Class 12 Chemistry Notes Chapter 13 (Amines) – Examplar Problems Book

Detailed Notes with MCQs of Chapter 13, Amines. This is a crucial chapter, especially considering its applications and the types of questions frequently asked in competitive government exams. We'll break it down systematically.

Amines: Structure, Classification, and Nomenclature

1. 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.
2. Structure: The nitrogen atom in amines is sp³ hybridized, resulting in a trigonal pyramidal geometry. The presence of an unshared pair of electrons (lone pair) makes amines basic and nucleophilic. The C-N-C or C-N-H bond angle is slightly less than the tetrahedral angle (109.5°) due to lone pair-bond pair repulsion (e.g., ~108° in trimethylamine).
3. Classification:
   • Primary (1°): One R/Ar group attached to N (R-NH₂, Ar-NH₂). Example: Methanamine (CH₃NH₂), Aniline (C₆H₅NH₂).
   • Secondary (2°): Two R/Ar groups attached to N (R₂NH, Ar₂NH, R-NH-Ar). Example: N-Methylethanamine (CH₃CH₂NHCH₃), Diphenylamine ((C₆H₅)₂NH).
   • Tertiary (3°): Three R/Ar groups attached to N (R₃N, Ar₃N, R₂NAr). Example: N,N-Dimethylmethanamine ((CH₃)₃N), N,N-Dimethylaniline (C₆H₅N(CH₃)₂).
   • Quaternary Ammonium Salts: Four R/Ar groups attached to N, carrying a positive charge ([R₄N]⁺X⁻). Example: Tetramethylammonium chloride ([(CH₃)₄N]⁺Cl⁻).
4. Nomenclature:
   • Common System: Alkylamines (e.g., Ethylamine, Diethylamine, Triethylamine). For aryl amines, aniline is the parent name.
   • IUPAC System: The '-e' of the parent alkane is replaced by '-amine' (Alkanamine). The position of the -NH₂ group is indicated by a number. For secondary and tertiary amines, the largest alkyl group is chosen as the parent chain, and other groups on N are designated using the prefix 'N-' (e.g., N-Methylpropan-2-amine, N,N-Diethylethanamine). Aniline is an accepted IUPAC name for C₆H₅NH₂. Its substituted derivatives are named accordingly (e.g., 4-Bromoaniline).

Methods of Preparation of Amines

1. Reduction of Nitro Compounds: Nitroalkanes or nitroarenes are reduced to corresponding primary amines using:
   • H₂/Pd, Pt, or Ni
   • Sn/HCl or Fe/HCl (Fe/HCl is preferred for aromatic amines as FeCl₂ formed gets hydrolyzed to release HCl, requiring less acid).
     Ar-NO₂ + 6[H] --(Fe/HCl)--> Ar-NH₂ + 2H₂O
2. Ammonolysis of Alkyl Halides: Reaction of alkyl or benzyl halides with an ethanolic solution of ammonia (nucleophilic substitution).
   • R-X + NH₃ --> R-NH₃⁺X⁻ --(NaOH)--> R-NH₂
   • Disadvantage: The primary amine formed can further react with R-X, leading to a mixture of 1°, 2°, 3° amines and quaternary ammonium salt. To obtain primarily 1° amine, use a large excess of ammonia. The order of reactivity of halides is R-I > R-Br > R-Cl. Aryl halides are much less reactive towards ammonolysis.
3. Reduction of Nitriles (Cyanides): Nitriles are reduced to primary amines using:
   • LiAlH₄ (Lithium Aluminium Hydride)
   • Catalytic hydrogenation (H₂/Ni, Pt or Pd)
     R-C≡N + 4[H] --(LiAlH₄ or H₂/Ni)--> R-CH₂-NH₂ (Adds one CH₂ group - step-up reaction)
4. Reduction of Amides: Amides are reduced to amines containing the same number of carbon atoms using LiAlH₄.
   R-CO-NH₂ + 4[H] --(LiAlH₄/H₂O)--> R-CH₂-NH₂
5. Gabriel Phthalimide Synthesis: Used exclusively for the preparation of pure primary aliphatic amines.
   • Phthalimide reacts with ethanolic KOH to form potassium phthalimide.
   • Potassium phthalimide reacts with an alkyl halide (SN2 reaction) to form N-alkylphthalimide.
   • Alkaline hydrolysis (or with hydrazine) of N-alkylphthalimide yields the primary amine.
   • Limitation: Aromatic primary amines cannot be prepared because aryl halides do not undergo nucleophilic substitution with the phthalimide anion easily.
6. Hoffmann Bromamide Degradation Reaction: Primary amides react with Br₂ in aqueous or ethanolic NaOH solution to yield primary amines having one carbon atom less than the parent amide.
   R-CO-NH₂ + Br₂ + 4NaOH --> R-NH₂ + Na₂CO₃ + 2NaBr + 2H₂O (Step-down reaction)

