. Alkyl groups (+I effect) increase electron density on nitrogen, making the lone pair more available for donation.
    • Basicity Order in Gas Phase: 3° > 2° > 1° > NH₃ (Only +I effect matters).
    • Basicity Order in Aqueous Solution: This is complex and depends on a combination of:
      • Inductive Effect (+I): Increases basicity (3° > 2° > 1°).
      • Solvation Effect (H-bonding with water): Stabilizes the conjugate acid (ammonium ion) by hydration. More H-atoms on N⁺ means better solvation. Order of stabilization: 1° > 2° > 3°. This factor decreases basicity as the alkyl group size increases.
      • Steric Hindrance: Bulky alkyl groups hinder the approach of a proton to the nitrogen atom and also hinder solvation. This effect decreases basicity, especially for 3° amines.
    • Combined Effect in Water:
      • For Methyl groups: (CH₃)₂NH (2°) > CH₃NH₂ (1°) > (CH₃)₃N (3°) > NH₃
      • For Ethyl groups: (C₂H₅)₂NH (2°) > (C₂H₅)₃N (3°) > C₂H₅NH₂ (1°) > NH₃
      • Generally, secondary amines are the strongest bases in aqueous solution.
    • Aromatic Amines vs Ammonia: Aromatic amines (like aniline) are much weaker bases than ammonia and aliphatic amines. The lone pair on nitrogen is delocalized into the benzene ring through resonance, making it less available for protonation. Anilinium ion (conjugate acid) is less resonance stabilized than aniline itself.
    • Effect of Substituents on Aniline Basicity:
      • Electron-donating groups (EDG) like -CH₃, -OCH₃ (usually at o/p positions) increase basicity.
      • Electron-withdrawing groups (EWG) like -NO₂, -SO₃H, -COOH, -X (halogens) decrease basicity. The effect is more pronounced at ortho and para positions.
• B. Alkylation: Reaction with alkyl halides (see Ammonolysis - leads to mixture).
• C. Acylation (Reaction with Acid Chlorides/Anhydrides):
  • 1° and 2° amines react with acid chlorides (RCOCl), acid anhydrides ((RCO)₂O), and esters (RCOOR') via nucleophilic acyl substitution to form amides. 3° amines do not react as they lack a replaceable hydrogen on nitrogen.
  • RNH₂ + R'COCl → RNHCOR' (N-Alkylalkanamide) + HCl
  • R₂NH + (R'CO)₂O → R₂NCOR' (N,N-Dialkylalkanamide) + R'COOH
  • Reaction is usually carried out in the presence of a stronger base (like pyridine) to neutralize the acid (HCl/RCOOH) formed, shifting equilibrium to the right.
  • Benzoylation (acylation with benzoyl chloride, C₆H₅COCl) is called the Schotten-Baumann reaction.
  • Acylation is used for protection of the amino group.
• D. Carbylamine Reaction (Isocyanide Test):
  • Specific test for primary amines (aliphatic or aromatic).
  • When a primary amine is heated with chloroform (CHCl₃) and alcoholic potassium hydroxide (KOH), it forms an isocyanide (or carbylamine) which has a very foul/offensive smell.
  • R-NH₂ + CHCl₃ + 3KOH (alc.) --(Heat)--> R-N≡C (Isocyanide) + 3KCl + 3H₂O
  • Secondary and tertiary amines do not give this test.
• E. Reaction with Nitrous Acid (HNO₂):
  • Nitrous acid (HNO₂) is unstable and prepared in situ (NaNO₂ + HCl) at low temperature (273-278 K or 0-5 °C). Its reaction differs with 1°, 2°, and 3° amines.
  • Primary Aliphatic Amines: React with HNO₂ to form unstable aliphatic diazonium salts, which readily decompose to form alcohol and liberate nitrogen gas (N₂).
    • R-NH₂ + HNO₂ --(NaNO₂ + HCl, 273-278K)--> [R-N₂⁺Cl⁻] --(H₂O)--> R-OH + N₂↑ + HCl
  • Primary Aromatic Amines: React with HNO₂ at low temperature (273-278 K) to form relatively stable aromatic diazonium salts. This reaction is called Diazotization.
    • Ar-NH₂ + HNO₂ + HCl --(273-278K)--> Ar-N₂⁺Cl⁻ (Benzene diazonium chloride) + 2H₂O
  • Secondary Amines (Aliphatic/Aromatic): React with HNO₂ to form N-nitrosamines, which are usually yellow oily liquids and insoluble in water.
    • R₂NH + HNO₂ → R₂N-N=O (N-Nitrosamine) + H₂O
    • N-Nitrosamines give Liebermann's nitroso test.
  • Tertiary Aliphatic Amines: React with HNO₂ to form soluble nitrite salts.
    • R₃N + HNO₂ → [R₃NH]⁺NO₂⁻ (Trialkylammonium nitrite)
  • Tertiary Aromatic Amines: Undergo electrophilic substitution at the para position of the ring to form p-nitroso compounds (green/blue).
    • e.g., N,N-Dimethylaniline + HNO₂ → p-Nitroso-N,N-dimethylaniline + H₂O
• F. Reaction with Arylsulphonyl Chloride (Hinsberg's Test):
  • Used to distinguish between 1°, 2°, and 3° amines and also for their separation.
  • Hinsberg's Reagent: Benzenesulphonyl chloride (C₆H₅SO₂Cl).
  • Primary Amine: Reacts to form N-alkylbenzenesulphonamide, which has an acidic hydrogen attached to nitrogen. Hence, it is soluble in alkali (KOH/NaOH).
    • C₆H₅SO₂Cl + RNH₂ → C₆H₅SO₂NHR (Soluble in alkali)
  • Secondary Amine: Reacts to form N,N-dialkylbenzenesulphonamide, which has no acidic hydrogen on nitrogen. Hence, it is insoluble in alkali.
    • C₆H₅SO₂Cl + R₂NH → C₆H₅SO₂NR₂ (Insoluble in alkali)
  • Tertiary Amine: Does not react with benzenesulphonyl chloride (no H on N).
  • (Note: Nowadays, p-toluenesulphonyl chloride is often used instead).
• G. Electrophilic Substitution in Aromatic Amines (Aniline):
  • The -NH₂ group is a powerful activating group and is ortho-, para- directing due to resonance (+R effect).
  • 1. Bromination: Aniline reacts with bromine water (aq. Br₂) at room temperature to give a white precipitate of 2,4,6-tribromoaniline. The reaction is very fast due to high activation.
    • C₆H₅NH₂ + 3Br₂ (aq) → C₆H₄Br₃NH₂ (2,4,6-Tribromoaniline) + 3HBr
    • To get monosubstituted product (e.g., p-bromoaniline), the activating effect of -NH₂ group must be reduced by protection via acetylation (reaction with acetic anhydride). The resulting acetanilide (-NHCOCH₃) is less activating. After bromination (major product is para), the acetyl group is removed by hydrolysis.
  • 2. Nitration: Direct nitration of aniline with conc. HNO₃ + conc. H₂SO₄ yields a mixture of ortho-, meta-, and para-nitroanilines. A significant amount of meta-product (~47%) is formed because, in the strongly acidic medium, aniline gets protonated to form the anilinium ion (C₆H₅NH₃⁺). The -NH₃⁺ group is deactivating and meta-directing. Oxidation products (tarry) are also formed.
    • To get p-nitroaniline as the major product, protect the -NH₂ group by acetylation first, then nitrate, and finally hydrolyze.
  • 3. Sulphonation: Aniline reacts with conc. H₂SO₄ to form anilinium hydrogensulphate, which upon heating at 453-473 K gives p-aminobenzenesulphonic acid (Sulphanilic acid). Sulphanilic acid exists as a dipolar ion (zwitterion).
  • Friedel-Crafts Reaction: Aniline does not undergo Friedel-Crafts reaction (alkylation or acylation) because it forms a salt with the Lewis acid catalyst (AlCl₃), deactivating the ring strongly.

