The Hook: The Colorful World of Azo Dyes
Look around you right now - your clothes, food packaging, cosmetics, even your pen ink likely contain azo dyes!
Mind-blowing statistics:
- 60-70% of all synthetic dyes are azo compounds
- Methyl Orange (pH indicator): Azo dye you use in chemistry lab
- Sunset Yellow (E110): Food coloring in soft drinks, candies
- Tartrazine (E102): Yellow food dye in chips, beverages
- Congo Red: Biological stain for detecting amyloid proteins
Historical importance:
- 1856: First synthetic dye (Mauveine) discovered by William Perkin
- 1884: Paul Griess discovered azo coupling
- Revolutionary: Freed textile industry from natural dyes (expensive, rare)
The chemistry: All azo dyes contain the -N=N- (azo) linkage connecting aromatic rings, formed through coupling reactions of diazonium salts!
JEE Question: Why do azo compounds show intense colors? How does pH affect their color?
The Core Concept
What is Azo Coupling?
Azo coupling is an electrophilic aromatic substitution reaction where a diazonium salt (electrophile) attacks an activated aromatic compound (nucleophile).
General Reaction:
$$\boxed{[\text{Ar-N}_2^+]\text{X}^- + \text{Ar'-H} \rightarrow \text{Ar-N=N-Ar'} + \text{HX}}$$Product: Azo compound (contains -N=N- group)
Key requirement: Ar’-H must be highly activated (electron-rich)
Coupling Partners
What Couples with Diazonium Salts?
Requirement: Aromatic ring must be highly activated (electron-rich)
Suitable coupling partners:
1. Phenols and Naphthols
Why reactive?
- -OH group is strongly activating (+R effect)
- Increases electron density on ring (especially ortho/para)
- Ring is electron-rich enough for electrophilic attack
Common partners:
- Phenol (C₆H₅OH)
- β-Naphthol (2-naphthol)
- α-Naphthol (1-naphthol)
- Substituted phenols
Conditions: Alkaline medium (pH 8-10)
Why alkaline?
- Phenol exists as phenoxide ion (C₆H₅O⁻)
- Phenoxide is MORE activated than phenol
- Better nucleophile
2. Aromatic Amines
Why reactive?
- -NH₂ or -NR₂ groups are strongly activating (+R effect)
- Lone pair delocalizes into ring
- Increases electron density
Common partners:
- Aniline (C₆H₅NH₂)
- N,N-Dimethylaniline (C₆H₅N(CH₃)₂)
- N-Methylaniline
- Naphthylamines
Conditions: Weakly acidic medium (pH 4-5)
Why acidic?
- In strong acid: -NH₂ becomes -NH₃⁺ (deactivating)
- In strong base: Diazonium salt decomposes
- Weak acid: Balance between activation and stability
For phenols: pH 8-10 (alkaline)
- Forms phenoxide ion (C₆H₅O⁻)
- More activated, better coupling
For amines: pH 4-5 (weakly acidic)
- Keeps -NH₂ free (not protonated)
- Maintains diazonium salt stability
Wrong pH consequences:
Phenol in acidic medium:
- No phenoxide ion
- Less activated
- Poor/no coupling
Amine in alkaline medium:
- Diazonium salt decomposes to phenol
- No coupling
JEE Memory: “Phenols are Proud (alkaline pH)” “Amines are Acidic (acidic pH)”
Interactive Demo: Visualize Coupling Reactions
See how diazonium salts couple with phenols and amines to form azo dyes.
