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48 Part I Mastering the Operations
Extraction and Evaporation
Separating the Components
of “Panacetin”
Separation Methods.
Operations OP-7 Heating OP-15 Gravity Filtration (SS) OP-17 Centrifugation (μS) OP-18 Extraction OP-19 Evaporation OP-4 Weighing OP-5 Measuring Volume OP-6 Making Transfers OP-16 Vacuum Filtration OP-26 Washing and Drying Solids
Before You Begin
1. Read the experiment carefully. 2. Read operations OP-15 (SS), OP-17 (μS), OP-18, OP-19, and the section titled “Heat Sources” in OP-7. Review the other listed operations as necessary.
Scenario
Your supervisor has been e-mailed the following message from a drug watchdog agency, the Association for Safe Pharmaceuticals (ASP). Greetings: Our roving agent in Southern California, Sam Surf, recently purchased some Panacetin—an analgesic drug preparation—at a drugstore in San Diego. According to the label on the bottle, the Panacetin tablets were manufactured in the United States by a legitimate pharmaceutical company, but Sam detected some discrep- ancies on the label and flaws in the tablets themselves that made him suspect they might be counterfeit. Such illegal knockoffs of a domestic drug can be manufactured cheaply elsewhere and smuggled into the United States, where they are sold at a big profit margin. The label on the bottle lists the ingredients per tablet as aspirin (200 mg), acetaminophen (250 mg), and sucrose (50 mg). Aspirin and acetaminophen are presumably the active ingredients, while sucrose is an inactive ingredient used to make the tablets more palatable to children. But counterfeit drugs may contain less of an active ingredi- ent than claimed, the wrong active ingredient, or no active ingredi- ents at all. We have reason to believe that Panacetin does contain
Operations used only for a standard scale procedure are indicated by (SS), and operations used only for a mi- croscale procedure are indicated by ( μ S).
See Experiment 1 if you don’t under- stand the purpose of the Scenarios.
EXPERIMENT 2
Experiment 2 Separating the Components of “Panacetin” 49
aspirin, sucrose, and another active component, but we’re not sure what that component is or whether the amounts listed on the label are accurate. The unknown component is probably a chemical rela- tive of acetaminophen, either acetanilide or phenacetin. Both of these kill pain as effectively as acetaminophen, so the consumer wouldn’t notice their presence in an analgesic drug. But acetanilide and phenacetin are banned in the United States because of their toxicity, and we would like to keep them off the market. We want your Consulting Chemists Institute to analyze this drug preparation to find out what percentages of aspirin, sucrose, and the un- known component it contains, and whether the unknown is acetanilide or phenacetin.You have two weeks to complete your investigation. Les Payne, Director of Operations, ASP
Applying Scientific Methodology
The Scenario presents two problems for you to solve in this experiment and the next: (1) Is the composition of Panacetin as stated on the label accu- rate? (2) What is the identity of the unknown component in Panacetin? You will concentrate on the first problem in this experiment, following the course of action described in the Directions. Because of possible material losses, you must allow some margin for error in deciding whether the per- centage composition derived from the label (10% sucrose, 40% aspirin, 50% unknown component) is accurate. If you follow the standard scale procedure, ranges of 8–12% sucrose, 35–45% aspirin, and 45–55% of the unknown component are close enough to indicate that the label is reason- ably accurate. If you follow the microscale procedure, acceptable ranges are 6–14% sucrose, 30–50% aspirin, and 40–60% of the unknown component. As in the previous experiment, you should start with a working hypothesis, gather and interpret evidence, change your hypothesis if the evidence doesn’t support it, arrive at a conclusion, and report your results.