7. Diazonium Salts (Ar-N₂⁺X⁻)

• Structure: Ar-N≡N⁺ X⁻

• Preparation: Diazotization of primary aromatic amines (see Reaction with HNO₂).

• Stability: Generally stable only at low temperatures (0-5 °C) in solution. They decompose if warmed or allowed to stand dry. Resonance stabilization makes them more stable than aliphatic diazonium salts.

• Importance: Very versatile intermediates in organic synthesis. The diazonium group (-N₂⁺) is an excellent leaving group (as stable N₂ gas) and can be replaced by various other groups.

• Synthetic Applications (Replacement Reactions):

  • 1. Replacement by Halide/Cyanide (Sandmeyer Reaction): Reaction of diazonium salt with CuCl/HCl, CuBr/HBr, or CuCN/KCN introduces -Cl, -Br, or -CN group respectively.
    • ArN₂⁺Cl⁻ --(CuCl/HCl)--> ArCl + N₂
    • ArN₂⁺Cl⁻ --(CuBr/HBr)--> ArBr + N₂
    • ArN₂⁺Cl⁻ --(CuCN/KCN)--> ArCN + N₂
  • 2. Replacement by Halide (Gattermann Reaction): Modification of Sandmeyer, uses Copper powder and HX instead of cuprous halide. Often gives lower yields.
    • ArN₂⁺Cl⁻ --(Cu/HCl)--> ArCl + N₂
    • ArN₂⁺Cl⁻ --(Cu/HBr)--> ArBr + N₂
  • 3. Replacement by Iodide (-I): Warming the diazonium salt solution with Potassium Iodide (KI). No copper catalyst needed.
    • ArN₂⁺Cl⁻ + KI --(Warm)--> ArI + KCl + N₂
  • 4. Replacement by Fluoride (-F) (Balz-Schiemann Reaction): Diazonium salt is reacted with fluoroboric acid (HBF₄) to precipitate diazonium fluoroborate (ArN₂⁺BF₄⁻), which on heating decomposes to give aryl fluoride.
    • ArN₂⁺Cl⁻ + HBF₄ → ArN₂⁺BF₄⁻ --(Heat)--> ArF + BF₃ + N₂
  • 5. Replacement by Hydrogen (-H) (Deamination): Reduction of diazonium salt using mild reducing agents like hypophosphorous acid (H₃PO₂) or ethanol (C₂H₅OH).
    • ArN₂⁺Cl⁻ + H₃PO₂ + H₂O → ArH + N₂ + H₃PO₃ + HCl
    • ArN₂⁺Cl⁻ + CH₃CH₂OH → ArH + N₂ + CH₃CHO (Acetaldehyde) + HCl
  • 6. Replacement by Hydroxyl group (-OH): Warming the diazonium salt solution with water or dilute acid.
    • ArN₂⁺Cl⁻ + H₂O --(Warm/H⁺)--> ArOH (Phenol) + N₂ + HCl
  • 7. Replacement by Nitro group (-NO₂): Diazonium fluoroborate is treated with aqueous sodium nitrite (NaNO₂) in the presence of copper powder.
    • ArN₂⁺BF₄⁻ + NaNO₂ --(Cu/Heat)--> ArNO₂ + NaBF₄ + N₂