Mechanism of Azo Coupling
Step-by-Step Mechanism
Example: Benzenediazonium chloride + Phenol (in NaOH)
Step 1: Formation of phenoxide ion
$$\text{C}_6\text{H}_5\text{OH} + \text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{O}^-\text{Na}^+ + \text{H}_2\text{O}$$Step 2: Electrophilic attack by diazonium ion
N≡N⁺
|
⊕ ← attacks para position
/ \
HO-⟨⟩-O⁻
Diazonium cation (N₂⁺) acts as electrophile, attacks electron-rich ortho/para position
Step 3: Formation of σ-complex (arenium ion)
O⁻ N≡N⁺
| |
⟨⟩⁺ ← sigma complex
|
H
Step 4: Deprotonation (rearomatization)
O⁻ O⁻
| |
⟨⟩-N=N-⟨⟩ + H⁺
Final product: p-Hydroxyazobenzene (yellow-orange dye)
$$\text{C}_6\text{H}_5\text{-N=N-C}_6\text{H}_4\text{-OH}$$Q: Why does coupling occur predominantly at para position?
Answer:
Para position is favored:
- Steric factors: Less crowded than ortho
- Electronic factors: Both ortho and para are activated, but para is more accessible
For phenoxide ion:
- Major product: p-Hydroxyazobenzene (para coupling)
- Minor product: o-Hydroxyazobenzene (ortho coupling, some steric hindrance)
For naphthols:
- β-Naphthol: Couples at α-position (position 1, adjacent to OH)
- More complex regioselectivity
JEE Note: Unless specified, assume para coupling for phenol derivatives
Important Azo Dyes and Their Preparation
1. Methyl Orange (Acid-Base Indicator)
Structure:
(CH₃)₂N-⟨⟩-N=N-⟨⟩-SO₃H
Full name: 4-Dimethylaminoazobenzene-4’-sulfonic acid
Preparation:
Step 1: Diazotization of sulfanilic acid
$$\text{H}_2\text{N-C}_6\text{H}_4\text{-SO}_3\text{H} \xrightarrow{\text{NaNO}_2/\text{HCl}, 0-5°\text{C}} [\text{⁺N}_2\text{-C}_6\text{H}_4\text{-SO}_3^-]$$(Sulfanilic acid exists as zwitterion)
Step 2: Coupling with N,N-dimethylaniline
$$[\text{⁺N}_2\text{-C}_6\text{H}_4\text{-SO}_3^-] + (\text{CH}_3)_2\text{N-C}_6\text{H}_5 \xrightarrow{\text{pH 4-5}}$$ $$(\text{CH}_3)_2\text{N-C}_6\text{H}_4\text{-N=N-C}_6\text{H}_4\text{-SO}_3\text{H}$$Properties:
- In acid (pH < 3.1): Red (protonated form)
- In base (pH > 4.4): Yellow (deprotonated form)
- Transition: pH 3.1-4.4 (orange)
Use: Acid-base indicator for titrations
Color change:
$$\text{Red (acidic)} \rightleftarrows \text{Orange} \rightleftarrows \text{Yellow (basic)}$$In acidic medium (pH < 3.1):
The -N=N- group gets protonated (or -N(CH₃)₂ gets protonated):
$$(\text{CH}_3)_2\text{N-C}_6\text{H}_4\text{-N=N-C}_6\text{H}_4\text{-SO}_3\text{H} \xrightarrow{+\text{H}^+}$$ $$(\text{CH}_3)_2\text{NH}^+-\text{C}_6\text{H}_4\text{-N=N-C}_6\text{H}_4\text{-SO}_3\text{H}$$Protonated form: RED (extended conjugation, lower energy gap)
In basic medium (pH > 4.4):
Unprotonated form:
$$(\text{CH}_3)_2\text{N-C}_6\text{H}_4\text{-N=N-C}_6\text{H}_4\text{-SO}_3^-$$Deprotonated form: YELLOW (different conjugation, higher energy gap)
JEE Key: Color depends on extent of conjugation and electron distribution!
