Question

Separation of Acidic, Basic, and Neutral Compounds:

This is for an Ochem Lab. 1st-8th is just the steps of the procedure we followed and the question corresponds to the 8th step in the procedure;​ In the first portion of the experiment, what is the purpose of washing the dichloromethane (layer C) with two 15 mL portions of water?

Background :

A mixture containing acidic, basic, and neutral compounds can be separated by acidic and basic properties. Commercial headache tablets, such as Excedrine, contain Aspirin (Acidic), Acetaminophen (Neutral), and Caffeine (Basic).

Procedure

1st)Weigh two headache tablets.

2nd)Crush using a mortal and pestle.

3rd)Dissolve the crushed tablets in 25mL of dichloromethane. Some solid binder may not dissolve so filter the dichloromethane using gravity filtration to remove the undissolved binder. This binder can be discarded.

4th)Transfer the dichloromethane solution to a separatory funnel and extract three times with 15 mL portions of 5% aqueous sodium bicarbonate.

5th)Combine the sodium bicarbonate layers in 100 mL beaker Labeled A.

6th)Extract the dichloromethane again but using three successive 15mL portions of 10% aqueous HCl.

7th)Combine the HCl layers in a beaker Labeled B.

8th)Lastly, wash* the dichloromethane with two 15mL portions of distilled water. The water will be the waste. Dry the dichloromethane over anhydrous calcium chloride. Label this C. 1.

Answer 1

Before the last step 8, you have followed extraction steps 4 and 5. In those steps you have done extraction with 5% NaHCO3 and 10% HCl solution respectively. So in these steps there is a chance that you might have left some salt solution in DCM layer. (100% removal of water layer is not practically possible!) Hence, to remove that traces amount of salt solution we give a final wash with distilled water and then dry DCM layer over anhydrous Na2SO4 or CaCl2. Further we evaporate DCM to get pure neutral compound (Acetaminophen).