• B. Coupling Reactions (Retention of Diazo Group):

  • Arenediazonium salts react as electrophiles with electron-rich aromatic compounds like phenols and anilines (electrophilic aromatic substitution). The reaction usually occurs at the para position (or ortho if para is blocked). This links two aromatic rings through the -N=N- azo group. The products are intensely coloured Azo Dyes.
  • Reaction with Phenol: Occurs in mildly alkaline medium (pH 9-10).
    • ArN₂⁺Cl⁻ + C₆H₅OH --(OH⁻, 273-278K)--> Ar-N=N-C₆H₄-OH (p-Hydroxyazobenzene - Orange dye) + HCl
  • Reaction with Aniline: Occurs in mildly acidic medium (pH 4-5).
    • ArN₂⁺Cl⁻ + C₆H₅NH₂ --(H⁺, 273-278K)--> Ar-N=N-C₆H₄-NH₂ (p-Aminoazobenzene - Yellow dye) + HCl

Multiple Choice Questions (MCQs)

1. Which of the following is the correct order of basicity for methyl substituted amines in aqueous solution?
   (a) (CH₃)₃N > (CH₃)₂NH > CH₃NH₂ > NH₃
   (b) CH₃NH₂ > (CH₃)₂NH > (CH₃)₃N > NH₃
   (c) (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃
   (d) (CH₃)₂NH > (CH₃)₃N > CH₃NH₂ > NH₃

2. Gabriel phthalimide synthesis is used for the preparation of:
   (a) Primary aromatic amines
   (b) Secondary aliphatic amines
   (c) Primary aliphatic amines
   (d) Tertiary amines

3. Which of the following tests is used to distinguish primary amines from secondary and tertiary amines?
   (a) Liebermann's nitroso test
   (b) Carbylamine test
   (c) Hinsberg's test
   (d) Both (b) and (c) can be used for distinction.

4. The reaction of benzene diazonium chloride with CuCl/HCl to form chlorobenzene is known as:
   (a) Gattermann reaction
   (b) Sandmeyer reaction
   (c) Balz-Schiemann reaction
   (d) Hoffmann bromamide reaction

5. Aniline reacts with bromine water at room temperature to give:
   (a) o-Bromoaniline
   (b) p-Bromoaniline
   (c) A mixture of o- and p-bromoaniline
   (d) 2,4,6-Tribromoaniline

6. Hinsberg's reagent is:
   (a) Benzenesulphonyl chloride
   (b) Benzoyl chloride
   (c) p-Toluenesulphonic acid
   (d) Phenyl isocyanide

7. The IUPAC name for CH₃CH(NH₂)CH₂CH₃ is:
   (a) Butan-3-amine
   (b) Butan-2-amine
   (c) 2-Aminobutane
   (d) 3-Aminobutane

8. Which reaction is suitable for converting acetamide (CH₃CONH₂) into methanamine (CH₃NH₂)?
   (a) Gabriel Phthalimide synthesis
   (b) Reduction with LiAlH₄
   (c) Hoffmann bromamide degradation
   (d) Ammonolysis

9. The boiling point order for isomeric amines is 1° > 2° > 3°. This is mainly due to:
   (a) Increasing molecular weight
   (b) Decreasing inductive effect
   (c) Decreasing extent of intermolecular hydrogen bonding
   (d) Increasing steric hindrance

10. Aniline is a weaker base than ethylamine primarily because:
    (a) The phenyl group has a +I effect.
    (b) The lone pair on nitrogen in aniline is delocalized into the benzene ring.
    (c) Ethylamine has greater steric hindrance.
    (d) Aniline is aromatic.

Answer Key:

1. (c)
2. (c)
3. (d) - Carbylamine is specific for 1°. Hinsberg distinguishes 1°, 2°, and 3°.
4. (b)
5. (d)
6. (a)
7. (b)
8. (c)
9. (c)
10. (b)

Study these notes thoroughly. Focus on understanding the reasons behind trends (like basicity, boiling points) and the scope and limitations of each preparation method and reaction. Diazonium salt chemistry is particularly rich in named reactions and synthetic applications, making it a favorite area for examiners. Good luck with your preparation!