20.1 AMINES – INTRODUCTION
⚛️ What are Amines?
- Definition: Organic compounds containing nitrogen linked with alkyl/aryl groups
- Derived from: Ammonia (NH₃) by replacing H atoms with R groups
- Key feature: Nitrogen has a lone pair of electrons
- General properties: Basic, nucleophilic, some are optically active
📝 Memorization Tip: “Amines = Ammonia’s children. Replace H with R groups!”
🔷 CLASSIFICATION OF AMINES
Primary (1°)
1 R group
R-NH₂
1 R group
R-NH₂
→
Secondary (2°)
2 R groups
R-NH-R’
2 R groups
R-NH-R’
→
Tertiary (3°)
3 R groups
R₃N
3 R groups
R₃N
| Type | Formula | Example | Name |
|---|---|---|---|
| Primary (1°) | R-NH₂ | CH₃-NH₂ | Methylamine |
| Secondary (2°) | R-NH-R’ | (CH₃)₂NH | Dimethylamine |
| Tertiary (3°) | R₃N | (CH₃)₃N | Trimethylamine |
🎯 Easy Trick: Count the carbons attached to N:
• 1 carbon = 1° amine
• 2 carbons = 2° amine
• 3 carbons = 3° amine
• 1 carbon = 1° amine
• 2 carbons = 2° amine
• 3 carbons = 3° amine
🏗️ STRUCTURE OF AMINES
Nitrogen: sp³ hybridized
Tetrahedral geometry
Lone pair in sp³ orbital
- Hybridization: sp³ (tetrahedral)
- Geometry: Nearly tetrahedral (107° in NH₃)
- Bonds: Three sigma bonds + one lone pair
- Optical activity: Possible if 3 different groups on N
20.2 PHYSICAL PROPERTIES
🔬 Key Properties
- Hydrogen bonding: N-H bonds are polar → strong H-bonds
- Boiling points: Higher than analogous alkanes
- Water solubility: Small amines are soluble (H-bond with water)
- Basicity: Due to lone pair on nitrogen
- Nucleophilicity: Can donate electron pair
💡 Why high BP? Hydrogen bonding! N-H···N bonds require more energy to break.
📊 Comparison Table
| Property | Primary Amines | Secondary Amines | Tertiary Amines |
|---|---|---|---|
| H-bonding | Strong (two H atoms) | Moderate (one H atom) | No H-bonding |
| Boiling Point | Highest | Intermediate | Lowest |
| Water Solubility | High | Moderate | Low |
| Basicity (aq) | Medium | Highest | Lowest |
20.3 PREPARATION OF AMINES
⚗️ 1. Reaction of Alkyl Halides with Ammonia
Ammonolysis Reaction
R-X + NH₃ → R-NH₂ (1°) → R₂NH (2°) → R₃N (3°) → R₄N⁺X⁻ (Quaternary salt)
Conditions: Alcoholic NH₃, sealed tube, 100°C
⚠️ Caution: Over-alkylation occurs! Use excess NH₃ to favor primary amine.
🔁 2. Reduction Methods
🎯 Memory Trick: “LiAlH₄ reduces everything to amines!”
| Starting Compound | Reducing Agent | Product |
|---|---|---|
| Nitriles (R-CN) | LiAlH₄ or H₂/Ni | Primary amines |
| Nitro compounds (R-NO₂) | Sn/HCl or H₂/Pd | Primary amines |
| Amides (R-CONH₂) | LiAlH₄ | Primary amines |
| Amides (Hofmann) | Br₂ + KOH | Primary amines (-1 carbon) |
🧪 Key Reaction Mechanisms
Nitrile Reduction
R-CN → R-CH₂NH₂
R-CN → R-CH₂NH₂
→
Nitro Reduction
R-NO₂ → R-NH₂
R-NO₂ → R-NH₂
→
Hofmann
R-CONH₂ → R-NH₂
R-CONH₂ → R-NH₂
20.4 BASICITY OF AMINES
⚖️ Why Amines are Basic?
- Lone pair: Nitrogen’s lone pair can accept H⁺
- Electron donation: Alkyl groups increase electron density on N
- pKb values: Lower pKb = stronger base
NH₃ + H⁺ → NH₄⁺ (ammonium)
CH₃-NH₂ + H⁺ → CH₃-NH₃⁺ (methylammonium)
📈 Basicity Order
In GAS Phase
3° > 2° > 1° > NH₃
3° > 2° > 1° > NH₃
↕
In AQUEOUS Phase
2° > 1° > 3° > NH₃
2° > 1° > 3° > NH₃
🤔 Why different orders?
