Exhaustive Guide: Amines - Prep & Basics | Class 12 Chemistry

Amines: Structure, Nomenclature & Preparation

Module 1 | CBSE Class 12 Chemistry | Organic Chemistry

Table of Contents

• 1. Introduction to Amines
• 2. Structure & Classification
  • 2.1 Structure of Amines
  • 2.2 Classification (1°, 2°, 3°)
• 3. Nomenclature (Common & IUPAC)
• 4. Methods of Preparation
  • 4.1 Reduction of Nitro Compounds
  • 4.2 Ammonolysis of Alkyl Halides
  • 4.3 Reduction of Nitriles and Amides
  • 4.4 Gabriel Phthalimide Synthesis
  • 4.5 Hoffmann Bromamide Degradation Reaction
• 5. NCERT Solved Examples
• 6. Previous Year Questions (PYQs)

1. Introduction to Amines

Amines constitute an important class of organic compounds derived by replacing one or more hydrogen atoms of ammonia (NH₃) molecules by alkyl/aryl group(s). In nature, they occur among proteins, vitamins, alkaloids, and hormones.

Biological & Medical Significance:
- Adrenaline and ephedrine: Secondary amino groups containing drugs used to increase blood pressure.
- Novocain: A synthetic amino compound used as an anesthetic in dentistry.
- Benadryl: A well-known antihistaminic drug that contains a tertiary amino group.

2. Structure & Classification

2.1 Structure of Amines

Like ammonia, the nitrogen atom of amines is trivalent and carries an unshared pair of electrons (lone pair). Nitrogen orbitals in amines are sp³ hybridized.

• Geometry: Due to the presence of the unshared electron pair, the geometry of amines is pyramidal (not tetrahedral).
• Bond Angle: The bond angle is slightly less than the regular tetrahedral angle of 109.5° due to lone pair-bond pair repulsion. For example, the C-N-C bond angle in trimethylamine is 108°.

2.2 Classification (1°, 2°, 3°)

Crucial Difference from Alcohols/Haloalkanes:
In alcohols and haloalkanes, 1°/2°/3° depends on the carbon atom to which the functional group is attached. In amines, classification depends strictly on the number of hydrogen atoms replaced directly on the Nitrogen atom of ammonia.

• Primary (1°) Amine: One hydrogen of NH₃ replaced by an alkyl/aryl group (R-NH₂).
• Secondary (2°) Amine: Two hydrogens of NH₃ replaced (R-NH-R').
• Tertiary (3°) Amine: All three hydrogens replaced (R₃N).

3. Nomenclature (Common & IUPAC)

Common Names: An aliphatic amine is named by prefixing alkyl group to amine, i.e., alkylamine. Secondary and tertiary amines are named by adding the prefixes 'di' or 'tri'.

IUPAC Names: Primary amines are named as alkanamines. The longest carbon chain is chosen, and the 'e' of alkane is replaced by 'amine'.

Naming 2° and 3° Amines (IUPAC): For secondary and tertiary amines, the longest chain dictates the parent alkanamine. The other alkyl groups are treated as substituents attached to the nitrogen atom and are indicated by the prefix N-.

Structure	Common Name	IUPAC Name
CH3-CH2-NH2	Ethylamine	Ethanamine
CH3-NH-CH2CH3	Ethylmethylamine	N-Methylethanamine
(CH3)3N	Trimethylamine	N,N-Dimethylmethanamine
C6H5-NH2	Aniline	Aniline (or Benzenamine)

4. Methods of Preparation

4.1 Reduction of Nitro Compounds

Nitro compounds are reduced to primary amines by passing hydrogen gas in the presence of finely divided nickel, palladium or platinum, OR by reduction with metals in acidic medium.