Related: Acid-base indicators
2. Orange II (Food Dye, E111)
Structure:
HO-⟨⟩-N=N-⟨⟩-SO₃Na
Preparation:
Step 1: Diazotization
$$\text{H}_2\text{N-C}_6\text{H}_4\text{-SO}_3\text{Na} \xrightarrow{\text{NaNO}_2/\text{HCl}} [\text{⁺N}_2\text{-C}_6\text{H}_4\text{-SO}_3\text{Na}]$$Step 2: Coupling with β-naphthol
$$[\text{⁺N}_2\text{-C}_6\text{H}_4\text{-SO}_3\text{Na}] + \text{β-naphthol} \xrightarrow{\text{NaOH}}$$ $$\text{HO-naphthyl-N=N-C}_6\text{H}_4\text{-SO}_3\text{Na}$$Color: Orange-red
Use: Textile dye, food coloring (now banned in many countries)
3. Congo Red (Biological Stain)
Structure: Contains two azo groups (-N=N-)
NH₂-⟨⟩-N=N-⟨naphthyl⟩-N=N-⟨⟩-NH₂
(with sulfonic acid groups)
Preparation:
- Requires benzidine (diamine)
- Two-step diazotization and coupling
- Complex synthesis
Properties:
- In acid: Blue-violet
- In base: Red
- Transition pH: 3.0-5.0
Uses:
- pH indicator
- Biological stain for amyloid proteins
- Detects cellulose fibers
4. Aniline Yellow (Para-aminoazobenzene)
Structure:
H₂N-⟨⟩-N=N-⟨⟩
Preparation:
Step 1: Diazotization of aniline
$$\text{C}_6\text{H}_5\text{NH}_2 \xrightarrow{\text{NaNO}_2/\text{HCl}} [\text{C}_6\text{H}_5\text{N}_2^+]\text{Cl}^-$$Step 2: Coupling with aniline (in weakly acidic medium)
$$[\text{C}_6\text{H}_5\text{N}_2^+]\text{Cl}^- + \text{C}_6\text{H}_5\text{NH}_2 \xrightarrow{\text{pH 4-5}} \text{C}_6\text{H}_5\text{-N=N-C}_6\text{H}_4\text{-NH}_2$$Color: Bright yellow
Note: This is an amine-amine coupling (both rings have NH₂)
Why Are Azo Compounds Colored?
The Science of Color
Color in azo compounds comes from:
1. Extended Conjugation
Azo group (-N=N-) extends conjugation:
⟨⟩-N=N-⟨⟩
Electrons delocalized over:
- Benzene ring 1
- Azo group (N=N)
- Benzene ring 2
Extended π-system allows lower energy transitions
2. Chromophore and Auxochrome
Chromophore: The azo group (-N=N-)
- Responsible for light absorption
- Creates basic color
Auxochrome: Substituents like -OH, -NH₂, -SO₃H
- Modify/intensify color
- Shift absorption wavelength
Example:
- Azobenzene (C₆H₅-N=N-C₆H₅): Yellow (λmax ≈ 440 nm)
- p-Hydroxyazobenzene: Orange-red (λmax shifted, deeper color)
- p-Aminoazobenzene: Yellow-orange (λmax modified)
3. Light Absorption
Visible light range: 400-700 nm
Azo compounds absorb:
- π → π* transitions
- n → π* transitions (from N lone pairs)
Wavelength absorbed determines color seen:
| Color Absorbed | Color Seen (Complementary) |
|---|---|
| Violet (400nm) | Yellow-green |
| Blue (480nm) | Yellow |
| Green (530nm) | Red |
| Yellow (580nm) | Blue |
| Orange (610nm) | Blue-green |
| Red (680nm) | Green |
Most azo dyes absorb in blue-green region → appear red/orange/yellow
Q: How does adding -OH group to azobenzene change its color?