Gas: Only inductive effect matters
Aqueous: Solvation effect + inductive effect
Tertiary amines less soluble → weaker in water
Gas: Only inductive effect matters
Aqueous: Solvation effect + inductive effect
Tertiary amines less soluble → weaker in water
🔢 pKb Values Comparison
| Compound | pKb | Relative Basicity |
|---|---|---|
| Ammonia (NH₃) | 4.76 | Weakest |
| Methylamine (CH₃NH₂) | 3.38 | Stronger than NH₃ |
| Dimethylamine ((CH₃)₂NH) | 3.27 | Strongest (aqueous) |
| Trimethylamine ((CH₃)₃N) | 4.19 | Weaker than 1° & 2° |
🎯 Exam Tip: Methylamine > Ammonia because CH₃ group pushes electrons toward N (+I effect)
20.5 REACTIONS OF AMINES
⚡ 1. With Acid Halides (Acylation)
Amine + Acid chloride → Amide
R-NH₂ + R’-COCl → R’-CONH-R + HCl
🔄 2. With Alkyl Halides (Alkylation)
Stepwise Alkylation
1° amine + R-X → 2° amine → 3° amine → Quaternary salt
🎨 3. With Nitrous Acid (Diazotization)
R-NH₂ + HNO₂ + HCl → R-N₂⁺Cl⁻ + 2H₂O
Temperature: 0-5°C (crucial!)
⚠️ Critical Temperature: Diazonium salts decompose above 10°C! Keep ice bath.
🌈 DIAZONIUM SALTS – Applications
| Property/Use | Details |
|---|---|
| Stability | Only aromatic diazonium salts are stable (benzene ring stabilizes) |
| Dyes | Orange-red azo dyes with phenol |
| Industries | Textile, cosmetics, paint, leather |
| Properties | Antibacterial, antifungal |
| Example | Benzenediazonium chloride: C₆H₅-N₂⁺Cl⁻ |
🎯 Memory Trick: “Diazo = Di (two) + azo (nitrogen). Two nitrogen atoms in N₂⁺ group!”
COMPLETE SOLUTIONS – MCQs & Short Questions
Multiple Choice Questions:
1. General formula for secondary amine is:
Answer: b. RNHR
Detailed Explanation:
- a. RNH₂ → Primary amine (one R group)
- b. RNHR → Secondary amine (two R groups)
- c. NH₃ → Ammonia (parent compound)
- d. RCONH₂ → Amide (not amine)
🎯 Trick: Count R groups attached to N:
1 R = primary, 2 R = secondary, 3 R = tertiary
1 R = primary, 2 R = secondary, 3 R = tertiary
2. General formula for nitriles is:
Answer: c. RCN
Detailed Explanation:
- a. RNH₂ → Primary amine
- b. RNO₃ → Nitrate ester
- c. RCN → Nitrile (-C≡N group)
- d. ROH → Alcohol
🎯 Memory Aid: “Nitrile = CN (like Cyanide but with R group)”
3. Which of the following is a reducing agent?
Answer: c. LiAlH₄
Detailed Explanation:
- a. KMnO₄ → Oxidizing agent (purple to colorless)
- b. K₂Cr₂O₇ → Oxidizing agent (orange to green)
- c. LiAlH₄ → Strong reducing agent (Lithium Aluminum Hydride)
- d. NaOH → Base, not reducing agent
🎯 Trick: “LiAlH₄ reduces everything! Nitriles, amides, nitro compounds → amines”
4. Which can donate an electron pair?
Answer: d. RNH₂
Detailed Explanation:
- a. RX → Alkyl halide (electrophile, accepts electrons)
- b. RCOR → Ketone (carbonyl carbon is electrophilic)
- c. RCHO → Aldehyde (electrophilic carbonyl)
- d. RNH₂ → Amine (has lone pair on N, nucleophile)
🎯 Key Point: Amines are nucleophiles because of the lone pair on nitrogen
Short Answer Questions:
(i) What are primary, secondary and tertiary alkyl amines?
Answer: Classification based on number of alkyl groups attached to nitrogen atom.
| Type | Definition | Formula | Example |
|---|---|---|---|
| Primary (1°) | One alkyl group attached to N | R-NH₂ | CH₃-NH₂ (methylamine) |
| Secondary (2°) | Two alkyl groups attached to N | R₂NH | (CH₃)₂NH (dimethylamine) |
| Tertiary (3°) | Three alkyl groups attached to N | R₃N | (CH₃)₃N (trimethylamine) |
🎯 Memory Trick: “1° = 1 R, 2° = 2 R, 3° = 3 R. Count the carbons!”
