Preparation of Organic Compounds Acetanilide

Introduction

The preparation of acetanilide is a fundamental organic chemistry laboratory experiment that demonstrates the principles of acetylation reactions and recrystallization techniques. This comprehensive guide provides detailed instructions for conducting and verifying this classic organic synthesis experiment in a pitch lab setting.

Aim

To synthesize acetanilide from aniline and acetic anhydride through acetylation reaction and verify the product through melting point determination and physical property analysis.

Apparatus Required

Glassware and Equipment:

• 100 mL round bottom flask
• Reflux condenser
• Dropping funnel
• Ice bath
• Büchner funnel
• Filter paper
• Beakers (100 mL, 250 mL)
• Measuring cylinder (10 mL, 50 mL)
• Glass rod
• Thermometer
• Hot plate with magnetic stirrer
• Magnetic stirrer bars

Chemical Reagents:

• Aniline (5 mL)
• Acetic anhydride (7 mL)
• Distilled water
• Activated charcoal (for decolorization)

Theory

Chemical Reaction

The preparation of acetanilide involves the acetylation of aniline:

C₆H₅NH₂ + (CH₃CO)₂O → C₆H₅NHCOCH₃ + CH₃COOH

Aniline + Acetic anhydride → Acetanilide + Acetic acid

Mechanism

1. Nucleophilic attack: The lone pair of electrons on the nitrogen atom of aniline attacks the electrophilic carbonyl carbon of acetic anhydride
2. Formation of tetrahedral intermediate: A tetrahedral intermediate is formed
3. Elimination: Acetic acid is eliminated, forming acetanilide

Physical Properties

• Molecular formula: C₈H₉NO
• Molecular weight: 135.16 g/mol
• Melting point: 114-116°C
• Appearance: White crystalline solid
• Solubility: Soluble in hot water, less soluble in cold water

Procedure

Step 1: Setup

1. Take 5 mL of aniline in a 100 mL round bottom flask
2. Cool the flask in an ice bath to prevent excessive heat generation
3. Set up the reflux condenser

Step 2: Acetylation

1. Slowly add 7 mL of acetic anhydride dropwise through the dropping funnel
2. Continue cooling during addition to control exothermic reaction
3. Stir the mixture for 15-20 minutes at room temperature
4. Heat the reaction mixture under reflux for 30 minutes

Step 3: Hydrolysis

1. Cool the reaction mixture to room temperature
2. Add 100 mL of distilled water slowly with constant stirring
3. Stir for 10 minutes to complete hydrolysis of excess acetic anhydride

Step 4: Crystallization

1. Cool the solution in an ice bath to promote crystallization
2. Scratch the sides of the beaker if necessary to induce crystallization

Step 5: Filtration and Purification

1. Filter the crystals using Büchner funnel
2. Wash crystals with small portions of cold water
3. Transfer crystals to a beaker and dissolve in minimum amount of hot water
4. Add a pinch of activated charcoal for decolorization
5. Filter the hot solution to remove charcoal
6. Allow slow crystallization by cooling the filtrate

Step 6: Final Collection

1. Filter the pure crystals using Büchner funnel
2. Wash with cold water
3. Dry the crystals and determine melting point

Observation Table

Result

The preparation of acetanilide was successfully completed through the acetylation of aniline with acetic anhydride. The final product obtained was characterized by:

• Physical appearance: White crystalline solid with needle-like structure
• Percentage yield: 82%
• Melting point: 115°C (literature value: 114-116°C)
• Purity: High (confirmed by sharp melting point)

Chemical Equation Verification:

C₆H₅NH₂ + (CH₃CO)₂O → C₆H₅NHCOCH₃ + CH₃COOH

Precautions

Safety Measures:

1. Work in fume hood: Both aniline and acetic anhydride are volatile and potentially toxic
2. Wear safety equipment: Safety goggles, gloves, and lab coat
3. Control addition rate: Add acetic anhydride slowly to prevent violent exothermic reaction
4. Maintain cooling: Use ice bath during reagent addition

Experimental Precautions:

1. Avoid moisture: Keep reagents dry as water interferes with the reaction
2. Proper stirring: Ensure homogeneous mixing throughout the reaction
3. Temperature control: Prevent overheating which may cause decomposition
4. Careful filtration: Handle delicate crystals gently to avoid loss

Viva Voce Questions and Answers

Q1: Why is ice bath used during the addition of acetic anhydride?

A: The reaction between aniline and acetic anhydride is highly exothermic. The ice bath helps control the temperature rise and prevents the decomposition of reactants and products.

Q2: What is the purpose of adding activated charcoal?

A: Activated charcoal is added to remove colored impurities present in the crude product, resulting in colorless, pure crystals.

Q3: Why is recrystallization necessary?

A: Recrystallization purifies the crude product by removing unreacted starting materials and by-products, ensuring high purity of final acetanilide.

Q4: What happens if excess water is added during hydrolysis?

A: Excess water can dissolve the formed acetanilide and reduce the yield of the product. It should be added in optimal quantity.

Q5: Why is the melting point of pure acetanilide sharp?

A: Pure compounds have definite melting points with sharp melting ranges, while impure compounds show broad melting ranges due to mixed melting phenomena.

Q6: What is the role of acetic anhydride in this reaction?

A: Acetic anhydride acts as the acetylating agent, providing the acetyl group (CH₃CO-) to convert aniline into acetanilide.

Q7: How can you increase the percentage yield?

A: The yield can be increased by:

• Using excess acetic anhydride
• Proper temperature control
• Efficient crystallization
• Minimizing product loss during filtration

Conclusion

The preparation of acetanilide experiment successfully demonstrates the principles of electrophilic aromatic substitution and acetylation reactions. The high percentage yield (82%) and sharp melting point (115°C) confirm the successful synthesis and purification of acetanilide. This experiment provides excellent training in organic synthesis techniques including:

• Controlled exothermic reactions
• Crystallization and recrystallization
• Vacuum filtration
• Melting point determination
• Product characterization

The experiment validates the theoretical knowledge of acetylation mechanisms and provides practical skills essential for advanced organic chemistry laboratory work.

SEO Keywords

• Preparation of acetanilide
• Organic chemistry lab experiment
• Acetylation of aniline
• Recrystallization technique
• Melting point determination
• Organic compound synthesis
• Laboratory experiment verification
• Chemical reaction mechanism
• Acetanilide preparation procedure
• Organic chemistry practical

This comprehensive guide serves as a complete reference for students and educators conducting the acetanilide preparation experiment, ensuring safety, accuracy, and successful results.