Answer:
Azobenzene: C₆H₅-N=N-C₆H₅ (Yellow, λmax ≈ 440 nm)
p-Hydroxyazobenzene: HO-C₆H₄-N=N-C₆H₅ (Orange-red, λmax ≈ 480 nm)
Reason:
1. Extended conjugation:
- -OH group has +R effect
- Lone pair on oxygen delocalizes into ring
- Further extends π-conjugation
- Lower energy gap (HOMO-LUMO)
2. Bathochromic shift (red shift):
- Absorption shifts to longer wavelength
- Blue light absorbed instead of violet
- Complementary color changes: yellow → orange/red
3. Intensified color:
- -OH is auxochrome
- Increases molar absorptivity (ε)
- Deeper, more intense color
JEE Key: More conjugation = Longer wavelength absorption = Deeper color
Related: Conjugation and aromaticity
Conditions for Successful Coupling
Essential Requirements
1. Activated aromatic ring (coupling partner)
- Must have strong EDG (-OH, -NH₂, -NR₂)
- Electron density high enough for electrophilic attack
2. Correct pH
- Phenols: pH 8-10 (alkaline)
- Amines: pH 4-5 (weakly acidic)
3. Temperature
- Usually 0-5°C (like diazonium salt formation)
- Low temperature maintains diazonium salt stability
4. Freshly prepared diazonium salt
- Never store
- Use immediately after preparation
- Decomposes on warming
What Does NOT Couple?
❌ Benzene: Not activated enough
- No EDG
- Too low electron density
- No coupling occurs
❌ Nitrobenzene: Deactivated ring
- -NO₂ is EWG
- Decreases electron density
- No coupling
❌ Chlorobenzene: Weakly deactivated
- -Cl has weak -I effect
- Not activated enough
❌ Toluene: Only weakly activated
- -CH₃ has weak +I effect
- Usually insufficient for coupling
✓ Must have strong EDG: -OH, -OR, -NH₂, -NHR, -NR₂
Test for Azo Dye Formation
Laboratory Test
Procedure:
Step 1: Prepare diazonium salt
$$\text{Aniline} + \text{NaNO}_2 + \text{HCl} \xrightarrow{0-5°\text{C}} \text{Benzenediazonium chloride}$$Step 2: Add coupling partner
- For phenol: Add NaOH, then add phenol
- For amine: Adjust pH to 4-5, add amine
Step 3: Observe
- Immediate appearance of bright color (orange, red, yellow)
- Indicates azo dye formation
Example:
Benzenediazonium chloride + β-naphthol (in NaOH):
Observation: Brilliant orange-red color appears immediately
Product: 1-(Phenylazo)-2-naphthol (Orange II precursor)
Typical question: “A colorless solution A is treated with NaNO₂ and HCl at 0-5°C. The resulting solution is added to an alkaline solution of β-naphthol, and a brilliant orange color appears. Identify A.”
Solution:
Analysis:
- Forms diazonium salt (NaNO₂/HCl, 0-5°C) → Must be aromatic primary amine
- Couples with β-naphthol in alkaline medium → Confirms diazonium salt
- Orange color appears → Azo dye formed
Compound A: Aniline (C₆H₅NH₂) or substituted aniline
Reactions:
$$\text{A} \xrightarrow{\text{NaNO}_2/\text{HCl}} [\text{Ar-N}_2^+]\text{Cl}^-$$ $$[\text{Ar-N}_2^+]\text{Cl}^- + \text{β-naphthol} \xrightarrow{\text{NaOH}} \text{Orange azo dye}$$This is a classic identification test for aromatic primary amines!