Painkilling Drugs, from Antifebrin to Tylenol
Analgesic drugs reduce pain; antipyretic drugs reduce fever. Some drugs, including aspirin and acetaminophen, do both. Many of the common over- the-counter analgesic–antipyretic drug preparations contain aspirin, aceta- minophen, or combinations of these substances with other ingredients. For example, acetaminophen is the active ingredient of Tylenol, and Extra Strength Excedrin contains aspirin, acetaminophen, and caffeine. From their molecular structures, you can see that acetaminophen is chemically related to both acetanilide and phenacetin, whose painkilling effects were discovered late in the nineteenth century. In 1886, two clinical assistants named Arnold Cahn and Paul Hepp were looking for something that would rid their patients of a particularly unpleasant intestinal worm. The trick was to find a drug that would kill the worm but not the patient, and their method—not a very scientific one—was to test the chemicals in their stockroom until they found one that worked. When they came across an ancient bottle labeled NAPHTHALENE, they tried it out on a patient who had every malady in the book, including worms. It didn’t faze the worms, but it reduced the patient’s fever dramatically. Before
Some compounds with analgesic–antipyretic properties
OCOCH 3 COOH
aspirin
NHCOCH 3
acetanilide NHCOCH (^3)
acetaminophen
NHCOCH 3
OH OCH 2 CH (^3) phenacetin
Experiment 2 Separating the Components of “Panacetin” 51
acid-base properties. Panacetin contains aspirin, sucrose, and an unknown component that may be either acetanilide or phenacetin. These substances have the following characteristics:
- Sucrose is insoluble in the organic solvent dichloromethane also called methylene chloride).
- Aspirin, acetanilide, and phenacetin are soluble in dichloromethane but relatively insoluble in water.
- Aspirin reacts with bases such as sodium bicarbonate to form a salt, sodi- um acetylsalicylate, which is insoluble in dichloromethane and soluble in water.
- Acetanilide and phenacetin are not converted to salts by sodium bicar- bonate. Mixing the Panacetin with dichloromethane should therefore dissolve the aspirin and the unknown component, leaving the sucrose behind as an insoluble solid that can be removed by gravity filtration or centrifugation. Aspirin can be removed from the dichloromethane solution by extraction with an aqueous solution of sodium bicarbonate. The base converts aspirin to its sodium salt, as shown in the margin. This salt will migrate from the dichloromethane layer, in which it is insoluble, to the aqueous layer, in which it is soluble. The unknown component will stay behind in the dichloromethane layer. After separation of the layers, aspirin can be recov- ered by the same method you used in Experiment 1 to prepare benzoic acid from its salt: precipitation from the aqueous layer with hydrochloric acid followed by vacuum filtration. The unknown component can then be isolated by evaporating the solvent from the dichloromethane solution. This separation process is summarized by the following flow diagram:
(CH 2 Cl 2 ,
mix with dichloromethane, filter
aspirin and unknown in dichloromethane
sucrose
Solid Residue Filtrate
add HCl, filter
evaporate
unknown in dichloromethane
sodium acetylsalicylate in water
aspirin unknown
Aqueous Layer Organic Layer
“Panacetin” (sucrose, aspirin, unknown)
extract with NaHCO (^3)
A Greener Way: Diethyl ether is less harmful to health and the environment than is dichloromethane, so it can be used as the solvent in place of dichloromethane. Just remember that an ether layer separates above an aque- ous layer rather than below it. If you use dichloromethane, it is best to evapo- rate the organic layer under vacuum and recover the solvent, which can then be recycled.
Minilab 2 in Part III will help you un- derstand how the extraction operation works; you can carry out this minilab at your instructor’s request.
OCOCH 3 COOH NaHCO 3 HCl aspirin
OCOCH 3 COONa
sodium acetylsalicylate
You can estimate the percentage composition of Panacetin from the masses of the dried components. Note that the actual composition may or may not be the same as the composition given in the Scenario. Careful work is required to obtain accurate results in this experiment; errors can arise from incomplete mixing with dichloromethane, incomplete extraction or precipitation of aspirin, incomplete drying of the recovered components, and losses in transferring substances from one container to another. Dichloromethane may be harmful to the environment, especially when released into groundwater. The Environmental Protection Agency (EPA) classifies it as a priority pollutant and has established a maximum contami- nant level (MCL) of 5 parts per billion (ppb) for its concentration in drinking water. See the “Chemistry and the Environment” section for information about priority pollutants, MCLs, and other environmental topics.
DIRECTIONS
Safety Notes Dichloromethane may be harmful if ingested, inhaled, or absorbed through
the skin. There is a possibility that prolonged inhalation of dichloro- methane may cause cancer. Minimize contact with the liquid, and do not breathe its vapors.
See “Chemical Hazards” in the “Laboratory Safety” section for an ex- planation of the hazard symbols.
