CHEM 3022 - Separation of a Three Component Mixture by Extraction

Purpose: to separate a mixture of benzoic acid, ethyl 4-aminobenzoate (a base) and 9-fluorenone (a neutral compound) by liquid-liquid extraction.

Reading: "Microscale" pp. 104-108, 113-118 and 124-126. McMurry sections 20.3, 25.6 and 24.4. Look up and record the melting points, boiling points and solubilities in water of each compound listed above in the Handbook (ethyl 4-aminobenzoate is found under benzoic acid, subheading 4-amino, subsubheading ethyl ester - also known as benzocaine).

Background: Compounds which are not very soluble in neutral water may dissolve if the pH is changed. By appropriate adjustment of the pH, organic acids and bases can be physically separated from organic neutral compounds.

The sections from McMurry in the reading give details on the molecular changes caused by pH adjustments, and the consequences of these changes on solubility. Study these sections carefully; you need to know how you will be controlling solubilities by your additions of HCl and NaOH to the mixtures.  You will be repeating this procedure on your mixture unknown; master the technique now.

General Separation Scheme for Acid, Base and Neutral Compound:
In carrying out the separation, you should follow your progress with the flow chart below and label your flasks.  Make a similar flow chart in your notebook and record your observations and any changes you make with each step along the way.

With this scheme, you should be able to isolate any acid (a carboxylic acid or a phenol), base, or neutral compound. The materials separated by this scheme are the "acid fraction", the "base fraction" and the "neutral fraction" respectively.

The term "to dry" an organic layer means to remove traces of water which would cause problems in IR and NMR analysis (such as ruining the salt plates in IR).   Drying is accomplished by passing the ether solution through a column of anhydrous Na2SO4 (Microscale, pp. 122-124).  The dry ether solution can now be concentrated by evaporation.

What would happen if one or more of the components of the mixture was a liquid, not a solid? In this case, crystals would not form where they are indicated in the scheme; instead an oil would separate either above or below the aqueous layer. It is usually best at this point to add some ether to dissolve the oil.  The ether layer, always the upper layer, would be separated and dried as described above then concentrated by evaporation.

This scheme may be modified to fit specific situations, for example, if it is known that a mixture consists only of an acid and a neutral compound, the scheme may be started with the addition of 3 M NaOH. On the other hand, if a mixture consists of only a base and a neutral, the 3 M NaOH addition may be skipped and the ether layer evaporated to yield the neutral fraction.

Procedure:  Obtain 2 mL of the ether solution which contains 50 mg each of: benzoic acid, ethyl para- aminobenzoate and 9-fluorenone.  You will be dispensed only 2 mL of the solution into a 5 mL conical vial.

Include a simple drawing of a vial or separatory funnel for each physical separation carried out below.  Indicate the aqueous and ether layers and the components of each (chemical formula).

Separation of the Basic Fraction: Cool the solution slightly in some ice-water and add 1 mL of 3 M hydrochloric acid, cap the vial (shiny side of the PTFE septum up) and shake the mixture thoroughly. Allow the vial to stand to allow the layers to separate; then slowly remove the cap and transfer the bottom (aqueous) layer to a labeled, 30 mL beaker with a pipet. (A small amount of crystalline material may form at the interface between the layers which dissolves upon repeating this extraction as follows.)   Repeat the extraction with an additional 1 mL of the 3 M acid solution and transfer the aqueous layer again to the same beaker.

Isolation of Ethyl 4-Aminobenzoate:  To the acidic aqueous solution in the 30 mL beaker, add 6 M NaOH dropwise until the solution's pH is above 10. Record your observations. Cool the beaker in an ice bath for about 10-15 minutes (while you are waiting, start separating the acidic fraction, below.) Collect the solid precipitate by reduced-pressure filtration using a Hirsch funnel (Microscale, Fig. 5.40 - use the correct size paper disk.) Wash the precipitate with two 0.5-mL portions of deionized water. Allow air to suck through the crystals and the filter paper to remove the water. Weigh your product and calculate the percentage recovery. If your birthday falls in the 1st 4 months of the year, obtain a melting point of the dried material and compare your result with the literature value.  Place your dried product in a sealed, plastic bag into your notebook.  Consult your instructor about obtaining NMR and IR data.

Separation of the Acidic Fraction:  To the remaining ether solution now add 2 mL of 3 M NaOH. The system is mixed as before and the aqueous layer is separated and transferred to a labeled, 50 mL beaker.  This step is repeated and the aqueous layer again removed and transferred to the same 50 mL beaker.

Isolation of Benzoic Acid:  To the aqueous alkaline solution, add 6 M HCl dropwise until the solution's pH is below 4. Record your observations. Cool the flask in an ice bath for about 10 minutes. Collect the precipitated benzoic acid with a Hirsch funnel as above. Wash the precipitate with two 1-mL portions of distilled water. Weigh the benzoic acid and calculate your percentage recovery. If your birthday falls in May, June, July or August, obtain the melting point of the dried material and compare your result with the literature value. Place your dried product in a sealed, plastic bag into your notebook.  Consult your instructor about obtaining NMR and IR data.

Separation of the Neutral Fraction - Isolation of 9-Fluorenone: The ether solution remaining should be "washed" to remove traces of HCl or NaOH. This is done by adding 1 mL of deionized water, capping and shaking the vial, than removing and discarding the water phase.

Prepare a drying pipet by placing a small wad of cotton into a pipet (Microscale Fig. 3.30b or c), then loading it with about 2 cm of anhydrous Na2SO4 (this was done before in the free radical chlorination experiment). Clamp the drying pipet vertically above a preweighed 30 mL beaker. With another pipet, transfer the ether solution and allow it to drain through the column of Na2SO4 into the beaker. Rinse the conical vial with 1 mL of ether and transfer it through the drying pipet into the beaker.

Allow the ether to evaporate by setting it in the hood. Obtain the weight of the isolated 9-fluorenone and calculate the percentage recovery. If your birthday falls in the last 4 months of the year, obtain a melting point of the material and compare your result with the literature value.  Place your dried product in a sealed, plastic bag into your notebook.  Consult your instructor about obtaining NMR and/or IR data.

Exchange melting point information with others in the class once you tabulate your data.

Material Melting Point Amount Percent Literature Value woman8.wmf (16010 bytes)
ACID

BASE

NEUTRAL

Cleanup: For equipment with organic material clinging to the surface, hold the object over a large beaker with a plastic glove or forceps and squirt acetone on it; then wash it with soap and water. Allow it to dry on a paper towel.  Dispose of acetone rinse in the organic waste container.

 

Created by Evan Thompson, revised by S. Phillips on 1/21/98.