Related: Properties of amines
Applications of Azo Compounds
1. Textile Dyes
Direct dyes:
- Azo compounds with -SO₃H groups
- Water-soluble
- Directly dye cotton, wool
Examples:
- Congo Red
- Direct Blue
- Chrysophenine
Process:
- Dissolve dye in water
- Immerse fabric
- Dye molecules adsorb onto fibers
- Fix with salt or heat
2. Food Colorings
Common food azo dyes:
| Dye | E Number | Color | Use |
|---|---|---|---|
| Tartrazine | E102 | Yellow | Soft drinks, chips |
| Sunset Yellow | E110 | Orange | Desserts, candies |
| Allura Red | E129 | Red | Beverages, sweets |
| Amaranth | E123 | Red | (Banned in USA) |
Health concerns:
- Some linked to hyperactivity in children
- Regulations vary by country
- Many being replaced with natural dyes
3. pH Indicators
Azo dyes that change color with pH:
Methyl Orange:
- Red (pH < 3.1) ↔ Yellow (pH > 4.4)
- Used in acid-base titrations
Methyl Red:
- Red (pH < 4.4) ↔ Yellow (pH > 6.2)
- More sensitive range than methyl orange
Congo Red:
- Blue (pH < 3.0) ↔ Red (pH > 5.0)
- Used in biological staining
4. Biological Stains
Congo Red:
- Stains amyloid proteins (disease marker)
- Used in Alzheimer’s research
Sudan dyes:
- Stain lipids (fats)
- Used in histology
5. Industrial Applications
Paper industry:
- Colored papers, cardboard
Leather industry:
- Dye leather goods
Cosmetics:
- Hair dyes
- Makeup products
Printing inks:
- Newspaper, magazines
Common Mistakes to Avoid
Wrong: “Couple phenol with diazonium salt in acidic medium”
Correct: “Phenol requires ALKALINE medium (pH 8-10)”
Reason:
- Phenol (C₆H₅OH) is weak acid (pKa ≈ 10)
- In base: Forms phenoxide (C₆H₅O⁻)
- Phenoxide is much more activated (better coupling)
Wrong: “Couple aniline with diazonium salt in alkaline medium”
Correct: “Aniline requires WEAKLY ACIDIC medium (pH 4-5)”
Reason:
- Strong base: Diazonium salt decomposes
- Strong acid: Aniline protonated to -NH₃⁺ (deactivated)
- Weak acid: Balance between both
JEE Memory: Phenols = alkaline, Amines = acidic
Wrong: “Benzene will couple with benzenediazonium chloride”
Correct: “Benzene is NOT activated enough to undergo coupling”
Requirements for coupling:
- Must have strong EDG (-OH, -NH₂, -NR₂)
- Benzene has no activating groups
- Electrophilic substitution requires very strong electrophile (diazonium ion is moderate)
Only highly activated rings couple: ✓ Phenol, naphthols ✓ Aniline, N,N-dimethylaniline ✓ Substituted phenols/amines with EDG
✗ Benzene ✗ Toluene (too weakly activated) ✗ Chlorobenzene (deactivated) ✗ Nitrobenzene (strongly deactivated)
Wrong: “Warming speeds up coupling reaction”
Correct: “Keep at 0-5°C to prevent diazonium salt decomposition”
If temperature rises:
$$[\text{Ar-N}_2^+]\text{Cl}^- \xrightarrow{>10°\text{C}} \text{Ar-OH} + \text{N}_2↑$$Diazonium salt decomposes to phenol!
JEE Note: Coupling must be done immediately after diazotization, keeping cold throughout
Practice Problems
Level 1: Foundation (NCERT)
Q: What happens when benzenediazonium chloride is treated with phenol in NaOH solution?
Solution:
Reaction:
Step 1: Formation of phenoxide
$$\text{C}_6\text{H}_5\text{OH} + \text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{O}^-\text{Na}^+ + \text{H}_2\text{O}$$Step 2: Coupling
$$[\text{C}_6\text{H}_5\text{N}_2^+]\text{Cl}^- + \text{C}_6\text{H}_5\text{O}^-\text{Na}^+ \rightarrow$$ $$\text{C}_6\text{H}_5\text{-N=N-C}_6\text{H}_4\text{-OH} + \text{NaCl}$$Product: p-Hydroxyazobenzene (orange-yellow azo dye)
Structure:
⟨⟩-N=N-⟨⟩-OH
Observation: Orange-yellow color appears
Conditions:
- Alkaline medium (NaOH)
- Temperature: 0-5°C
- Coupling at para position (major product)
Q: Write the preparation of methyl orange from sulfanilic acid.