Physical Properties

1. Solubility: Lower aliphatic amines (1°, 2°) are soluble in water due to their ability to form hydrogen bonds with water molecules. Solubility decreases with increasing molar mass (larger hydrophobic alkyl part). Aromatic amines are generally insoluble in water.
2. Boiling Points: Amines have higher boiling points than non-polar compounds (like alkanes) of comparable molar mass due to intermolecular H-bonding.
   • The order of boiling points for isomeric amines is: Primary (1°) > Secondary (2°) > Tertiary (3°). This is because 1° amines have two H-atoms on N for H-bonding, 2° have one, and 3° have none (no H on N).
   • Boiling points of amines are lower than those of alcohols or carboxylic acids of similar molar mass because N-H bonds are less polar than O-H bonds (Nitrogen is less electronegative than Oxygen).

Chemical Reactions

1. Basic Character of Amines:

   • Amines act as Lewis bases due to the lone pair of electrons on the nitrogen atom.
   • R-NH₂ + H₂O ⇌ R-NH₃⁺ + OH⁻
   • Basicity constant, Kb = [RNH₃⁺][OH⁻] / [RNH₂]. Higher Kb or lower pKb indicates a stronger base (pKb = -log Kb).
   • Comparison:
     • Aliphatic Amines vs. Ammonia: Aliphatic amines are generally stronger bases than ammonia due to the electron-releasing inductive effect (+I effect) of alkyl groups, which increases electron density on nitrogen.
     • Aromatic Amines vs. Ammonia: Aromatic amines (like aniline) are weaker bases than ammonia because the lone pair on nitrogen is delocalized into the benzene ring through resonance, making it less available for protonation.
     • Order of Basicity (Important):
       • In Gaseous Phase: Based purely on +I effect: 3° > 2° > 1° > NH₃.
       • In Aqueous Solution: The order is influenced by a combination of +I effect, steric hindrance (affects solvation of the conjugate acid), and solvation effect (H-bonding with water stabilizes the conjugate acid).
         • 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.
     • Effect of Substituents on Aniline Basicity:
       • Electron-donating groups (EDG) like -OCH₃, -CH₃ increase basicity (usually stronger at para > ortho).
       • Electron-withdrawing groups (EWG) like -NO₂, -SO₃H, -COOH, -X decrease basicity (usually stronger at para > ortho).

2. Alkylation: Reaction with alkyl halides to form 2°, 3° amines and finally quaternary ammonium salts (see Ammonolysis).

3. Acylation: Reaction of 1° and 2° amines with acid chlorides (RCOCl), anhydrides ((RCO)₂O), or esters (RCOOR') to form N-substituted amides. This reaction is carried out in the presence of a stronger base (like pyridine) to neutralize the HCl formed.
   R-NH₂ + R'-COCl --(Base)--> R-NH-CO-R' + HCl (N-alkylalkanamide)
   R₂NH + R'-COCl --(Base)--> R₂N-CO-R' + HCl (N,N-dialkylalkanamide)

   • Tertiary amines do not undergo acylation due to the absence of replaceable hydrogen on nitrogen.
   • Application: Protection of the amino group during electrophilic substitution reactions of aromatic amines. Acetylation reduces the activating effect of the -NH₂ group and prevents polysubstitution or oxidation. The acetyl group can be removed later by hydrolysis.

4. Carbylamine Reaction (Isocyanide Test):

   • Only primary amines (aliphatic or aromatic) react with chloroform (CHCl₃) and alcoholic KOH upon heating to form isocyanides (carbylamines), which have a very unpleasant smell.
     R-NH₂ + CHCl₃ + 3KOH --(Heat)--> R-NC + 3KCl + 3H₂O
   • This reaction is used as a test for primary amines. Secondary and tertiary amines do not give this test.

5. Reaction with Nitrous Acid (HNO₂): (HNO₂ is unstable, prepared in situ from NaNO₂ + HCl)

   • Primary Aliphatic Amines: React with HNO₂ to form unstable aliphatic diazonium salts, which decompose to yield nitrogen gas and a mixture of alcohols, alkenes.
     R-NH₂ + HNO₂ --(NaNO₂ + HCl)--> [R-N₂⁺Cl⁻] --(H₂O)--> R-OH + N₂ + HCl (Major product is alcohol)
   • Primary Aromatic Amines: React with HNO₂ at low temperatures (0-5°C or 273-278 K) to form stable arene diazonium salts. This reaction is called diazotisation.
     Ar-NH₂ + HNO₂ + HCl --(273-278 K)--> Ar-N₂⁺Cl⁻ + 2H₂O
   • Secondary Amines (Aliphatic/Aromatic): React with HNO₂ to form N-nitrosamines, which are yellow oily liquids and are carcinogenic.
     R₂NH + HNO₂ --> R₂N-N=O + H₂O (N-Nitrosamine)
   • 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 or blue).
     Ar-NR₂ + HNO₂ --> p-(O=N)-Ar-NR₂ + H₂O

6. Reaction with Arylsulphonyl Chloride (Hinsberg's Test): Used to distinguish between 1°, 2°, and 3° amines. The reagent is benzenesulphonyl chloride (C₆H₅SO₂Cl).

