Separating Acids and Neutral Compounds by Solvent Extraction

Written by Robert

Purpose:  To use solvent extraction techniques to separate a mixture consisting of a carboxylic acid, a phenol, and a neutral compound and to determine the melting points of said chemicals.

Description:

• For this experiment, we used the following materials: (4) 100 mL beakers, 400 mL beaker with ice, Buchner funnel, 125 mL Erlenmeyer Flask, 250 mL filter flask, filter paper, glass stirring rod, 25 mL graduated cylinder, hot plate, melting point capillary tubes, disposable pipets, pH test paper, 125 mL separatory funnel, support ring, support stand, thermometer, acetanilide (neutral compound), 2-napthol (phenol), benzoic acid (carboxylic acid), and standard safety equipment.
• Our samples of benzoic acid, acetanilide, and 2-napthol were carefully measured out to roughly 0.500 grams, 0.250 grams, and 0.500 grams respectively, and were recorded. 25 mL of MTBE was then added to a 100 mL beaker along with the three solids. The solution was then mixed and transferred to the separatory funnel.
• The extraction of the benzoic acid began by adding 10 mL of 0.5M aqueous NaHCO₃, gently mixing the two layers in the funnel by inversion, and occasionally opening the stop cock to vent the built up fumes. This process was continued until the funnel could be vigorously shaken with no sign of any gas venting from the funnel. The funnel was then placed on the support ring and allowed to separate layers. The glass stopper was then removed from the top of the funnel, and the stop cock was opened to release the aqueous layer into a clean 100 mL beaker, while being sure not to transfer any of the top layer. This was repeated two more times, and them 5 mL of water was added to the separatory funnel where the water settled, and was drained into the 100 mL beaker with the NaHCO₃ extract. The NaHCO₃ extract was then set aside for use later during the lab.
• The extraction of our 2-napthol then began by taking the separatory funnel with the ether solution in it and adding 10 mL of 0.5M NaOH. Follow the same steps from the benzoic acid extraction using NaOH, and save the ether layer remaining in the separatory funnel.
• The isolation of the benzoic acid then began by carefully adding 3M HCl to the NaHCO₃ solution until no more precipitate is formed. Filter paper and a weigh boat were weighed, and recorded. The precipitate was then filtered with a Buchner funnel, and then transferred to the weigh boat where they would dry. After drying for a week, the crystals were weighed and recorded.
• The isolation of the 2-napthol then began by weighing out a clean 100 mL beaker, and recording the weight. Then, the NaOH solution was transferred to the 100 mL beaker which was set into a water bath at about 60°C to remove any remaining MTBE that might inhibit precipitation. 3M HCl was then added to the solution until it became acidic, and the phenol was cooled in an ice bath to crystallize the phenol. The precipitate was then extracted using a Buchner funnel and set out to dry on a filter paper and weigh boat. After drying, the phenol was then weighed and recorded.
• The isolation of the Acetanilide then began by transferring the remaining ether-acetanilide solution from the separatory funnel to a clean 125 mL Erlenmeyer flask. Roughly 1 gram of Na₂SO₄ was added to the flask and was stoppered and set for 5 minutes to remove and traces of water in the solution. A clean 100 mL beaker was then weighed and recorded, and the solution was then decanted into the beaker. The beaker with solution was then taken to a water bath to evaporate the solution away to leave the remaining acetanilide. After the acetanilide dried, it was weighed and recorded.
• The 3 crystals were then transferred to capillary tubes where their melting point ranges were observed and recorded.

Results:

• Preparation of crystals:
  • Weight of acetanilide: 0.258 g
  • Weight of benzoic acid: 0.504 g
  • Weight of 2-napthol: 0.502 g
  • Final crystal weights:
    • Weight of acetanilide: 0.249 g
    • Weight of benzoic acid: 0.218 g
    • Weight of 2-napthol: 0.371 g
    • Chemical melting points:
      • Acetanilide: 90.2-103.5 °C
      • Benzoic acid: 121.9-122.8 °C
      • 2-napthol: 120.1-121.0 °C

Equipment Uncertainties and Indeterminate Errors:

• A few errors that could have occurred during this experiment include:
  • Loss of crystals through the various extraction processes.
  • Accidental transfer of the incorrect upper layer during extraction.
  • Crystals not dry during final weighing process.