Solution:
Step 1: Diazotization of sulfanilic acid
Sulfanilic acid exists as zwitterion: ⁺H₃N-C₆H₄-SO₃⁻
$$\text{H}_2\text{N-C}_6\text{H}_4\text{-SO}_3\text{H} + \text{NaNO}_2 + 2\text{HCl} \xrightarrow{0-5°\text{C}}$$ $$[\text{Cl-}^+\text{N}_2\text{-C}_6\text{H}_4\text{-SO}_3\text{H}]$$(Sulfanilic acid diazonium chloride)
Step 2: Coupling with N,N-dimethylaniline
$$[\text{⁺N}_2\text{-C}_6\text{H}_4\text{-SO}_3\text{H}] + (\text{CH}_3)_2\text{N-C}_6\text{H}_5 \xrightarrow{\text{pH 4-5}}$$ $$(\text{CH}_3)_2\text{N-C}_6\text{H}_4\text{-N=N-C}_6\text{H}_4\text{-SO}_3\text{H}$$Product: Methyl orange (orange dye, used as indicator)
Conditions:
- Step 1: 0-5°C, acidic
- Step 2: pH 4-5 (weakly acidic for amine coupling)
Properties:
- Red in acid (pH < 3.1)
- Yellow in base (pH > 4.4)
Level 2: JEE Main
Q: Which of the following will couple with benzenediazonium chloride?
(A) Benzene (B) Nitrobenzene (C) Phenol (in NaOH) (D) Toluene
Solution: (C) Phenol (in NaOH)
Analysis:
(A) Benzene:
- No activating group
- Not electron-rich enough
- No coupling ✗
(B) Nitrobenzene:
- -NO₂ is strong EWG (deactivating)
- Ring is electron-deficient
- No coupling ✗
(C) Phenol (in NaOH):
- Forms phenoxide ion (C₆H₅O⁻)
- -O⁻ is very strong activating group
- Highly electron-rich ring
- Coupling occurs ✓
(D) Toluene:
- -CH₃ is weak activating group
- Not activated enough for coupling with diazonium salt
- No coupling ✗
Correct answer: (C)
Requirement: Must have strong EDG (-OH, -NH₂, -NR₂) for coupling
Q: Explain why coupling of benzenediazonium chloride with: (a) Phenol is done in alkaline medium (b) Aniline is done in weakly acidic medium
Solution:
(a) Phenol in alkaline medium (pH 8-10):
Reason:
In alkaline medium:
$$\text{C}_6\text{H}_5\text{OH} + \text{OH}^- \rightarrow \text{C}_6\text{H}_5\text{O}^- + \text{H}_2\text{O}$$Phenoxide ion (C₆H₅O⁻) formed:
- -O⁻ is much stronger activating group than -OH
- Increases electron density on ring significantly
- Better nucleophile for electrophilic substitution
- Faster, more efficient coupling
In acidic medium:
- Phenol remains as -OH (weaker activator)
- Coupling is slow or doesn’t occur
(b) Aniline in weakly acidic medium (pH 4-5):
Reason:
Balance required:
If too acidic (pH < 3):
$$\text{C}_6\text{H}_5\text{NH}_2 + \text{H}^+ \rightarrow \text{C}_6\text{H}_5\text{NH}_3^+$$Anilinium ion (-NH₃⁺) is deactivating → No coupling
If alkaline (pH > 7):
$$[\text{C}_6\text{H}_5\text{N}_2^+]\text{Cl}^- + \text{OH}^- \rightarrow \text{C}_6\text{H}_5\text{OH} + \text{N}_2↑$$Diazonium salt decomposes → No coupling
At pH 4-5 (weakly acidic):
- Aniline remains as -NH₂ (activated)
- Diazonium salt remains stable
- Both coupling partners available
- Successful coupling occurs
JEE Key: pH must be optimized for both partners!