(ii) What are diazonium salts?
Answer: Ionic compounds containing R-N₂⁺ X⁻ group, where R = aryl group, X = halide.
- Formula: R-N₂⁺X⁻ (e.g., C₆H₅-N₂⁺Cl⁻)
- Preparation: Primary aromatic amine + NaNO₂ + HCl at 0-5°C
- Stability: Only aromatic diazonium salts are stable (benzene ring stabilizes)
- Decomposition: Above 10°C → phenol + N₂ gas
- Uses: Azo dye synthesis, pigments
- Properties: Antibacterial, antifungal, colored compounds
⚠️ Important: Aliphatic diazonium salts are too unstable for practical use!
(iii) Amines are more basic than ammonia.
Answer: Due to +I effect of alkyl groups increasing electron density on nitrogen.
NH₃ → pKb = 4.76
CH₃-NH₂ → pKb = 3.38
Explanation:
- Inductive Effect: Alkyl groups are electron-donating (+I)
- Electron density: Increased on nitrogen atom
- Lone pair: More available for protonation
- Basicity order: 2° > 1° > NH₃ > 3° (in aqueous phase)
🎯 Trick: “Alkyl groups = electron push = more basic!”
(iv) What are amines?
Answer: Organic derivatives of ammonia where one or more H atoms are replaced by alkyl/aryl groups.
Derived from
NH₃ (ammonia)
NH₃ (ammonia)
→
By replacing
H with R groups
H with R groups
→
To form
R-NH₂, R₂NH, R₃N
R-NH₂, R₂NH, R₃N
Key Features:
- Contain nitrogen with lone pair
- Basic and nucleophilic
- sp³ hybridized N
- Can form hydrogen bonds
- Higher BP than analogous alkanes
(v) Amines have high melting/boiling points compared to analogous alkanes. Why?
Answer: Due to intermolecular hydrogen bonding between N-H bonds.
N-H···N Hydrogen bonds
Stronger than London forces
Detailed Explanation:
- Hydrogen bonding: N-H bonds are polar (N: 3.0, H: 2.1 electronegativity)
- Strength: H-bonds (10-40 kJ/mol) vs London forces (1-10 kJ/mol)
- More energy needed: To break H-bonds during boiling/melting
- Comparison:
- Butane (C₄H₁₀): BP = -1°C (only London forces)
- Butylamine (C₄H₉NH₂): BP = 78°C (H-bonding present)
🎯 Trick: “Hydrogen bonding = higher BP. Look for N-H bonds!”
(vi) Methylamine is a stronger base than ammonia. Explain
Answer: Methyl group is electron-donating (+I effect), increasing electron density on N.
Mechanism:
- CH₃ group pushes electrons toward N (+I effect)
- Electron density on N increases
- Lone pair becomes more available
- Easier to donate to H⁺
- Forms more stable CH₃NH₃⁺ ion
🎯 Memory Aid: “CH₃ pushes electrons → more basic!”
(vii) How would you prepare Diazonium salt?
Answer: React primary aromatic amine with NaNO₂ and HCl at 0-5°C.
Step 1
Cool solution to 0-5°C (ice bath)
Cool solution to 0-5°C (ice bath)
→
Step 2
Add NaNO₂ to primary aromatic amine
Add NaNO₂ to primary aromatic amine
→
Step 3
Add dilute HCl slowly
Add dilute HCl slowly
C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O
Important Conditions:
- Temperature: 0-5°C (critical! Above 10°C = decomposition)
- Amine type: Primary aromatic (aliphatic too unstable)
- Acid: HCl or H₂SO₄
- Product: Benzenediazonium chloride (colorless crystals)
⚠️ Exam Alert: Temperature control is often asked! Must mention 0-5°C.
