Chemistry Organic: How to Solve: Amines (Basicity, Hofmann Degradation, Carbylamine Test, Azo-Dye Test) – IIT JEE Guide

How to Solve: Amines (Basicity, Hofmann Degradation, Carbylamine Test, Azo-Dye Test) – IIT JEE Guide

Introduction

Mastering amines unlocks 5-7 marks in IIT JEE—enough to push you from a 90 to a 99+ percentile. These reactions appear in organic synthesis, mechanism-based questions, and qualitative analysis, making them a high-yield topic for both Main and Advanced.

WHAT YOU NEED TO KNOW FIRST

1. Electron-donating vs. electron-withdrawing groups (EDG vs. EWG) – Affects basicity and reactivity.
2. Nucleophilicity vs. basicity – Amines act as both; know the difference.
3. Reaction mechanisms (SN2, elimination, diazotization) – Hofmann degradation and carbylamine test rely on these.

KEY TERMS & FORMULAS

1. Basicity of Amines

• Formula: ( K_b = \frac{[RNH_3^+][OH^-]}{[RNH_2]} )
• ( K_b ) = Basicity constant (higher ( K_b ) = stronger base)
• MEMORISE THIS: Order of basicity in gas phase: ( 3° > 2° > 1° > NH_3 )
• In aqueous solution: ( 2° > 1° > 3° > NH_3 ) (due to solvation effects)

2. Hofmann Degradation

• Reaction: ( RCONH_2 \xrightarrow{Br_2, NaOH} RNH_2 ) (amide → amine with one less carbon)
• Key Step: Exhaustive methylation followed by elimination (anti-Zaitsev product).
• MEMORISE THIS: Hofmann elimination gives the less substituted alkene.

3. Carbylamine Test (Isocyanide Test)

• Reaction: ( RNH_2 + CHCl_3 + 3KOH \rightarrow RNC + 3KCl + 3H_2O )
• Positive test: Foul-smelling isocyanide (RNC) formed.
• MEMORISE THIS: Only 1° amines give this test (2° and 3° do not).

4. Azo-Dye Test (Diazotization & Coupling)

• Step 1 (Diazotization): ( ArNH_2 + NaNO_2 + 2HCl \xrightarrow{0-5°C} ArN_2^+Cl^- + NaCl + 2H_2O )
• Step 2 (Coupling): ( ArN_2^+Cl^- + Ar'OH \rightarrow ArN=N-Ar'OH ) (colored azo dye)
• MEMORISE THIS: Only aromatic 1° amines undergo diazotization.

STEP-BY-STEP METHOD

Step 1: Identify the Type of Amine

• 1° (Primary): ( RNH_2 ) (e.g., ( CH_3NH_2 ))
• 2° (Secondary): ( R_2NH ) (e.g., ( (CH_3)_2NH ))
• 3° (Tertiary): ( R_3N ) (e.g., ( (CH_3)_3N ))
• Aromatic vs. Aliphatic: Check if N is directly attached to benzene ring.

Step 2: Predict Basicity

• Aliphatic amines: ( 2° > 1° > 3° > NH_3 ) (in water)
• Aromatic amines: Much less basic than aliphatic (lone pair delocalized into ring).
• EDG (e.g., -CH₃) increases basicity, EWG (e.g., -NO₂) decreases it.

Step 3: Apply Hofmann Degradation (If Given an Amide)

1. Exhaustive methylation: Treat amide with ( CH_3I ) (excess) + ( Ag_2O ).
2. Elimination: Heat → forms less substituted alkene + amine.
3. Final product: Amine with one less carbon than starting amide.

Step 4: Carbylamine Test (For 1° Amines)

1. Mix amine with CHCl₃ + KOH (alc.).
2. Heat gently → foul-smelling isocyanide (RNC) confirms 1° amine.
3. 2° and 3° amines do NOT give this test.

Step 5: Azo-Dye Test (For Aromatic 1° Amines)

1. Diazotization: Cool amine + ( NaNO_2 + HCl ) (0-5°C).
2. Coupling: Add phenol/napthol → colored azo dye (orange/red).
3. Only aromatic 1° amines work (aliphatic amines give unstable diazonium salts).

WORKED EXAMPLES

Example 1 – Basic (Basicity Comparison)

Question: Arrange in increasing order of basicity: ( NH_3, CH_3NH_2, (CH_3)_2NH, (CH_3)_3N )

Solution:
1. Identify amine types: All aliphatic.
2. Apply basicity order (aqueous): ( NH_3 < (CH_3)_3N < CH_3NH_2 < (CH_3)_2NH )
3. Why? Solvation effects favor 2° > 1° > 3° in water.

Answer: ( NH_3 < (CH_3)_3N < CH_3NH_2 < (CH_3)_2NH )

Example 2 – Medium (Hofmann Degradation)

Question: Predict the product when propanamide undergoes Hofmann degradation.

Solution:
1. Starting amide: ( CH_3CH_2CONH_2 )
2. Exhaustive methylation: Forms quaternary ammonium salt.
3. Elimination: Gives ethene (less substituted alkene) + ( CH_3NH_2 ).
4. Final product: Methylamine (CH₃NH₂) (one less carbon than starting amide).

Answer: ( CH_3NH_2 )

Example 3 – Exam-Style (Carbylamine Test)

Question: Which of the following will give a positive carbylamine test? (A) ( CH_3NHCH_3 ) (B) ( C_6H_5NH_2 ) (C) ( (CH_3)_3N ) (D) ( CH_3CH_2NH_2 )

Solution:
1. Carbylamine test works only for 1° amines.
2. Check options: - (A) 2° amine → No - (B) Aromatic 1° amine → Yes (but test is for aliphatic 1° amines) - (C) 3° amine → No - (D) Aliphatic 1° amine → Yes
3. Trick: Aromatic 1° amines do not give carbylamine test (only aliphatic 1° amines do).

Answer: (D) ( CH_3CH_2NH_2 )

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP (Night Before Exam)

"Amines are 5-7 marks in JEE, so nail this in 60 seconds. First, basicity: in water, 2° > 1° > 3° > NH₃. Aromatic amines? Weak bases—lone pair on N is delocalized. Hofmann degradation: amide → amine with one less carbon, and it gives the less substituted alkene. Carbylamine test: only 1° amines (aliphatic) give foul-smelling isocyanide. Azo-dye test: only aromatic 1° amines couple with phenols to give colored dyes. Traps? Hofmann ≠ Curtius, carbylamine ≠ aromatic amines, and always check phase (gas vs. aqueous) for basicity. Now go crush those questions!