Level 3: JEE Advanced
Q: Starting from benzene, outline the preparation of Orange II dye (structure: HO-naphthyl-N=N-C₆H₄-SO₃Na).
Solution:
Retrosynthetic analysis: Orange II ← β-Naphthol + Sulfanilic acid diazonium salt
Route 1: Prepare sulfanilic acid diazonium salt
Step 1: Benzene → Nitrobenzene
$$\text{C}_6\text{H}_6 \xrightarrow{\text{HNO}_3/\text{H}_2\text{SO}_4} \text{C}_6\text{H}_5\text{NO}_2$$Step 2: Nitrobenzene → Aniline
$$\text{C}_6\text{H}_5\text{NO}_2 \xrightarrow{\text{Sn/HCl}} \text{C}_6\text{H}_5\text{NH}_2$$Step 3: Aniline → Sulfanilic acid
$$\text{C}_6\text{H}_5\text{NH}_2 \xrightarrow{\text{conc. H}_2\text{SO}_4, 180°\text{C}} \text{H}_2\text{N-C}_6\text{H}_4\text{-SO}_3\text{H}$$(Sulfonation)
Step 4: Diazotization
$$\text{H}_2\text{N-C}_6\text{H}_4\text{-SO}_3\text{H} \xrightarrow{\text{NaNO}_2/\text{HCl}, 0-5°\text{C}} [\text{⁺N}_2\text{-C}_6\text{H}_4\text{-SO}_3^-]$$Route 2: β-Naphthol (given or prepared from naphthalene)
Step 5: Coupling
$$[\text{⁺N}_2\text{-C}_6\text{H}_4\text{-SO}_3\text{Na}] + \text{β-naphthol} \xrightarrow{\text{NaOH, 0-5°\text{C}}}$$ $$\text{HO-naphthyl-N=N-C}_6\text{H}_4\text{-SO}_3\text{Na}$$Product: Orange II (orange-red azo dye)
Conditions summary:
- All steps at controlled temperatures
- Coupling in alkaline medium (phenol derivative)
- 0-5°C for diazonium salt stability
JEE Insight: Multi-step synthesis combining several concepts!
Q: Compound A has molecular formula C₁₂H₁₁N₃ and shows the following properties:
- Gives orange color with HCl
- Reacts with acetyl chloride to give a product with molecular formula C₁₄H₁₃N₃O
- Contains azo linkage
Identify compound A and explain the color change.
Solution:
Analysis:
Molecular formula: C₁₂H₁₁N₃
- Contains 3 nitrogen atoms
- One is likely azo group (-N=N-)
- Other is likely -NH₂ group
Clue 1: Azo linkage present
- Must have -N=N-
Clue 2: Reacts with acetyl chloride
- Has free -NH₂ group (forms amide)
- C₁₂H₁₁N₃ + CH₃COCl → C₁₄H₁₃N₃O
- Gain: C₂H₂O (acetyl group, -COCH₃)
Clue 3: Orange color with HCl
- Protonation changes color (typical for azo dyes)
Structure deduction:
Must be: H₂N-C₆H₄-N=N-C₆H₅
Compound A: p-Aminoazobenzene (para-phenylazoaniline)
Verification:
- Formula: C₁₂H₁₁N₃ ✓
- Has -NH₂ (reacts with acetyl chloride) ✓
- Has azo group ✓
Acetylation:
$$\text{H}_2\text{N-C}_6\text{H}_4\text{-N=N-C}_6\text{H}_5 + \text{CH}_3\text{COCl} \rightarrow$$ $$\text{CH}_3\text{CO-NH-C}_6\text{H}_4\text{-N=N-C}_6\text{H}_5$$(C₁₄H₁₃N₃O) ✓
Color change explanation:
In neutral form: Yellow-orange
- Normal conjugation
With HCl (protonated): Orange-red
$$\text{H}_2\text{N-C}_6\text{H}_4\text{-N=N-C}_6\text{H}_5 + \text{HCl} \rightarrow$$ $$[\text{H}_3\text{N}^+-\text{C}_6\text{H}_4\text{-N=N-C}_6\text{H}_5]\text{Cl}^-$$- Protonated -NH₃⁺ is electron-withdrawing
- Changes conjugation pattern
- Shifts absorption wavelength
- Deeper color (orange → red)
JEE Key: Protonation affects conjugation and color in azo dyes!
