Extraction

Reading: Mayo pp. 104-108 and 124-126. McM sections 2.7-2.8 and the first two paragraphs of Section 20.3. Look up in the Handbook and record here the melting points of benzoic acid _______ and 9-fluorenone _______.

Preparation: Liquid-liquid extraction is one of the most used separation methods in chemistry. It takes advantage of chemical (rather than physical) properties of compounds.

In this laboratory activity you will carry out the separation of a carboxylic acid, benzoic acid, from a neutral compound, fluorenone from an amine. (The amine 4-aminobenzoic acid, ethyl ester and will be symbolized R-NH2.) As a preliminary exercise, you will first separate benzoic acid (symbolized by R-COOH) from a neutral compound. The compounds to be separated are all solids and will be initially dissolved in ether.

The key to success in this laboratory is not just to follow the directions but also to understand the chemical reactions you are controlling. The instructor will outline schemes of the separations that you need to copy into your notebook to keep track of the chemical processes involved in each separation.

 

Separation of A Carboxylic Acid From a Neutral Compound.

From the graph in Mayo, Fig 5.24 most compounds having 5 or more carbon atoms are essentially insoluble in water. This rule applies to carboxylic acids (Mayo, p. 124 and McM p. 819).

Note: both references have the about the same drawing for an organic – carboxylic acid. Here the acid is abbreviated as a general organic acid as R-COOH. In Mayo, p. 124 the "R" is a benzene ring with a chlorine atom attached; in McM p. 51 the "R" is specified as CH3, and later in McM p. 819 the "R" represents the rest of the molecule, so as to generalize the discussion for any organic carboxylic acid.

Using the Brønsted-Lowry definition, an organic acid is a compound that reacts to donate a proton (= H+) to a base such as OH- ion. A typical example of this is found at the bottom of McM p. 819:

R-COOH   +   OH-   ®   R-COO-   +   H2O
Organic acid               Conjugate base        
Insoluble in H2O               Soluble in H2O        

As can be seen from this reaction, the result of the loss of a proton causes the formation of the carboxylic acid’s conjugate base with a negative charge. This charge now allows the compound to dissolve in water. A carboxylic acid’s property of forming an ionic conjugate base allows us to separate carboxylic acids from other compounds that don’t have this chemical property.

In actual practice, the acid + non-acid mixture is first dissolved in ether, an organic solvent that dissolves most organic compounds (most compounds will dissolve in ether or in water, but not in both.)

1. To the ether containing this mixture is added aqueous NaOH. The OH- ions convert the acid to the water soluble conjugate base. The water and ether will separate into two layers; the water, being more dense, will form the bottom layer. The mixture is carefully shaken to allow completion of the reaction above. The conjugate base ions will dissolve in the water phase while the non-acidic organic compounds will remain dissolved in the ether phase.

2. The bottom layer of water containing the R-COO2- ions is transferred to a 2nd container. These negatively charged ions are balanced with Na+ ions left from the reaction of NaOH.

3. This aqueous layer is purified by removing any non-acids from it.

4. By adding enough HCl to this water solution to neutralize the NaOH, the reaction above is reversed and the conjugate base is converted back to the acid. The resulting cloudiness is due to the re-formation of the benzoic acid which as always is not very soluble in water. The crude acid is isolated by filtration. Thus the solid carboxylic acid is isolated.

5. The non-acid compound remaining in the ether layer is then purified and isolated. It is first purified by addition of pure water, then the ether layer is separated from the water and dried (i.e. traces of water are removed – see Note below) and then the ether is removed by evaporation.

In the preliminary experiment below, you will separate benzoic acid (R- = C6H5- ) from 9-fluorenone, which is a neutral compound and not an acid. You will be able to follow your progress in this separation since 9-fluorenone is yellow. The steps 1-5 above are explained – step by step - in further detail in the procedures below.

 

Separation of an Amine From a Carboxylic Acid + Neutral Compound.

