Acid / Base Extractions

Purpose: This experiment will make use of extraction techniques to separate a mixture of an acid, a base, and a neutral compound into its individual components. The relative purity of the isolated components will be determined by Thin Layer Chromatography (TLC)

Introduction: View and take notes on the Extractions link web page and read  pages Chapter 15 on Extraction and Washing in lab text.

Organic Chemists generally must separate the compound of interest from a variety of impurities and byproducts that result from a synthesis. One of the most useful techniques is known as solvent extraction. In this procedure, differences in solubilities are exploited to separate components of a mixture based on their tendencies to dissolve in either organic or aqueous solvents.

Most organic solvents are immiscible with water, that is, they form a separate phase when mixed with water. Depending on the density (d) of the organic solvent, it will either float on top of the water (d_org <d_aq) or sink (d_org >d_aq). Thus it is important to know and record the density of solvents used in a procedure lest the wrong layer be discarded. Most neutral organic substances are soluble in organic solvents and are insoluble in water. The exceptions are organic molecules containing a high percentage of polar groups capable of hydrogen bonding to water (lower alcohols, ketones, carboxylic acids etc.)

The relative solubility in water of organic molecules containing acid or basic functional groups can be manipulated by changing the pH of the aqueous environment. In today’s experiment, you will investigate the solubility characteristics of three substances and exploit these characteristics to separate them from a mixture. The substances are shown in the following illustration.

At neutral pH, none of the compounds are significantly soluble in water at room temperature. However, in a sufficiently acidic environment (low pH), p-nitroaniline will form the corresponding ionic ammonium salt as shown. Being ionic, this substance is significantly soluble in water.

Conversely, in a basic environment (high pH), benzoic acid will form ionic sodium benzoate as shown.

The ionic sodium benzoate is significantly soluble in water.

Azobenzene lacks a basic or acidic functional group, and its solubility in water is not significantly effected by changes in pH.

The following procedure exploits these solubility changes to separate the three components from a initial mixture containing all three.

Procedure:

As part of your pre-lab activities, prepare a reagent table listing relevant properties of the three components and the solvents used in the experiment.

* Note: Wear gloves during this experiment. If any of the chemicals used in this experiment do come in contact with your skin, wash the affected area with soap and water.  Also, some of these chemicals will stain your clothes permanently.  Therefore, it is suggested that you wear something that you don't mind possibly ruining.

1. In a 50-mL Erlenmeyer flask, place the sample of benzoic acid, p-nitroaniline, and azobenzene).

2. Record the exact amount that you weigh. Completely dissolve this mixture in 35 mL of methylene chloride with swirling.

3. Using a funnel, pour the solution into a 125 mL separatory funnel (make sure that the stopcock is closed first).

4. Extract the solution with two 25-mL portions of 6 M hydrochloric acid. To do this, you will first pour 25 mL of 6 M HCl into the separatory funnel containing your methylene chloride solution. Stopper the funnel, invert, and gently shake the funnel with frequent venting. With stopcock closed, upright the separatory funnel and place it in the holder. You will see two layers in the funnel. Determine which layer is the aqueous and which is the organic.

5. Loosen the stopper and then drain the bottom layer into a labeled beaker. Then drain the remaining layer into a separate labeled beaker. With the stopcock closed, pour the organic layer back into the separatory funnel. Add an additional 25 mL of 6 M HCl to the solution in the funnel. Repeat the shaking/venting process as before and drain the layers into the appropriate beakers with the aqueous layer being added to the aqueous layer collected from the first extraction process. Once these steps are completed, you have performed the “extraction of the solution with two 25-mL portions”.

6. Pour the organic solution back into the separatory funnel and this time extract it with two 25-mL portions of 3 M sodium hydroxide solution following the extraction procedure from above. Place the aqueous layer from this set of extractions into a third labeled beaker (separate from the aqueous layers from the extraction with acid).

7. At this point, the organic layer may still contain some aqueous solution. To “dry” the solution (or remove the aqueous portion), add approximately 1 g of anhydrous magnesium sulfate to the organic layer and swirl.

8. While the organic solution is drying over magnesium sulfate, cool each of the aqueous extract layers in an ice-water bath. Neutralize the “acid extract” (the aqueous layer from your first set of extractions) with 6 M sodium hydroxide until the solution is distinctly basic to litmus paper. Neutralize the “basic extract” (the aqueous layer from your second set of extractions) with 6 M HCl until the solution is distinctly acidic to litmus paper. Upon neutralization of each solution, a precipitate should appear. Be sure to verify the appearance in your ‘observations’ (But you know that!)

9. Collect the precipitates separately by vacuum filtration. Wash each of the precipitated solids (while on the Buchner funnel) with cold distilled water. Pre-weigh two small labeled watch glasses and place the solid materials collected on each of them. Place these samples in an oven (at 90° C) to dry for approximately 1 hour.

10. Separate the “organic solution” from the magnesium sulfate by gravity filtration into a small labeled pre-weighed beaker. Under a hood, gently heat the methylene chloride solution on a hot plate at low heat (setting ~3) until the methylene chloride evaporates.

11. After the samples have dried, reweigh each of the watch glasses and beaker to obtain the weights of each of the separated materials recovered.

12. Perform a TLC analysis on each isolated substance. (See link for procedure)

The conclusion section of your report should address the following concepts. You may add others as you see fit.

1. How were you able to separate the three components in this mixture?  Create a flow chart that outlines the process and shows the fate of each component.

2. What was your percent recovery for each of the three materials?

3. If your percent recovery is not exactly 100%, what could be some reasons for this?

4. Why is anhydrous magnesium sulfate added to the organic layer?

5. Why was the water used for washing the precipitates on the funnels specified to be cold?

6. What is the total mass of the three separate compounds? Does this amount differ from the initial mass of the mixture? Why?

7. What information regarding the purity of each isolated component was obtained by TLC? Does the TLC indicate the components were completely separated? What other technique might further purify each component?

8. On the basis of what you learned in this experiment, plus the information that phenol is soluble in aqueous sodium hydroxide but not in sodium hydrogen carbonate (sodium bicarbonate), whereas benzoic acid is soluble in both solutions, show how you could separate a mixture containing phenol, benzoic acid, p-nitroanaline, and azobenzene. (Complete your answer in the form a flow chart.)