Amines: Classification and Nomenclature

Complete guide to amine classification, IUPAC nomenclature, structural features, and naming conventions for JEE Main and Advanced

11 min read

The Hook: From Rotten Fish to Life-Saving Drugs

Connect: Real Life → Chemistry

Ever noticed the pungent smell of decaying fish? That’s trimethylamine, a simple tertiary amine! But not all amines smell bad - many are life-saving medicines:

  • Adrenaline (epinephrine): Emergency hormone, contains 2° amine
  • Dopamine: Neurotransmitter, primary amine structure
  • Antihistamines: Allergy medicines, contain tertiary amines
  • DNA/RNA: Contain nitrogen bases (modified amines)
  • Proteins: Built from amino acids with NH₂ groups

Here’s the JEE question: Why are amines basic in nature? And why does basicity change from primary to tertiary amines?

The Core Concept

What are Amines?

Amines are derivatives of ammonia (NH₃) where one or more hydrogen atoms are replaced by alkyl or aryl groups.

Think of it this way:

  • Ammonia: NH₃
  • Replace one H with R → Primary amine (R-NH₂)
  • Replace two H with R → Secondary amine (R₂NH)
  • Replace all three H with R → Tertiary amine (R₃N)
JEE Weightage
Amines: 3-4 questions in JEE Main, 2-3 in JEE Advanced Focus areas: Classification, nomenclature, basicity comparison, Gabriel synthesis, carbylamine test

Classification of Amines

Based on Number of Alkyl/Aryl Groups

graph TD
    A[Amines] --> B[Primary 1°]
    A --> C[Secondary 2°]
    A --> D[Tertiary 3°]
    A --> E[Quaternary]
    B --> B1[R-NH₂]
    C --> C1[R₂NH or R-NH-R']
    D --> D1[R₃N]
    E --> E1[R₄N⁺X⁻]

1. Primary Amines (1°)

General Formula: R-NH₂ or Ar-NH₂

Structure: One alkyl/aryl group attached to nitrogen

Examples:

  • CH₃-NH₂ (Methylamine)
  • C₂H₅-NH₂ (Ethylamine)
  • C₆H₅-NH₂ (Aniline)

2. Secondary Amines (2°)

General Formula: R₂NH or R-NH-R'

Structure: Two alkyl/aryl groups attached to nitrogen

Examples:

  • (CH₃)₂NH (Dimethylamine)
  • CH₃-NH-C₂H₅ (N-Methylethylamine)
  • (C₆H₅)₂NH (Diphenylamine)

3. Tertiary Amines (3°)

General Formula: R₃N

Structure: Three alkyl/aryl groups attached to nitrogen

Examples:

  • (CH₃)₃N (Trimethylamine) - fishy smell!
  • (C₂H₅)₃N (Triethylamine)
  • C₆H₅-N(CH₃)₂ (N,N-Dimethylaniline)

4. Quaternary Ammonium Salts

General Formula: R₄N⁺X⁻

Structure: Four alkyl/aryl groups, positive charge on nitrogen

Examples:

  • (CH₃)₄N⁺Cl⁻ (Tetramethylammonium chloride)
  • [C₆H₅CH₂-N(CH₃)₃]⁺Br⁻ (Benzyltrimethylammonium bromide)
Critical Distinction for JEE

Quaternary ammonium salts are NOT amines!

  • No lone pair on nitrogen
  • Not basic
  • Ionic compounds
  • Good phase-transfer catalysts

JEE Trap: “Classify (CH₃)₄N⁺Cl⁻” Wrong answer: Tertiary amine Correct answer: Quaternary ammonium salt (not an amine)

Based on Alkyl vs Aryl Groups

Aliphatic Amines

Nitrogen attached to alkyl groups only

Examples:

  • CH₃NH₂ (Methylamine)
  • (C₂H₅)₂NH (Diethylamine)
  • (CH₃)₃N (Trimethylamine)

Aromatic Amines

Nitrogen attached directly to benzene ring

Examples:

  • C₆H₅-NH₂ (Aniline)
  • C₆H₅-NH-CH₃ (N-Methylaniline)
  • (C₆H₅)₂NH (Diphenylamine)
Key Difference

C₆H₅-NH₂ (Aniline) → Aromatic amine (N directly on ring)

C₆H₅-CH₂-NH₂ (Benzylamine) → Aliphatic amine (N on CH₂, not ring)

This difference affects:

  • Basicity (benzylamine > aniline)
  • Reactivity patterns
  • Electrophilic substitution positions

Related: Basicity of Amines

Interactive Demo: Visualize Amine Structures

Explore the different types of amine structures and their classifications.