The strategy of separation by pH control is now extended to separating an organic base – an amine – from a neutral compound from an organic acid. Most amines are insoluble in water, but are soluble in organic solvents like ether. In this laboratory you will separate benzoic acid (the carboxylic acid) from ethyl-4-aminobenzoate (the amine) from 9-fluorenone (the neutral compound)

Amines can be separated from non-amine compounds by converting them to their conjugate acids by the addition of HCl. The reaction below is taken from Mayo, p.124 (bottom) where R- is a benzene ring, C6H5-. The Brønsted-Lowry property of a base – of being able to accept a proton – distinguishes it from non-basic compounds. Again, a compound that is not soluble in water is converted to an ion that is soluble in water:

R-NH2   +   HCl   ®   R-NH3+   +   Cl-
Amine               Conjugate acid        
water insoluble               water soluble        

If a mixture of a base, and acid, and a neutral compound is to be separated, it is customary to first remove the organic base first. This will leave a mixture of carboxylic acid + neutral that can be separated in the same manner as outlined in steps 1-5 above. A scheme for the overall separation is found in Mayo, bottom of p. 125.

A. The three-component mixture is first dissolved in ether. Water containing HCl is added and the mixture is shaken. This allows the R-NH2 to become converted to R-NH3+ and dissolve in the water.

B. The mixture is carefully shaken to allow completion of the reaction above.

C. The water containing the R-NH3+ is transferred to a 2nd container and purified to remove any non-amine compounds.

D. The amine is isolated. By adding aqueous NaOH the HCl is neutralized and the conjugate acid is converted back to the amine. The resulting cloudiness is due to the re-formation of the amine which is not very soluble in water. The crude amine is isolated by filtration.

E. The ether layer contains the carboxylic acid and the neutral compounds. These fractions are separated into its components by adding 1 mL of 3 N aqueous NaOH and continuing with steps 2-5 above.

The rest of the laboratory work has to do with details and the purification of the compounds.

Note: the term "to dry" an organic layer means to remove traces of water from the compounds you separate.  Drying is accomplished by passing the ether solution through a pipette containing anhydrous Na2SO4 (Mayo, pp. 122-124).  Traces of water are retained in the pipette by reacting with the anhydrous Na2SO4. while the ether solution passes through. The purified compound can then be isolated by evaporation of the ether.

 

Procedure – separation of a carboxylic acid from a neutral compound: 

For preliminary trials, first separate an acid from a neutral substance.

Keep ether solutions cold to prevent evaporation and pressure buildup; do this by keeping the containers with ether in a pan of ice when they are not being used.

1. Into the 5 ml conical vial transfer 2 ml of an ether solution containing 100 mg each of: benzoic acid and 9-fluorenone. To the vial add 1 ml of 3 N aqueous NaOH.

2. The vial is topped with a Teflon disk (shiny side facing out) and secured with a screw cap. Thoroughly shake the vial to allow completion of the reaction and the dissolution of the compounds in their respective solvents. Allow the layers to separate.

3. Place an empty 3 ml vial alongside the 5 ml vial on the bench top and using a pipette, carefully transfer only the bottom layer to the smaller vial. If both layers enter the pipette, discharge its contents into the larger vial and try again. To the 3 ml vial add one ml of pure ether. Cap and shake its contents. As before, transfer the lower layer, this time to a 10 ml beaker. By now the contents of the beaker should be very pale or colorless (that is, free of 9-fluorenone).

4. To the beaker add – dropwise – 6N aqueous HCl until the mixture is acidic to pH paper. (Use a stirring rod to transfer a droplet of the water to the pH paper for this test) Calculating from the volume and concentration of the NaOH previously added, at least how many mL of this HCl will be needed? The appearance of the white solid benzoic acid should signal the neutralization of the NaOH by the HCl. Cool the mixture of benzoic acid in water to allow formation of all the solid. Transfer the solid onto a Hirsch funnel (paper disk and suction) using a small spatula. Tamp the crude, solid benzoic acid to allow the water to be removed, then weigh this solid before discarding it in the trash.

