Class 12 Chemistry Chapter 13 Notes: Amines | Important Questions & PYQs

 Class 12 Chemistry Chapter 13 Notes: Amines Notes

🔹 13.1 Structure of Amines

• Amines are derivatives of ammonia (NH₃)

• One or more hydrogen atoms of NH₃ are replaced by alkyl / aryl groups

• Nitrogen atom is sp³ hybridised

• Shape: Trigonal pyramidal

• Contains one lone pair of electrons → basic nature

---

🔹 13.2 Classification of Amines

Based on number of alkyl/aryl groups attached to N:

1. Primary (1°) amine: R–NH₂

2. Secondary (2°) amine: R–NH–R′

3. Tertiary (3°) amine: R–N(R′)–R″

---

🔹 13.3 Nomenclature of Amines

(A) IUPAC System

• Suffix –amine is used

• Longest carbon chain attached to N is selected

Example:

• CH₃CH₂NH₂ → Ethanamine

• (CH₃)₂NH → Dimethylamine

(B) Common System

• Alkyl groups named first + “amine”

• Example: CH₃NH₂ → Methylamine

---

🔹 13.4 Preparation of Amines (Important Methods)

✔ Reduction Methods

• Nitro compounds → amines (Sn/HCl, Fe/HCl)

• Nitriles → amines (LiAlH₄)

✔ Hoffmann Bromamide Reaction

• Amide → primary amine

• One carbon less

RCONH₂ + Br₂ + KOH → RNH₂ + CO₂

✔ Gabriel Phthalimide Synthesis

• Used to prepare pure primary aliphatic amines

• Not applicable to aromatic amines

---

🔹 13.5 Physical Properties of Amines

✔ Boiling Point

• Primary > Secondary > Tertiary

• Due to H-bonding

✔ Solubility

• Lower amines soluble in water

• Solubility decreases with increase in molecular mass

---

🔹 13.6 Basic Nature of Amines (VERY IMPORTANT)

✔ Reason:

• Due to lone pair of electrons on N

✔ Order of Basic Strength (Aqueous Solution)

Aliphatic amines > NH₃ > Aromatic amines

Reason:

• +I effect of alkyl groups increases basicity

• In aniline, lone pair is delocalised into benzene ring → less basic

✔ pKb Relation

• Lower pKb = stronger base

---

🔹 13.7 Chemical Reactions of Amines

✔ Reaction with Acids

RNH₂ + HCl → RNH₃⁺Cl⁻

✔ Alkylation

• Excess alkyl halide → quaternary ammonium salt

✔ Acylation

• Amine + acetyl chloride → amide

• Used to protect –NH₂ group

---

🔹 13.8 Diazonium Salts

✔ Preparation (Diazotisation)

• Primary aromatic amine + NaNO₂ + HCl

• Temperature: 273–278 K

ArNH₂ → ArN₂⁺Cl⁻

✔ Stability

• Aromatic diazonium salts are stable

• Aliphatic diazonium salts are unstable

---

🔹 13.9 Reactions of Diazonium Salts

✔ Sandmeyer Reaction

• –N₂⁺ replaced by Cl, Br, CN

✔ Coupling Reaction

• Forms azo dyes

• Occurs mainly at para position

---

🔹 13.10 Importance of Diazonium Salts

• Preparation of:

  • Phenols

  • Haloarenes

  • Cyanobenzene

  • Azo dyes

---

🔹 IMPORTANT NAME REACTIONS (PYQ FAVOURITE)

✔ Carbylamine Reaction

• Only 1° amines

• Foul smelling isocyanides formed

✔ Hoffmann Bromamide Reaction

• Amide → primary amine (–1 carbon)

✔ Gabriel Phthalimide Synthesis

• Pure primary aliphatic amines

---

🔹 DISTINGUISHING TESTS (HIGH SCORING)

✔ Hinsberg Test

Amine	Observation
Primary	Soluble in KOH, ppt on acidification
Secondary	Insoluble in KOH
Tertiary	No reaction, soluble after acid

✔ Nitrous Acid Test

Amine Type	Observation
1° aliphatic	N₂ gas evolved
1° aromatic	Diazonium salt
2°	Yellow nitrosoamine
3°	Salt / nitroso derivative

---

🔹 WHY QUESTIONS (Board Favourite)

✔ Why aniline is less basic than methylamine?