Long Questions:
3. Discuss the methods of preparation of amines
Answer: Four main methods with mechanisms and conditions.
| Method | Reactants | Conditions | Product | Mechanism |
|---|---|---|---|---|
| 1. Ammonolysis | R-X + NH₃ | Alcoholic, 100°C, sealed tube | Mixture (1°,2°,3°,4°) | Nucleophilic substitution |
| 2. Nitrile reduction | R-CN + LiAlH₄ | Dry ether, then H₂O | 1° amine (R-CH₂NH₂) | Reduction (C≡N → CH₂NH₂) |
| 3. Nitro reduction | R-NO₂ + Sn/HCl | Heating | 1° amine (R-NH₂) | Reduction (NO₂ → NH₂) |
| 4. Hofmann | Amide + Br₂/KOH | Cold, then heat | 1° amine (-1 C) | Degradation (loss of CO) |
🎯 Selection Guide:
• Want same C chain? → Nitrile reduction
• Want -1 C? → Hofmann degradation
• From aromatic? → Nitro reduction
• Simple alkyl? → Ammonolysis
• Want same C chain? → Nitrile reduction
• Want -1 C? → Hofmann degradation
• From aromatic? → Nitro reduction
• Simple alkyl? → Ammonolysis
4. What are nitriles? How they can be reduced.
Answer: Nitriles are R-C≡N compounds; reduced by LiAlH₄ or catalytic hydrogenation.
Nitriles: R-C≡N
Contains -C≡N triple bond
Reduction Methods:
Method 1: LiAlH₄
R-CN → R-CH₂NH₂
R-CN → R-CH₂NH₂
OR
Method 2: H₂/Ni
R-CN → R-CH₂NH₂
R-CN → R-CH₂NH₂
Detailed Mechanisms:
- LiAlH₄ reduction:
- In dry ether, then H₂O workup
- C≡N reduced to CH₂-NH₂
- Carbon chain remains intact
- Catalytic hydrogenation:
- H₂ gas with Ni/Pt/Raney Ni catalyst
- Milder conditions
- Same product: primary amine
🎯 Memory Trick: “Nitrile + LiAlH₄ = Primary amine with extra CH₂”
5. Identify differences between different types of amines
Answer: Differences in structure, basicity, physical properties, and reactions.
| Aspect | Primary (1°) | Secondary (2°) | Tertiary (3°) |
|---|---|---|---|
| Formula | R-NH₂ | R₂NH | R₃N |
| H-bonding | Strong (2 H) | Moderate (1 H) | None |
| Boiling Point | Highest | Medium | Lowest |
| Basicity (aq) | Medium | Highest | Lowest |
| With HNO₂ | Diazonium salt | N-nitrosamine | No reaction |
| Optical Activity | If chiral C | If chiral C | If 3 different R |
| Preparation from R-X | Excess NH₃ | Controlled alkylation | Excess R-X |
🎯 Quick Comparison:
• 1°: Can form diazonium salts
• 2°: Strongest base in water
• 3°: Can be optically active without chiral carbon
• 1°: Can form diazonium salts
• 2°: Strongest base in water
• 3°: Can be optically active without chiral carbon
6. Evaluate importance of diazonium salts in everyday life
Answer: Crucial for dye industry, medicines, and analytical chemistry.
Why so important?
- Color variety: Wide range of colors possible
- Stability: Azo dyes are light-fast and wash-fast
- Antimicrobial: Natural antibacterial properties
- Cost-effective: Relatively cheap to produce
- Versatile: Can be modified for different applications
🎯 Real-world Connection: The orange safety vests, red carpets, and blue jeans all use azo dyes from diazonium chemistry!
7. Make a comparison between primary amines and secondary amines
Answer: Comprehensive comparison based on structure, properties, and reactivity.
| Parameter | Primary Amines (1°) | Secondary Amines (2°) |
|---|---|---|
| Formula | R-NH₂ | R₂NH |
| No. of R groups | 1 | 2 |
| No. of N-H bonds | 2 | 1 |
| Hydrogen Bonding | Stronger (more H) | Weaker (less H) |
| Boiling Point | Higher | Lower |
| Basicity (aqueous) | Moderate | Highest |
| Reason for basicity | +I from 1 alkyl group | +I from 2 alkyl groups |
| With HNO₂ | Diazonium salts | N-nitrosamines |
| With R-COCl | Secondary amide | Tertiary amide |
| With R-X | Forms 2° amine | Forms 3° amine |
| Solubility in water | Higher | Lower |
| Odor | Fishy (weaker) | More pungent |
| Preparation method | Ammonolysis (excess NH₃) | Alkylation of 1° amine |
🎯 Key Takeaway:
• 1° amines: Better H-bonding = higher BP
• 2° amines: More alkyl groups = stronger base in water
• 1° amines: Better H-bonding = higher BP
• 2° amines: More alkyl groups = stronger base in water