Based on Simple vs Mixed

Simple Amines

All alkyl/aryl groups are identical

Examples:

  • (CH₃)₂NH - both groups are methyl
  • (C₆H₅)₃N - all three groups are phenyl

Mixed Amines

Different alkyl/aryl groups attached

Examples:

  • CH₃-NH-C₂H₅ (methyl and ethyl)
  • C₆H₅-N(CH₃)₂ (one phenyl, two methyl)

IUPAC Nomenclature of Amines

Primary Amines (Aliphatic)

Method 1: Alkanamine System (Preferred)

Rule: Replace -e of alkane with -amine

StructureAlkaneIUPAC Name
CH₃-NH₂MethaneMethanamine
C₂H₅-NH₂EthaneEthanamine
CH₃-CH₂-CH₂-NH₂PropanePropan-1-amine

Method 2: Alkylamine System (Common)

Rule: Name alkyl group + amine

  • CH₃-NH₂ → Methylamine
  • C₂H₅-NH₂ → Ethylamine
  • (CH₃)₂CH-NH₂ → Isopropylamine

Secondary and Tertiary Amines

Two systems:

System 1: N-substituted derivative

Rule: Largest group = parent, others prefixed with N-

Examples:

Secondary:

  • CH₃-NH-C₂H₅ → N-Methylethanamine

    • Parent: Ethanamine (larger group)
    • Substituent: Methyl on nitrogen (N-methyl)

  • C₆H₅-NH-CH₃ → N-Methylaniline or N-Methylbenzenamine

Tertiary:

  • (CH₃)₂N-C₂H₅ → N,N-Dimethylethanamine

    • Parent: Ethanamine
    • Two methyl groups on nitrogen (N,N-dimethyl)

  • C₆H₅-N(CH₃)₂ → N,N-Dimethylaniline

Memory Trick: When to Use 'N-'

“N for Nitrogen-attached”

Use N- prefix when naming groups attached to nitrogen (not carbon)

  • N-Methylethanamine → Methyl on Nitrogen
  • 2-Methylpropanamine → Methyl on Carbon (no N- prefix)

JEE Tip: Count the N- prefixes to determine if it’s 2° or 3°:

  • One N- → Secondary amine
  • Two N- → Tertiary amine

Complex Amines with Functional Groups

Priority Order (Decreasing):

$$-COOH > -SO₃H > -COOR > -COCl > -CONH₂ > -CHO > C=O > -OH > -NH₂$$

When NH₂ is the principal group:

$$\text{CH₃-CH(NH₂)-COOH}$$

Wrong: 2-Aminopropanoic acid Correct: 2-Aminopropanoic acid ✓ (COOH has priority)

But if COOH is principal: IUPAC: 2-Aminopropanoic acid (amino as prefix) Common: Alanine (amino acid)

Aromatic Amines

Aniline and Derivatives

Parent compound: C₆H₅-NH₂ = Aniline (Benzenamine)

Substituted anilines:

  1. Substituent on benzene ring:

    • Use numbers (ortho = 2, meta = 3, para = 4)

    Examples:

    • 2-Methylaniline or o-Toluidine
    • 4-Nitroaniline or p-Nitroaniline

  2. Substituent on nitrogen:

    • Use N- prefix

    Examples:

    • N-Methylaniline
    • N,N-Diethylaniline

  3. Both positions substituted:

    • Number ring substituents, use N- for nitrogen substituents

    Example:

        CH₃
     |
C₆H₄—NH—CH₃
(para position)

    IUPAC: N,4-Dimethylaniline or N-Methyl-p-toluidine

Common Naming Mistakes

Mistake #1: Forgetting N- prefix

Wrong: Dimethylaniline Correct: N,N-Dimethylaniline

The methyl groups are on nitrogen, not the ring!

Mistake #2: Confusing benzylamine with aniline

C₆H₅-NH₂ = Aniline (aromatic) C₆H₅-CH₂-NH₂ = Benzylamine (aliphatic)

Structure matters for basicity and reactions!