   • Primary Amines: React to form N-alkylbenzenesulphonamide, which is soluble in alkali (like KOH or NaOH) because the hydrogen attached to nitrogen is acidic.
     C₆H₅SO₂Cl + RNH₂ --> C₆H₅SO₂NHR --(KOH)--> [C₆H₅SO₂NR]⁻K⁺ (Soluble salt)
   • Secondary Amines: React to form N,N-dialkylbenzenesulphonamide, which is insoluble in alkali as it has no acidic hydrogen on nitrogen.
     C₆H₅SO₂Cl + R₂NH --> C₆H₅SO₂NR₂ (Insoluble in KOH)
   • Tertiary Amines: Do not react with benzenesulphonyl chloride.
   • Note: Nowadays, p-toluenesulphonyl chloride is often used instead.

7. Electrophilic Substitution in Aromatic Amines (Aniline): The -NH₂ group is a powerful activating group and is ortho-, para- directing.

   • Bromination: Aniline reacts with bromine water (aqueous Br₂) at room temperature to give a white precipitate of 2,4,6-tribromoaniline. Polysubstitution occurs due to high activation.
     C₆H₅NH₂ + 3Br₂(aq) --> 2,4,6-Tribromoaniline + 3HBr
   • To get monosubstituted product (e.g., p-bromoaniline): Protect the -NH₂ group by acetylation first (forms acetanilide), then brominate (p-bromoacetanilide is major), followed by hydrolysis (acidic or alkaline) to remove the acetyl group.
   • Nitration: Direct nitration with conc. HNO₃ + conc. H₂SO₄ leads to:
     • Oxidation products (tarry).
     • Significant amount of meta-nitroaniline (~47%) along with ortho (~2%) and para (~51%). This is because in the strongly acidic medium, aniline gets protonated to form the anilinium ion (C₆H₅NH₃⁺), which has a deactivating, meta-directing effect.
     • To get p-nitroaniline as the major product: Protect the -NH₂ group by acetylation -> nitration -> hydrolysis.
   • Sulphonation: Aniline reacts with concentrated H₂SO₄ to form anilinium hydrogensulphate, which upon heating (180-200°C or 453-473 K) rearranges to form p-aminobenzenesulphonic acid (Sulphanilic acid). Sulphanilic acid exists as a dipolar ion (zwitterion).

Diazonium Salts (Ar-N₂⁺X⁻)

1. Preparation: Diazotisation of primary aromatic amines (e.g., aniline) with NaNO₂/HCl at 0-5°C.
2. Stability: Arene diazonium salts are stable for a short time in solution at low temperatures. Their stability is due to resonance delocalization of the positive charge over the benzene ring.
3. Importance: Very useful synthetic intermediates. The diazonium group (N₂⁺) is an excellent leaving group (leaves as N₂ gas) and can be replaced by various nucleophiles.

Chemical Reactions of Diazonium Salts:

A. Reactions involving Displacement of Nitrogen:

1. Replacement by Halide or Cyanide:
   • Sandmeyer Reaction: Using cuprous salts (CuCl/HCl, CuBr/HBr, CuCN/KCN).
     ArN₂⁺X⁻ --(CuCl/HCl)--> ArCl + N₂
ArN₂⁺X⁻ --(CuBr/HBr)--> ArBr + N₂
ArN₂⁺X⁻ --(CuCN/KCN)--> ArCN + N₂
   • Gattermann Reaction: Using copper powder and corresponding halogen acid (HCl or HBr). Yields are generally lower than Sandmeyer.
     ArN₂⁺X⁻ --(Cu/HCl)--> ArCl + N₂ + CuX
ArN₂⁺X⁻ --(Cu/HBr)--> ArBr + N₂ + CuX
2. Replacement by Iodide Ion: Warming the diazonium salt solution with potassium iodide (KI). No copper salt needed.
   ArN₂⁺X⁻ + KI --(Warm)--> ArI + N₂ + KX
3. Replacement by Fluoride Ion (Balz-Schiemann Reaction): Diazonium salt is treated with fluoroboric acid (HBF₄) to precipitate diazonium fluoroborate, which on heating decomposes to yield aryl fluoride.
   ArN₂⁺X⁻ + HBF₄ --> ArN₂⁺BF₄⁻ --(Heat)--> ArF + BF₃ + N₂
4. Replacement by Hydrogen (Deamination): Reduction using mild reducing agents like hypophosphorous acid (H₃PO₂) or ethanol (CH₃CH₂OH).
   ArN₂⁺X⁻ + H₃PO₂ + H₂O --> ArH + N₂ + H₃PO₃ + HX
ArN₂⁺X⁻ + CH₃CH₂OH --> ArH + N₂ + CH₃CHO + HX
5. Replacement by Hydroxyl Group (-OH): Warming the diazonium salt solution with water or dilute acid.
   ArN₂⁺X⁻ + H₂O --(Warm)--> ArOH + N₂ + HX (Preparation of phenols)
6. Replacement by Nitro Group (-NO₂): Diazonium fluoroborate is treated with aqueous sodium nitrite solution in the presence of copper powder.
   ArN₂⁺BF₄⁻ + NaNO₂ --(Cu/Heat)--> ArNO₂ + N₂ + NaBF₄

