Preparation of Organic Compounds Aniline yellow or 2-Naphthol aniline dye

Introduction

The preparation of organic compounds, particularly synthetic dyes, represents a fundamental practical exercise in organic chemistry laboratories. Among these, Aniline Yellow or 2-Naphthol aniline dye preparation stands as a classic example of diazotization and azo coupling reactions. This experiment demonstrates the synthesis of an important azo dye through electrophilic aromatic substitution.

Aim

To prepare Aniline Yellow (p-hydroxyazobenzene) dye in the laboratory by diazotization of aniline followed by coupling with 2-naphthol, and to study its properties.

Apparatus Required

Glassware and Equipment:

• 250 mL Beaker
• 100 mL Measuring cylinder
• Glass rod for stirring
• Ice bath or ice container
• Dropping funnel
• Test tubes (3 nos.)
• Bunsen burner
• Wire gauze
• Tripod stand
• Filter paper
• Buchner funnel
• Conical flask (250 mL)

Chemicals Required:

• Aniline (5 mL)
• Concentrated HCl (10 mL)
• Sodium nitrite (NaNO₂) (3 g)
• 2-Naphthol (5 g)
• Sodium hydroxide (NaOH) pellets (10 g)
• Distilled water
• Sodium acetate (CH₃COONa) (5 g)

Theory

Aniline Yellow is synthesized through a two-step process involving diazotization and azo coupling reactions.

Chemical Reaction:

Step 1: Diazotization Reaction

C₆H₅NH₂ + NaNO₂ + 2HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O + NaCl

Step 2: Azo Coupling Reaction

C₆H₅N₂⁺Cl⁻ + C₁₀H₇OH → C₆H₅-N=N-C₁₀H₆OH + HCl

Mechanism:

1. Diazotization: Aniline is first converted to its hydrochloride salt using HCl, then treated with sodium nitrite at low temperature (0-5°C) to form benzene diazonium chloride.

2. Coupling Reaction: The diazonium salt acts as an electrophile and couples with 2-naphthol (activated by NaOH) to form the azo compound, Aniline Yellow.

Properties of Aniline Yellow:

• Orange-yellow crystalline solid
• Insoluble in water
• Soluble in organic solvents
• Used as a dye for fabrics

Procedure

Preparation of Diazonium Salt Solution:

1. Take 5 mL of aniline in a 250 mL beaker
2. Add 10 mL of concentrated HCl dropwise while stirring
3. Cool the mixture in an ice bath to 0-5°C
4. Prepare a solution of 3 g sodium nitrite in 15 mL distilled water
5. Add sodium nitrite solution dropwise to the cooled aniline hydrochloride mixture with constant stirring
6. Maintain temperature below 5°C throughout the addition

Preparation of Coupling Component:

1. Dissolve 5 g of 2-naphthol in 20 mL of 10% NaOH solution in a separate beaker
2. Cool this solution to 0-5°C in ice bath
3. Add 5 g of sodium acetate to maintain pH

Coupling Reaction:

1. Slowly add the diazonium salt solution to the 2-naphthol solution with continuous stirring
2. Maintain the temperature at 0-5°C
3. Stir the mixture for 15-20 minutes
4. Allow the precipitated dye to settle

Filtration and Drying:

1. Filter the precipitated Aniline Yellow using Buchner funnel
2. Wash the precipitate with cold water
3. Dry the solid and weigh the final product

Observation Table

Result

The preparation of Aniline Yellow dye was successfully completed through diazotization of aniline followed by coupling with 2-naphthol. The theoretical yield was calculated to be approximately 6.8 g, while the actual yield obtained was 5.2 g, giving a percentage yield of 76.5%.

Key Observations:

• Orange-yellow precipitate formed immediately upon mixing
• Product was insoluble in water but soluble in organic solvents
• Crystalline solid obtained after drying
• Characteristic azo compound properties confirmed

Precautions

Safety Measures:

1. Temperature Control: Maintain reaction temperature below 5°C to prevent decomposition of diazonium salts
2. Proper Ventilation: Conduct experiment in well-ventilated area or fume hood due to evolution of nitrogen oxides
3. Chemical Handling: Handle concentrated HCl and sodium nitrite with care using appropriate PPE
4. Gradual Addition: Add sodium nitrite solution slowly to control exothermic reaction

Technical Precautions:

1. Keep all solutions cold throughout the process
2. Avoid excess nitrous acid which can destroy the diazo compound
3. Ensure complete dissolution of 2-naphthol in alkaline medium
4. Thorough washing of final product to remove unreacted chemicals

Viva Questions and Answers

Q1: What type of reaction occurs in the preparation of Aniline Yellow?

A: Electrophilic aromatic substitution (azo coupling reaction)

Q2: Why is the reaction temperature kept below 5°C?

A: Diazonium salts are unstable at higher temperatures and decompose with evolution of nitrogen gas

Q3: What is the role of sodium acetate in the reaction?

A: It maintains the pH of the reaction medium and prevents the destruction of diazonium salt by excess acid

Q4: Why is aniline first converted to its hydrochloride salt?

A: To make the amino group less nucleophilic and prevent side reactions during diazotization

Q5: What is the structural feature responsible for the color of Aniline Yellow?

A: The extended conjugation system including the azo group (-N=N-) and aromatic rings

Q6: What is the IUPAC name of Aniline Yellow?

A: p-hydroxyazobenzene or 4-[(phenyl)diazenyl]naphthalen-2-ol

Q7: How can you test the purity of the prepared dye?

A: By determining melting point, thin layer chromatography (TLC), or comparing with standard sample

Q8: What are the industrial applications of azo dyes?

A: Textile dyeing, food coloring (approved ones), cosmetic industry, and biological staining

Conclusion

The preparation of Aniline Yellow dye successfully demonstrates the fundamental principles of diazotization and azo coupling reactions in organic chemistry. This experiment provides valuable insights into the synthesis of synthetic dyes and their structural properties. The orange-yellow color of the final product confirms the formation of the azo compound, validating the theoretical concepts of electrophilic aromatic substitution reactions.

Applications and Significance

Aniline Yellow, though primarily prepared for educational purposes, represents the class of azo dyes that revolutionized the textile and coloring industry. Understanding its preparation mechanism helps students grasp important organic chemistry concepts including:

• Electrophilic aromatic substitution
• Diazo chemistry
• Conjugated systems and their optical properties
• Industrial dye synthesis principles

This experiment serves as a foundation for understanding more complex synthetic procedures in organic chemistry and industrial processes.