R-NO₂ + 3H₂ →(Pd/Ethanol) R-NH₂ + 2H₂O
Ar-NO₂ + Sn / HCl → Ar-NH₂

NCERT Reasoning: Why is Fe + HCl preferred over Sn + HCl?
Reduction with Iron scrap and Hydrochloric acid is preferred because the FeCl₂ formed gets hydrolyzed to release HCl during the reaction. Thus, only a very small amount of HCl is required to initiate the reaction.

4.2 Ammonolysis of Alkyl Halides

When an alkyl or benzyl halide is allowed to react with an ethanolic solution of ammonia, it undergoes a nucleophilic substitution reaction where the halogen atom is replaced by an amino group (-NH₂).

NH₃ + R-X → R-NH₃⁺X^- →(NaOH) R-NH₂ (1° amine)

Disadvantage of Ammonolysis: The primary amine formed behaves as a nucleophile and can further react with the alkyl halide. This leads to the formation of a complex mixture of primary, secondary, tertiary amines, and quaternary ammonium salts.
To maximize the yield of Primary Amine: A large excess of Ammonia is used.

4.3 Reduction of Nitriles and Amides

• Nitriles: Reduction with LiAlH₄ or catalytic hydrogenation (H₂/Ni) produces primary amines. This reaction is highly useful for ascending the amine series (adding one carbon).
  R-C≡N →(LiAlH₄) R-CH₂-NH₂
• Amides: The amides on reduction with LiAlH₄ yield amines having the same number of carbon atoms.
  R-CONH₂ →(LiAlH₄ / H₂O) R-CH₂-NH₂

4.4 Gabriel Phthalimide Synthesis (Highly Tested)

This is a legendary method used exclusively for the preparation of pure Primary (1°) Aliphatic Amines.

Steps:
1. Phthalimide reacts with ethanolic KOH to form the potassium salt of phthalimide.
2. This salt acts as a nucleophile and is heated with an alkyl halide (R-X) to form N-alkylphthalimide via S_N2.
3. Alkaline hydrolysis (with aq. NaOH) of N-alkylphthalimide gives the corresponding primary amine.

Potassium Phthalimide + R-X → N-alkylphthalimide →(aq. NaOH) R-NH₂ (1° Amine) + Sodium phthalate

Board Question: Why can't Aniline be prepared by Gabriel Phthalimide Synthesis?
Aromatic primary amines (like aniline) cannot be prepared by this method because aryl halides (Ar-X) do not undergo nucleophilic substitution with the anion formed by phthalimide under normal conditions (due to partial double bond character of the C-X bond in haloarenes).

4.5 Hoffmann Bromamide Degradation Reaction

Hoffmann developed a method for the preparation of primary amines by treating an amide with bromine (Br₂) in an aqueous or ethanolic solution of sodium hydroxide (NaOH).

The "Step-Down" Reaction: In this degradation reaction, migration of an alkyl or aryl group takes place from the carbonyl carbon to the nitrogen atom. The amine so formed contains one carbon less than the starting amide.

R-CO-NH₂ + Br₂ + 4NaOH → R-NH₂ + Na₂CO₃ + 2NaBr + 2H₂O

5. NCERT Solved Examples (Step-by-Step)

NCERT Example 13.1: Write IUPAC names of the following compounds and classify them into primary, secondary and tertiary amines.
(i) (CH₃)₂CHNH₂
(ii) CH₃(CH₂)₂N(CH₃)₂

Solution:
(i) Structure: A 3-carbon chain with the -NH₂ group on the second carbon.
IUPAC Name: Propan-2-amine.
Classification: Only one hydrogen of NH₃ is replaced, so it is a Primary (1°) amine.

(ii) Structure: The longest chain attached to Nitrogen is 3 carbons (propyl). Two methyl groups are also attached to the Nitrogen.
IUPAC Name: N,N-Dimethylpropan-1-amine.
Classification: All three hydrogens of NH₃ are replaced, so it is a Tertiary (3°) amine.