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

• Diazonium salts act as electrophiles and react with electron-rich aromatic compounds like phenols and anilines (electrophilic aromatic substitution).
• The reaction occurs at the para position (usually) of the phenol or aniline.
• The product is an azo compound (Ar-N=N-Ar'), which contains the -N=N- (azo) group linking two aromatic rings. Azo compounds are often intensely coloured (dyes).
• Conditions:
  • Reaction with Phenols: Occurs in mildly alkaline medium (pH 9-10).
    ArN₂⁺Cl⁻ + Phenol --(OH⁻, pH 9-10)--> p-Hydroxyazobenzene (Orange dye) + Cl⁻ + H₂O
  • Reaction with Anilines: Occurs in mildly acidic medium (pH 4-5).
    ArN₂⁺Cl⁻ + Aniline --(H⁺, pH 4-5)--> p-Aminoazobenzene (Yellow dye) + Cl⁻ + H₂O

Multiple Choice Questions (MCQs)

1. Which of the following is a secondary amine?
   (a) Aniline
   (b) Triethylamine
   (c) N-Methylaniline
   (d) Iso-propylamine

2. The reaction of an amide with Br₂ and aqueous NaOH to give a primary amine with one carbon atom less than the amide is known as:
   (a) Gabriel Phthalimide synthesis
   (b) Hoffmann Bromamide degradation
   (c) Carbylamine reaction
   (d) Sandmeyer reaction

3. Arrange the following in decreasing order of basicity in aqueous solution: NH₃, CH₃NH₂, (CH₃)₂NH, (CH₃)₃N
   (a) (CH₃)₃N > (CH₃)₂NH > CH₃NH₂ > NH₃
   (b) (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃
   (c) CH₃NH₂ > (CH₃)₂NH > (CH₃)₃N > NH₃
   (d) (CH₃)₂NH > (CH₃)₃N > CH₃NH₂ > NH₃

4. Which of the following tests can be used to distinguish between ethylamine (1°) and diethylamine (2°)?
   (a) Tollen's test
   (b) Fehling's test
   (c) Carbylamine test
   (d) Iodoform test

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. The reagent used in the Gattermann reaction for converting benzene diazonium chloride to chlorobenzene is:
   (a) CuCl/HCl
   (b) Cu powder/HCl
   (c) Cu₂O/HCl
   (d) CuCl₂/HCl

7. Acetylation of aniline is done before nitration to:
   (a) Increase the activating effect of the amino group
   (b) Obtain the ortho-nitro derivative predominantly
   (c) Prevent oxidation and obtain the para-nitro derivative as the major product
   (d) Make the reaction proceed faster

8. Which of the following amines cannot be prepared by Gabriel Phthalimide synthesis?
   (a) Ethanamine
   (b) Propan-1-amine
   (c) Aniline
   (d) Benzylamine

9. The reaction of benzene diazonium chloride with phenol in a weakly basic medium results in the formation of:
   (a) Azoxybenzene
   (b) p-Hydroxyazobenzene
   (c) Chlorobenzene
   (d) Phenyl benzoate

10. N-Ethyl-N-methylpropan-1-amine is a:
    (a) Primary amine
    (b) Secondary amine
    (c) Tertiary amine
    (d) Quaternary ammonium salt

Answer Key for MCQs:

1. (c)
2. (b)
3. (b)
4. (c)
5. (d)
6. (b)
7. (c)
8. (c)
9. (b)
10. (c)

Study these notes carefully, focusing on the reaction conditions, reagents, product structures, and especially the comparative aspects like basicity and distinguishing tests. Good luck with your preparation!