Extraction

Introduction

Extraction is the process by which a substance is dissolved into a solvent, either directly from the solid state or by partitioning between two phases. The power of the separation depends on the difference in solubility between the compound of interest and the solubility of any impurities. This method is often used to separate a family or a small class of compounds from bulk impurities. W e will extract a base, caffeine, from plant material by liquid/liquid extraction. Organic amines are protonated below their pKa and become cationic, very soluble in water but insoluble in organic solvents like dichloromethane. Above their pKa organic amines are neutral making them more soluble in organic solvents. Organic acids are neutral below their pKa and anionic above, in the anionic form they are soluble in water, in the neutral form, soluble in organic solvents. Consider these facts carefully when reading the procedure for the isolation of caffeine and in answering the questions below.

Procedure

1. Record the masses of your two tea bags.

2. Perform the microscale isolation of caffeine from tea described in Williamson on pages 163-164. You will have access to a centrifuge and should therefore shake the test tube vigorously during the extraction. An intractable emulsion will form, break the phases by centrifugation. After you isolate the caffeine by sublimation weigh the product.

3. Give your sublimed product to your instructor.

Data Analysis

Calculate the percent yield of the caffeine from the tea.

Post-lab Questions

1. Why did you add sodium carbonate to the tea before extracting?

2. Devise a flow scheme to separate caffeine from salicylic acid, using dichloromethane/water. (Remember that caffeine is a base and that certain pH’s one of the compounds will be charged and the other neutral, dichloromethane is incapable of dissolving ions because they are too polar).

3. What is a partition coefficient? Is the partition coefficient of caffeine in water/dichloromethane constant at all pH’s?

4. Is neutral caffeine completely non-polar? Would hexane have worked as well as dichoromethane as the extraction solvent?

Extraction Pre-Lab

Name_________________

Watch the extraction video at: http://www.oid.ucla.edu/Webcast/Chemistry/

and then answer the following questions.

1. What physical property of the solvents is crucial to know when performing an extraction? Why?

2. Define partition coefficient.

3. If you have an organic compound that is somewhat soluble in water that you are trying to extract from an aqueous solution into an organic solvent, what experimental procedure can you use to improve the partition coefficient for the extraction?