organic chemistry lab

SEPARATION OF ANALGESIC POWDER COMPONENTS OBJECTIVES: to purify Aspirin, Acetaminophen, and Caffeine from an Over the Counter analgesic medication utilizing a combination of liquid-liquid extraction and column chromatography.

TIME ALLOCATED: ONE LAB PERIOD

Introduction

Isolation of compounds from a mixture is a common step in organic chemistry laboratories. Separation and purification can be conducted utilizing various techniques that take into consideration the properties of the compounds, such as polarity, acidity, and solubility. Liquid-liquid extraction and chromatography are two of the most utilized purification techniques.

Excedrin is a commonly used analgesic medication that contains three active ingredients, acetaminophen, caffeine and Aspirin

In addition, Excedrin contain also an inactive ingredient (insoluble in organic solvents), used for binding together the other three components.

Aspirin, Acetaminophen and Caffeine have different polarity ad acidity properties that can be exploited for isolation. Caffeine is soluble in organic solvents, while Aspirin and Acetaminophen can be protonated and deprotonated adjusting the pH of the solution, therefore can be dissolved in an aqueous media.

TECHNIQUES

Column Chromatography.

The term defines a class of separation techniques that operate based on the differential partitioning of the analyte (compound to be separated) between a stationary phase and a mobile phase. In this case the stationary phase is polar (silica) and the mobile phase is liquid. In organic chemistry lab, common practice is to conduct planar chromatography (TLC) in conjunction with column chromatography. The distribution of the analyte or analytes between the two phases depends on absorption, ionic interactions, solubility, dispersion and polarity. Column chromatography uses a cylindrical glass column packed with stationary phase (most common is silica), an appropriate eluent is chosen and is poured in the column, and the elution can be conducted by gravity or with the aid of pressure. Tubes are utilized to collect small volume of

eluent passing through the column and containing the analytes.

MATERIAL AND METHODS

Reagents/Chemicals

• Goody’s powder packets

• Hexane, ethyl acetate, acetone, glacial acetic acid

• Original standards of Acetaminophen, Aspirin and Caffeine

Equipment

• TLC plate

• Column for chromatography

• Test tubes

• Flasks

• Capillary for spotting TLC

• Silica

EXPERIMENTAL

During this experiment you will separate and purify Acetaminophen, Aspirin and Caffeine exploiting their functional groups and properties.

1) Transfer the Goody’s powder into a flask and add 20 ml of ethyl acetate

2) Filter the insoluble material (starchy inactive ingredient) and keep the filtrate that contains the three desired active components.

3) Add tip of a spatula of silica for column in point 5ii. Evaporate solvent using rotary evaporator.

4) At this point you will perform column chromatography.

5) Prepare the column:

i. Weigh 3 g of silica and make a "slurry" (see instructions on D2L) using 10 mL of a 1:1 mixture of hexane: ethyl acetate. Swirl the flask slowly and pour the slurry into the column (make sure the column is closed at the bottom). Let the slurry deposit into the column for a minute, then drain the solvent until It reaches the silica level

ii. Load your silica-absorbed crude

iii. let the solvent drain until it reaches the level of the silica.

2

iv. flash the column with 45 ml of 1:1 hexane :ethylacetate In 3 tubes (15 mL each)

v. add 45 mL 1:2 hexane: ethyl acetate and collect 3 tubes.

6) Perform TLC analysis (using 1:2 hexane: ethyl acetate with 1% acetic acid) of the fractions you have collected to determine the purity of the compounds. You will be given pure standard of Acetaminophen, Aspirin and Caffeine to compare with your fractions.

7) Combine the test tubes containing pure compounds. Evaporate the solvent using rotary evaporator. Keep the product for NMR next week.

TECHNICAL NOTES ON COLUMN PREPARATION

Columns can be packed (prepared) using two different methods: dry method and wet method.

➢ Dry method: the column is filled first with the dry stationary phase, then the eluent is poured on top and flushed through the column until is completely wet.

➢ Wet method: a slurry is prepared adding, in a beaker, to the weighted silica 1.5 times the amount of eluent. The slurry is poured into the column and the eluent is drained until the eluent reaches the silica surface

In both preparation the silica should be flat and free of bubbles.

Sample loading:

The sample can be loaded in two ways;

➢ It can be dissolved in a small amount of the solvent mix you will use to run your column (or dichloromethane). This is an option only if your sample is completely soluble in a small volume of the chosen eluent (small volume=200-500 µL)

➢ It can be absorbed on a small amount of silica before loading onto the column. A spoon of silica can be added to a solution of your sample, the solvent is evaporated and the silica-absorbed- sample is loaded onto the column.

Other components in the preparation of the column:

➢ Usually sand or cotton are added at the bottom of the column, if frits is not present.

➢ After loading the sample and extra layer of silica can be added on top to protect the sample.

TECHNICAL NOTES ON TLC PREPARATION

The figure shows a sketch of a TLC plate and the formula for the calculation of the Retention Factor (Rf).

Reminder: Retention Factor Rf is calculated for compound B as follows Rf

B = D

b /D

s

Rf B

is the retention factor of

compound B D

b is the distance traveled by

compound B in cm D

s is the distance traveled by the solvent in cm

An optimal Rf Is ~0.3-0.7

REAL LIFE EXAMPLE

POST-LAB ASSIGNMENT

• Complete the flow chart with all the entry and data recorded during the lab section and the worksheet.

• Complete the writing assignment “Discussion” following the instructions provided.