5. Combine the yellow ether layers into the 5 ml conical vial. Add 1ml of water, cap and shake its contents. Remove and discard the water layer with a pipette first because it is easier to transfer the lower layer. Then prepare a drying pipette: a clean, dry pipette is loaded with a cotton plug as in Mayo, fig 3.30b, and 2 cm of Na2SO4 is added above the plug. This pipette is clamped vertically above a dry, preweighed 30 or 50 ml beaker. Transfer the yellow ether layer to the filter pipette; rinse the 5 ml vial with small amounts of ether and transfer the rinsings to the filter pipette. Add a wooden splinter to the filtered solution and allow the ether to evaporate on the steam bath. Reweigh the beaker to determine the amount of 9-flourenone, then discard this yellow solid in the trash.

Summarize your results for the separated compounds from the Carboxylic Acid - Neutral mixture using the following table:

Compound Isolated Benzoic Acid 9-Fluorenone
weight of isolated    
% recovery    

 

Procedure – separation of an amine from a mixture of a carboxylic acid + neutral compound:

The preliminary steps for this separation is outlined above in steps A-E. These steps are detailed in the procedures that follow. Again draw a schematic of the layers and the containers. Indicate what is in each layer and to what container each layer is transferred.

For steps A-E you will separate ethyl 4-aminobenzoate (the amine) from a mixture of benzoic acid (the carboxylic acid) plus 9-fluorenone (the neutral compound). Following step E, you will repeat steps 1-5 to separate benzoic acid from 9-fluorenone.

A. Transfer 2 ml of the amine (100 mg) + carboxylic acid (100 mg) + neutral (100 mg) mixture that is already dissolved in ether to your 5 ml conical vial. To this add 1 ml of 3 M aqueous HCl.

B. Stopper the vial and thoroughly shake its contents to complete the reaction that selectively converts the amine to its water soluble conjugate acid.

C. Allow the layers to settle in a beaker of ice. Carefully transfer just the bottom (water) layer to a 3 ml conical vial. To the smaller vial add 1 ml of pure ether, cap and shake its contents. Transfer just the lower layer from this vial to a 10 ml beaker.

D. To this beaker add 6 M aqueous NaOH until the solution is basic to pH paper. The appearance of a white cloudiness indicates that the re-formation of the amine, ethyl 4–aminobenzoate. Cool the mixture and allow all the solid to congeal. Using a small steel spatula carefully scrape the solid into a Hirsch funnel that is prepared as before. Tamp the crude amine and allow the passage of air to remove water from it.

E. The yellow ether solutions remaining in the vials are re-combined into the 5 ml vial. Add 1 ml of pure water, shake the contents and then allow the layers to settle. Remove and discard the bottom aqueous layer. The remaining ether layer contains both the carboxylic acid (benzoic acid) and the yellow neutral compound (9-fluorenone). This mixture can now be separated into its components using steps 1-5 above.

All solids obtained from this separation of the amine-carboxylic acid -neutral compound method are weighed and their melting points are taken. The benzoic acid and the ethyl 4-aminobenzoate are likely to be rather impure because they were not subjected to the drying process. To assess the effect of this process, dissolve all the ethyl 4-aminobenzoate you obtained in 2 ml of pure ether and pass the solution through another filter – drying pipette into a preweighed container. After evaporation of the ether, determine the weight and melting point of the purified benzoic acid.

Compare the melting point and mass of the ethyl 4-aminobenzoate you isolated before and after drying.

Summarize your results for the separated compounds from the Amine - Carboxylic Acid - Neutral mixture using the following table:

Compound Isolated Ethyl 4-aminobenzoate Dried Ethyl 4-aminobenzoate Benzoic Acid 9-Fluorenone
mp of isolated        
mp of pure (Handbook)

92 oC

92 oC    
weight of isolated        
% recovery        

 References:

"Mayo": Mayo, D.W., Pike, R.M., Butcher, S.S. and Trumper, P.K. Microscale Techniques for the Organic Laboratory; Wiley: New York, 1991.

"McM:" McMurry, J. Organic Chemistry, 5th ed., Brooks/Cole Publishing Co. Pacific Grove, CA. 1999.

 

Created by Evan Thompson, revised by Shane Phillips on 1/21/98 then J. Almy January, 2001.