• Lone pair delocalised in benzene ring

✔ Why primary amines have higher boiling point than tertiary?

• More H-bonding

✔ Why aromatic amines are weaker bases?

• Resonance stabilisation of lone pair

---

🔥 EXAM HOTSPOTS

✔ Basic strength order
✔ Hinsberg test
✔ Diazotisation reaction
✔ Gabriel synthesis limitation
✔ Name reactions
✔ Reason-based questions

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What are amines?
   Amines are derivatives of ammonia in which one or more hydrogen atoms are replaced by alkyl or aryl groups.

2. What is the hybridisation of nitrogen in amines?
   Nitrogen atom is sp³ hybridised.

3. Why are amines basic in nature?
   Due to the presence of a lone pair of electrons on nitrogen.

4. Classify amines on the basis of degree.
   Primary (1°), Secondary (2°) and Tertiary (3°) amines.

5. Which amines have highest boiling point?
   Primary amines.

6. Why do primary amines have higher boiling point than tertiary amines?
   Due to extensive intermolecular hydrogen bonding.

7. Which reaction converts amide into amine with one carbon less?
   Hoffmann bromamide reaction.

8. Which synthesis gives pure primary aliphatic amines?
   Gabriel phthalimide synthesis.

9. Why are aromatic amines weaker bases than aliphatic amines?
   Because lone pair on nitrogen is delocalised into benzene ring.

10. Which amines give carbylamine reaction?
    Only primary amines.

---

📝 Long Answer Questions

1. Explain classification and nomenclature of amines.

Classification of Amines

Based on number of alkyl/aryl groups attached to nitrogen:

• Primary (1°) amine: R–NH₂

• Secondary (2°) amine: R–NH–R′

• Tertiary (3°) amine: R–N(R′)–R″

Nomenclature

(a) IUPAC system

• Longest carbon chain attached to nitrogen is selected

• Suffix –amine is used

Example:
CH₃CH₂NH₂ → Ethanamine

(b) Common system

• Alkyl groups named first followed by word “amine”

Example:
CH₃NH₂ → Methylamine

---

2. Explain preparation of amines by important methods.

(a) Reduction of nitro compounds

• Nitro compounds reduced to amines using Sn/HCl or Fe/HCl

(b) Reduction of nitriles

• Nitriles reduced to primary amines using LiAlH₄

(c) Hoffmann bromamide reaction

• Converts amide into primary amine with one carbon less

• RCONH₂ + Br₂ + KOH → RNH₂

(d) Gabriel phthalimide synthesis

• Used for preparation of pure primary aliphatic amines

• Not suitable for aromatic amines

---

3. Explain basic nature of amines.

Reason for basicity

• Presence of lone pair of electrons on nitrogen atom.

Order of basic strength (aqueous solution):
Aliphatic amines > NH₃ > Aromatic amines

Explanation

• Alkyl groups show +I effect, increasing electron density on nitrogen.

• In aniline, lone pair is delocalised due to resonance, decreasing basicity.

Relation with pKb

• Lower pKb value indicates stronger base.

---

❓ FAQs (Concept Clarity)

1. Why is aniline less basic than methylamine?
   Because lone pair on nitrogen is delocalised into benzene ring in aniline.

2. Why does Gabriel synthesis not give aromatic amines?
   Because aryl halides do not undergo nucleophilic substitution easily.

3. Why are aliphatic amines soluble in water?
   Due to hydrogen bonding with water molecules.

4. Why are diazonium salts prepared at low temperature?
   Because they decompose at higher temperature.

5. Why are aromatic diazonium salts more stable than aliphatic ones?
   Due to resonance stabilisation of diazonium ion.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

🧪 NCERT Class 12 Chemistry Notes (Updated)
👉 Visit AiradheNotes