Question

Separation of Acidic, Basic, and Neutral Compounds:

Why is it important to treat solution A with HCL before extracting with dichloromethane?What would happen if this step were eliminated?

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.

9th)Dry the dichloromethane over anhydrous calcium chloride. Label this C.       

^* Washing implies performing an extraction with water.

To solution A

Add concentrated HCl to the sodium bicarbonate layers from the initial extraction (A) until the mixture is homogeneous and acidic as determined using litmus paper

. Return this mixture to the separatory funnel. Add 15mL of dichloromethane. Caution- the HCI and bicarbonate react to form a gas. Pressure will build up in the sep funnel. Venting and cautious mixing is crucial. Separate the dichloromethane and repeat the extraction with two more 15mL portions of dichloromethane (a total of three 15mL extractions).

Combine the organic extracts and label the beaker A1. Dry it over anhydrous sodium sulfate, separate the dichloromethane from the drying agent and evaporate the dichloromethane on a hotplate under the hood to give aspirin. Obtain the melting point.

To solution B

Add solid sodium bicarbonate to the HCl solutions obtained in the second extraction (B) until the mixture is homogeneous and basic as determined using litmus paper.

Add 15mL of dichloromethane. Caution- the HCl and bicarbonate react to form a gas. Pressure will build up in the sep funnel. Venting and cautious mixing is crucial. Separate the organic layer and repeat the extraction with two more 15mL portions of dichloromethane (a total of three 15mL extractions).

Combine the organic extracts and label the beaker B1. Dry it over anhydrous sodium sulfate, separate the dichloromethane from the drying agent and evaporate the solvent on a hotplate under the hood to give caffeine. Obtain the melting point.

To solution C

When the dichloromethane is dried, separate it from the drying agent and evaporate it on a hotplate under the hood. The solid that remains is acetaminophen. Obtain the melting point

Why is it important to treat solution A with HCl before extracting with dichlormethane? What might happen if this step is eliminated?

Answer 1

Aqueous sodium bicarbonate solution was added to the dichloromethane solution and a solvent extraction was performed. Sodium bicarbonate forms an acid salt with aspirin and dissolves aspirin. Consequently, aspirin is transferred to the aqueous layer while the dichloromethane layer contains acetoaminophen and caffeine.

The sodium bicarbonate solution is separated and treated with HCl. This step is important to neutralize the unreacted sodium bicarbonate. Moreover, HCl helps precipitate aspirin from the bicarbonate solution as a solid. The precipitated aspirin is then dissolved by adding dichloromethane and isolated by solvent evaporation.

If the above step is skipped, aspirin will not completely separate out from the bicarbonate solution. Aspirin is soluble in both sodium bicarbonate and dichloromethane; however, aspirin is insoluble in HCl. Hence, the solution must be made acidic to ensure complete recovery of aspirin from the bicarbonate layer.