Mistake #3: Wrong numbering in disubstituted anilines

For 4-chloro-2-methylaniline:

  • NH₂ group is always position 1
  • Number to give substituents lowest locants
  • If substituents are on N and ring, ring gets numbers

Structural Features of Amines

Geometry and Hybridization

Nitrogen in amines:

  • Hybridization: sp³
  • Geometry: Pyramidal (not planar!)
  • Bond angle: ~107° (slightly less than tetrahedral 109.5°)
  • Lone pair: One pair on nitrogen
       R
        \
         N—R
        /
      :  (lone pair)

Why pyramidal?

  • 4 electron pairs around N (3 bonds + 1 lone pair)
  • VSEPR theory: Lone pair repulsion > bond repulsion
  • Lone pair compresses bond angle from 109.5° to ~107°
JEE Concept Check

Q: What is the hybridization of nitrogen in trimethylamine?

Solution:

  • Three C-N bonds
  • One lone pair
  • Total: 4 electron pairs
  • Hybridization: sp³
  • Geometry: Pyramidal

JEE Trap: Don’t confuse with sp² (planar) - amines are always sp³!

Special Case: Aniline

Aniline structure shows resonance:

  NH₂             NH₂⁺            NH₂⁺            NH₂⁺
   |               ||              ||              ||
   ⟨⟩    ←→      ⟨⟩⁻    ←→      ⟨⟩⁻    ←→      ⟨⟩⁻

Effects:

  1. Partial double bond character to C-N bond
  2. Electron density decreases on nitrogen
  3. Basicity decreases (lone pair less available)
  4. Benzene ring activation for electrophilic substitution

Related: Basicity comparison

Comparison Table: Ammonia vs Amines

PropertyNH₃1° Amine2° Amine3° Amine
FormulaNH₃RNH₂R₂NHR₃N
H on N3210
Lone pairs1111
Hybridizationsp³sp³sp³sp³
H-bondingStrongStrongModerateWeak*
Boiling point-33°CHigherHigherLower than 1°, 2°
Basicity (gas)ReferenceR₃N > R₂NH > RNH₂ > NH₃
Basicity (aq)ReferenceR₂NH > RNH₂ > R₃N > NH₃

*Tertiary amines cannot form H-bonds with themselves (no N-H), but can accept H-bonds

Quick Identification Tests

Carbylamine Test (Isocyanide Test)

For: Primary amines only

$$\boxed{\text{R-NH}_2 + \text{CHCl}_3 + 3\text{KOH} \xrightarrow{\Delta} \text{R-N≡C} + 3\text{KCl} + 3\text{H}_2\text{O}}$$

Product: Isocyanide (R-N≡C) - foul smell!

Result:

  • 1° amines → Offensive smell (positive test)
  • 2° and 3° amines → No smell (negative test)
Memory Trick: Carbylamine Test

“Primary Pupils Can Smell”

  • Primary amines only
  • Carbylamine test
  • Smell of isocyanide (foul!)

JEE Application: Used to distinguish 1° amines from 2° and 3° amines

Related: Preparation of Amines

Hinsberg’s Test

Reagent: Benzenesulfonyl chloride (C₆H₅SO₂Cl)

Distinguishes: 1°, 2°, and 3° amines

Amine TypeReactionSolubility in KOH
PrimaryForms sulfonamideSoluble (acidic H)
SecondaryForms sulfonamideInsoluble (no acidic H)
TertiaryNo reactionNot applicable

Reactions:

1° Amine:

$$\text{R-NH}_2 + \text{C}_6\text{H}_5\text{SO}_2\text{Cl} \rightarrow \text{C}_6\text{H}_5\text{SO}_2\text{-NH-R} + \text{HCl}$$

Product soluble in KOH (has acidic N-H)

2° Amine:

$$\text{R}_2\text{NH} + \text{C}_6\text{H}_5\text{SO}_2\text{Cl} \rightarrow \text{C}_6\text{H}_5\text{SO}_2\text{-NR}_2 + \text{HCl}$$

Product insoluble in KOH (no acidic H)

3° Amine: No reaction (no N-H to react)

Common Mistakes to Avoid

Mistake #1: Confusing Degree of Amine

Wrong: C₆H₅-NH-CH₃ is a primary amine (has one phenyl group)

Correct: It’s a secondary amine (two groups on nitrogen)

Rule: Count groups attached to nitrogen, not types of groups!