NCERT Example 13.2: Write chemical equations for the following conversions:
(i) CH₃-CH₂-Cl into CH₃-CH₂-CH₂-NH₂

Solution:
We need to add one carbon atom and convert it to an amine.
Step 1: React alkyl halide with KCN to form a nitrile (adds 1 carbon).
CH₃-CH₂-Cl + KCN (ethanol) → CH₃-CH₂-CN (Propanenitrile) + KCl
Step 2: Reduce the nitrile to a primary amine using LiAlH₄ or H₂/Ni.
CH₃-CH₂-CN →(LiAlH₄) CH₃-CH₂-CH₂-NH₂ (Propan-1-amine).

6. Previous Year Questions (PYQs) & Exhaustive Question Bank

Part A: Conceptual & Reasoning (1-2 Marks)

[CBSE 2018, 2021, NCERT]

Q1. Why cannot aromatic primary amines be prepared by Gabriel phthalimide synthesis?

Answer: The successful step in Gabriel synthesis is the nucleophilic substitution (S_N2) of an alkyl halide by the phthalimide anion. To prepare an aromatic primary amine (like aniline), an aryl halide (like chlorobenzene) must be used. However, aryl halides do not undergo nucleophilic substitution easily due to the partial double bond character of the C-X bond arising from resonance. Hence, the reaction fails.

[CBSE 2017, 2019]

Q2. During the reduction of nitro compounds to amines, why is the reduction with iron scrap and hydrochloric acid preferred over tin and hydrochloric acid?

Answer: When Iron (Fe) reacts with HCl, it forms FeCl₂. This FeCl₂ undergoes hydrolysis with the water produced in the reaction to regenerate HCl continuously. Therefore, only a tiny catalytic amount of HCl is needed to initiate the reaction, making it highly cost-effective and efficient for industrial use compared to Sn/HCl where a stoichiometric amount of acid is required.

Part B: Assertion-Reason Type (1 Mark)

[CBSE Sample Paper 2024]

Q3. Assertion (A): Ammonolysis of alkyl halides is not a suitable method for the preparation of pure primary amines.
Reason (R): Ammonolysis yields a complex mixture of primary, secondary, and tertiary amines along with quaternary ammonium salts.

Answer: Both Assertion and Reason are correct, and Reason is the correct explanation for Assertion. The primary amine formed initially is nucleophilic and continues to react with remaining alkyl halide molecules, leading to over-alkylation and a mixture of products that are difficult to separate.

Part C: Synthesis and Name Reactions (3 Marks)

[CBSE 2015, 2022]

Q4. Write the chemical equations for the following conversions:
(a) Benzamide to Aniline
(b) Ethanoic acid to Methanamine

Answer:
(a) Benzamide to Aniline: This requires stepping down one carbon atom. We use the Hoffmann Bromamide Degradation.
C₆H₅CONH₂ (Benzamide) + Br₂ + 4NaOH → C₆H₅NH₂ (Aniline) + Na₂CO₃ + 2NaBr + 2H₂O.

(b) Ethanoic acid to Methanamine:
Step 1: Convert acid to amide by reacting with ammonia and heating.
CH₃COOH + NH₃ →(Δ) CH₃CONH₂ (Ethanamide) + H₂O.
Step 2: Step down the carbon chain using Hoffmann Bromamide.
CH₃CONH₂ + Br₂ + 4NaOH → CH₃NH₂ (Methanamine) + Na₂CO₃ + 2NaBr + 2H₂O.

© 2026 ChemCA.in - The Premier Digital Resource for Chemistry.
This module is strictly mapped to the latest rationalised NCERT syllabus for Class 12 Chemistry.
Coming up in Module 2: Physical Properties, Basic Character of Amines (pKb trends), and Carbylamine Reaction.

🏠 Home 📝 Quiz 📘 Revision Notes ⬇ Downloads 📚 Class Notes ▶ Video Lectures

Get new posts by email

Powered by

📚 Also Read

Lecture Notes