Notes on Recrystallization of Acetanilide

Introduction

• Instructor: Sully Isaeva, Chemistry Department
• Focus: Recrystallization of acetanilide
• Importance of purifying chemicals in organic chemistry lab

Recrystallization Overview

• Definition: A method to purify solid organic compounds.
• Steps Involved:
  1. Choose a suitable recrystallization solvent.
  2. Dissolve the sample in minimal boiling solvent.
  3. Filter the hot solution to remove insoluble impurities.
  4. Cool the solution to induce crystallization.
  5. Separate the solid from the solution via vacuum filtration.
  6. Wash the solid with cold solvent to remove traces of mother liquor.
  7. Dry the solid to eliminate solvent traces.

Choosing the Right Solvent

• Criteria for Selection:
  • Compound should be highly soluble in hot solvent, limited solubility in cold.
  • Impurities should be soluble in cold or insoluble in hot solvent.
  • Boiling point: 50-120 °C (lower than 50 °C evaporates quickly, higher than 120 °C difficult to remove).
  • Solvent should not react with the product.
  • Prefer solvent with lower toxicity.
• Removing Colored Impurities:
  • Use activated charcoal in hot solution.

Experiment Procedure

• Sample: Dissolve acetanilide in water.
• Filtration Types:
  • Hot Filtration: Removes insoluble impurities; maintain temperature to prevent crystallization in the filter paper.
  • Cold Filtration: Perform post-cooling to collect purified crystals.

Hot Filtration Steps

1. Use a preheated Lennmeyer flask with boiling chips.
2. Heat the flask and pour acetanilide-charcoal suspension through fluted filter paper.
3. Avoid premature crystallization; keep the funnel and solutions hot.

Cooling Process

• Cool the flask to room temperature before placing it in an ice water bath to control crystal size and purity.

Vacuum Filtration Steps

1. Use Wagner funnel with disc filter paper.
2. Wet the filter paper with cold solvent to secure it.
3. Pour cooled suspension through the funnel to collect crystals.

Melting Point Determination

• Purpose: Assess the purity of crystallized solid.
• Melting Point Range: Interval between the start and end of melting.
  • Pure compounds have narrow ranges (0.5-2 °C).
  • Impurities depress and broaden melting point range (e.g., cholesterol: pure 148-149 °C, contaminated 100-125 °C).

Safety Precautions

• Always wear lab coat, gloves, and safety goggles.

Reagents and Equipment

• Reagents: Acetanilide, activated charcoal, water, boiling chips.
• Equipment: 125 ml flask, hot plate, glass rod, funnel, filter paper, vacuum setup.

Experiment Steps

1. Weigh 2 grams of impure acetanilide; place in a flask with boiling chips and DI water (30 ml).
2. Heat and stir until dissolved, adding water as necessary (up to 10 ml).
3. Add 0.1 gram of activated charcoal and bring to a second boil.
4. Set up for hot filtration; ensure no crystallization occurs in the filter.
5. Allow the extract to cool, then transfer to ice bath for cold filtration.
6. Remove boiling chips and press crystals to eliminate water.
7. Dry crystals in an oven at 70 °C.
8. Pack crystals into a capillary tube and measure melting point.

Conclusion

• Crude acetanilide melts between 100-116 °C; purification improves melting point range.
• Thanks for your attention!