Quick Revision Box
| Topic | Key Points | Conditions |
|---|---|---|
| Coupling partners | Phenols, naphthols, aromatic amines | Must have strong EDG |
| pH for phenols | pH 8-10 (alkaline) | Forms phenoxide (more activated) |
| pH for amines | pH 4-5 (weakly acidic) | Keeps NH₂ free, N₂⁺ stable |
| Temperature | 0-5°C | Prevents decomposition |
| Regioselectivity | Para position (major) | Less steric hindrance |
| Color origin | Extended conjugation | π → π* transitions |
| Chromophore | -N=N- (azo group) | Absorbs visible light |
| Auxochrome | -OH, -NH₂, -SO₃H | Modifies/intensifies color |
| Methyl Orange | Red (acid) ↔ Yellow (base) | pH 3.1-4.4 |
Connection to Other Topics
Prerequisites:
- Diazonium Salts - Preparation, stability
- Properties of Amines - Aromatic amines
- Aromatic Chemistry - Electrophilic substitution
Related Topics:
- Phenols - Coupling partners
- Electronic Effects - +R, -R effects
- Spectroscopy - UV-Vis, color
Applications:
- Textile chemistry
- Analytical chemistry (indicators)
- Organic synthesis
- Pharmaceutical chemistry
Summary
1. Coupling Reaction:
General reaction:
$$[\text{Ar-N}_2^+]\text{Cl}^- + \text{Ar'-H} \rightarrow \text{Ar-N=N-Ar'} + \text{HCl}$$Product: Azo compound (colored dyes)
2. Coupling Partners:
Must be highly activated: ✓ Phenols, naphthols → pH 8-10 (alkaline) ✓ Aromatic amines → pH 4-5 (weakly acidic)
Won’t couple: ✗ Benzene (not activated) ✗ Nitrobenzene (deactivated) ✗ Toluene (too weakly activated)
3. pH Control (Critical!):
Phenols:
- Alkaline (pH 8-10)
- Forms phenoxide (C₆H₅O⁻)
- More activated, better coupling
Amines:
- Weakly acidic (pH 4-5)
- Keeps -NH₂ free (not -NH₃⁺)
- Maintains diazonium salt stability
4. Important Azo Dyes:
Methyl Orange:
- From sulfanilic acid + N,N-dimethylaniline
- pH indicator (red ↔ yellow)
- pH range: 3.1-4.4
Orange II:
- From sulfanilic acid + β-naphthol
- Orange-red textile dye
5. Color Chemistry:
Why colored?
- Extended conjugation (π-system)
- Chromophore: -N=N- group
- Auxochromes: -OH, -NH₂, -SO₃H
Structure affects color:
- More conjugation → Deeper color
- EDG → Bathochromic shift (red shift)
- EWG → Hypsochromic shift (blue shift)
6. JEE Strategy:
✓ Remember pH requirements (phenols = alkaline, amines = acidic) ✓ Only strongly activated rings couple ✓ Temperature: Always 0-5°C ✓ Know methyl orange preparation ✓ Understand color-structure relationship
7. Applications:
- Textile dyes (60-70% of all dyes)
- Food colorings (E numbers)
- pH indicators (methyl orange, congo red)
- Biological stains
- Industrial applications
“Azo coupling transforms diazonium salts into the vibrant world of colored compounds - from textiles to pH indicators, it’s chemistry that colors our world!”
Congratulations! You’ve completed the nitrogen compounds chapter. Master these concepts to excel in JEE organic chemistry!