JEE Tip:

  • Primary = one R group on N
  • Secondary = two R groups on N
  • Tertiary = three R groups on N
Mistake #2: Benzylamine vs Aniline Confusion

These are DIFFERENT compounds:

Aniline: C₆H₅-NH₂

  • Aromatic amine
  • N directly on ring
  • Weakly basic
  • Resonance with ring

Benzylamine: C₆H₅-CH₂-NH₂

  • Aliphatic amine
  • N on CH₂ (not ring)
  • Strongly basic
  • No resonance with ring

JEE Consequence: Different basicity order, different reactions!

Mistake #3: Wrong IUPAC Name

For CH₃-NH-C₂H₅:

Wrong names:

  • ❌ Methylethylamine (too ambiguous)
  • ❌ Ethylmethylamine (wrong order)

Correct IUPAC: N-Methylethanamine

  • Parent: Ethanamine (larger group)
  • Substituent: N-Methyl

Rule: Largest alkyl group = parent, others get N- prefix

Practice Problems

Level 1: Foundation (NCERT)

Problem 1: Classification

Q: Classify the following amines as 1°, 2°, or 3°: (a) (CH₃)₂CHNH₂ (b) C₆H₅NHCH₃ (c) (C₂H₅)₃N (d) C₆H₅CH₂NH₂

Solution:

(a) (CH₃)₂CH-NH₂ → Primary (1°)

  • One alkyl group (isopropyl) on nitrogen

(b) C₆H₅-NH-CH₃ → Secondary (2°)

  • Two groups on nitrogen (phenyl + methyl)

(c) (C₂H₅)₃N → Tertiary (3°)

  • Three ethyl groups on nitrogen

(d) C₆H₅-CH₂-NH₂ → Primary (1°)

  • One benzyl group on nitrogen
  • This is aliphatic, not aromatic!

Key Point: Count groups on nitrogen, not carbons in the group!

Problem 2: IUPAC Nomenclature

Q: Give IUPAC names for: (a) CH₃-CH₂-NH₂ (b) (CH₃)₂NH (c) CH₃-NH-C₂H₅

Solution:

(a) Ethanamine or Ethylamine

  • Parent: Ethane
  • Replace -e with -amine

(b) N-Methylmethanamine or Dimethylamine

  • Can use either methyl as parent
  • Other methyl gets N- prefix

(c) N-Methylethanamine

  • Parent: Ethanamine (larger group)
  • Substituent: N-Methyl

Common mistake: Don’t write “Methylethylamine” - ambiguous!

Level 2: JEE Main

Problem 3: Aromatic vs Aliphatic

Q: Which of the following is an aromatic amine? (A) C₆H₅CH₂NH₂ (B) C₆H₅NHCH₃ (C) CH₃NH₂ (D) (C₆H₅CH₂)₂NH

Solution: (B) C₆H₅NHCH₃

Explanation:

  • Aromatic amine = N directly attached to benzene ring
  • (A) is benzylamine - N on CH₂ (aliphatic)
  • (B) is N-methylaniline - N on ring (aromatic) ✓
  • (C) is methylamine - no aromatic ring
  • (D) is dibenzylamine - N on CH₂ (aliphatic)

JEE Trick: Look for N directly bonded to aromatic ring!

Problem 4: Hinsberg Test

Q: An amine forms a product with benzenesulfonyl chloride which is soluble in KOH. The amine is: (A) Primary (B) Secondary (C) Tertiary (D) Quaternary salt

Solution: (A) Primary

Reasoning:

  • Product soluble in KOH → must have acidic H
  • Primary amine: R-NH-SO₂-C₆H₅ (has acidic N-H) ✓
  • Secondary amine: R₂N-SO₂-C₆H₅ (no acidic H, insoluble)
  • Tertiary amine: No reaction

Equation:

$$\text{RNH}_2 + \text{C}_6\text{H}_5\text{SO}_2\text{Cl} \rightarrow \text{RNH-SO}_2\text{-C}_6\text{H}_5$$

The N-H hydrogen is acidic, reacts with KOH → soluble salt

Level 3: JEE Advanced

Problem 5: Complex Nomenclature

Q: Write the IUPAC name for the compound:

     CH₃
      |
  NH₂-C-CH₂-CH₃
      |
     CH₃

Solution:

Structure: (CH₃)₂C(NH₂)-C₂H₅

IUPAC Name: 2-Methylbutan-2-amine

Steps:

  1. Find longest carbon chain containing NH₂: 4 carbons (butane)
  2. Number to give NH₂ lowest position: NH₂ on C-2
  3. Identify substituents: Two methyl groups also on C-2
  4. Combine: 2-Methyl (substituent) + butan-2-amine

Alternative approach: Parent: Butan-2-amine Substituent: 2-Methyl Result: 2-Methylbutan-2-amine

Common name: tert-Butylamine (but IUPAC is preferred for JEE)

Problem 6: Structure Determination

Q: An organic compound with molecular formula C₄H₁₁N does not react with benzenesulfonyl chloride but reacts with methyl iodide to form a quaternary salt. Identify the compound.

Solution:

Analysis:

  • Does not react with C₆H₅SO₂Cl → Tertiary amine
  • Reacts with CH₃I to form quaternary salt → Confirms tertiary

Possible structures for C₄H₁₁N (tertiary):

  1. (CH₃)₃N-CH₃ - Wrong formula
  2. (CH₃)₂N-C₂H₅ → C₄H₁₁N ✓
  3. CH₃-N(C₂H₅)₂ → C₅H₁₃N - Wrong

Answer: N,N-Dimethylethanamine or (CH₃)₂N-C₂H₅

Verification:

$$(\text{CH}_3)_2\text{N-C}_2\text{H}_5 + \text{CH}_3\text{I} \rightarrow [(\text{CH}_3)_3\text{N}^+-\text{C}_2\text{H}_5]\text{I}^-$$

Quaternary ammonium salt formed ✓

Quick Revision Box

TopicKey PointsJEE Formula/Rule
Primary (1°)One R on NR-NH₂
Secondary (2°)Two R on NR₂NH or R-NH-R'
Tertiary (3°)Three R on NR₃N
QuaternaryFour R on N⁺R₄N⁺X⁻ (not amine!)
IUPAC NamingLargest group = parentN- prefix for substituents on N
AromaticN directly on ringAniline (C₆H₅-NH₂)
AliphaticN not on ringBenzylamine (C₆H₅-CH₂-NH₂)
Carbylamine1° amines onlyOffensive smell
HinsbergDistinguishes 1°, 2°, 3°1° soluble in KOH
Hybridizationsp³, pyramidal~107° bond angle

When to Use What: Decision Tree

Need to identify amine type?
│
├─ Count groups on nitrogen
│  ├─ One group → Primary (1°)
│  ├─ Two groups → Secondary (2°)
│  ├─ Three groups → Tertiary (3°)
│  └─ Four groups → Quaternary salt (not amine)
│
├─ For IUPAC naming
│  ├─ Primary → Alkanamine
│  ├─ Secondary/Tertiary → N-substituted derivative
│  └─ Multiple N-substituents → N,N-disubstituted
│
└─ To distinguish experimentally
   ├─ Carbylamine test → Only 1° gives smell
   └─ Hinsberg test → 1° soluble, 2° insoluble, 3° no reaction

Connection to Other Topics

Prerequisites:

Related Topics:

Applications:

Summary

Key Takeaways

1. Classification System:

  • Primary (1°): R-NH₂ (one group on N)
  • Secondary (2°): R₂NH (two groups on N)
  • Tertiary (3°): R₃N (three groups on N)
  • Quaternary: R₄N⁺X⁻ (NOT an amine - ionic)

2. Aromatic vs Aliphatic:

  • Aniline (C₆H₅-NH₂): N on ring → aromatic
  • Benzylamine (C₆H₅-CH₂-NH₂): N on CH₂ → aliphatic
  • This affects basicity significantly!

3. IUPAC Nomenclature:

  • Primary: Alkanamine (e.g., ethanamine)
  • Secondary/Tertiary: Largest = parent, others N-substituted
  • Use N- prefix for groups on nitrogen

4. Identification Tests:

  • Carbylamine: Only 1° amines give foul smell
  • Hinsberg: 1° soluble in KOH, 2° insoluble, 3° no reaction

5. Structure:

  • All amines: sp³ hybridization, pyramidal
  • One lone pair on nitrogen (basis of basicity)
  • Bond angle ~107° (less than tetrahedral)

“Amines are ammonia’s organic relatives - classification depends on how many hydrogens were replaced!”

Next, explore preparation methods to understand Gabriel synthesis and reduction reactions!