Standard Scale
Separation of Sucrose. Accurately weigh [OP-4] about 3.00 g of Panacetin and transfer it to a clean, dry 125-mL Erlenmeyer flask. Add 50 mL of dichloromethane to the flask. Stir the mixture thoroughly with a glass stirring rod to dissolve as much solid as possible, and use the rod to break up any lumps or granules. Using a preweighed fluted filter paper, filter the mixture by gravity [OP-15] into a small flask, saving the filtrate (the liquid that goes through the filter paper) for the next step. Set the filter paper aside, being careful not to lose any of the sucrose, and reweigh it when it is completely dry. Record the mass of the sucrose in your laboratory notebook. If requested, submit the sucrose to your instructor in a tared (preweighed) and labeled vial. Separation of Aspirin. Transfer [OP-6] the filtrate to a separatory fun- nel and extract [OP-18] it with two separate 30-mL portions of 5% sodium bicarbonate. For each extraction, use a stirring rod to stir the liquid layers until any fizzing subsides before you stopper and shake the separatory funnel. Because the dichloromethane layer will be on the bottom, you will have to transfer each layer to a different container (label the containers) and return the dichloromethane layer to the separatory funnel before the second extraction. Combine the two aqueous extracts in the same container and save the dichloromethane layer for the following step, “Isolation of the Unknown Component.” Slowly add 7.0 mL of 6 M hydrochloric to the combined aqueous extracts while stirring with a glass rod. Test the pH of the solution as
Take Care! Avoid contact with dichloromethane; do not breathe its vapors.
Stop and Think: Does all of the solid dissolve? If not, why not?
Stop and Think: What components of Panacetin are in the filtrate?
0 Reactivity
Fire
Health dichloromethane
Take Care! A gas is evolved, so don’t shake too vigorously.
Stop and Think: Which component of Panacetin is in the aqueous layer? In the dichloromethane layer? If you don’t shake the sepa- ratory funnel long enough, how might that affect your results?
Observe and Note: Can you detect any evidence for a chemical reac- tion upon adding HCl? If so, de- scribe it in your lab notebook.
52 Part I Mastering the Operations
with the first one. Save the dichloromethane solution in the vial for the following step, “Isolation of the Unknown Component.” Slowly add 1.0 mL of 6 M hydrochloric acid to the combined aqueous extracts while stirring with a glass rod. Test the pH of the solution with pH paper as described in Experiment 1, and add more acid, if necessary, to bring the pH to 2 or lower. Cool the mixture in an ice/water bath for at least five minutes, collect the aspirin by vacuum filtration [OP-16] with a Hirsch funnel, and wash it on the filter [OP-26a] with cold water. Let the aspirin dry on the filter for a few minutes with the aspirator running, then dry [OP-26b] it to constant mass. Weigh the aspirin and record its mass in your lab notebook. Submit the aspirin to your instructor in a vial labeled as described in Experiment 1. Isolation of the Unknown Component. Under the hood, evaporate [OP-19] the solvent from the dichloromethane solution while heating [OP-7] it in a warm water bath. Discontinue evaporation when only a solid residue remains in the vial or when no more solvent evaporates. Let the unknown component dry [OP-26b] to constant mass. Weigh it before you begin Experiment 3. Calculate your percent recovery, dividing the sum of the masses of all components by the mass of Panacetin that you started with. Calculate the approximate percentage composition of Panacetin, based on the total mass of components recovered. (These percentages should add up to 100%.) In your report, be sure to include the information specified in the “Report” section of Experiment 1 and any other information requested by your instructor.
Exercises
1. (a) Describe any evidence that a chemical reaction occurred when you added 6 M HCl to the solution of sodium acetylsalicylate. (b) Explain why the changes that you observed took place. 2. Describe and explain the possible effect on your results of the follow- ing experimental errors or variations. In each case, specify the compo- nent(s) whose percentage(s) would be too high or too low. (a) After adding dichloromethane to Panacetin, you didn’t stir or shake the mixture long enough. (b) During the extraction, you failed to mix the aqueous and organic layers thoroughly. (c) You mistakenly extracted the dichloromethane solution with 5% HCl rather than 5% . (d) Instead of using pH paper, you neutralized the solution to pH 7 using litmus paper. 3. Although acetanilide and phenacetin are not appreciably acidic, aceta- minophen (like aspirin) is a stronger acid than water. What problem would you encounter if the unknown component were acetaminophen rather than acetanilide or phenacetin, and you extracted the aspirin with 5% NaOH? Explain, giving equations for any relevant reactions. 4. Acetaminophen is a weaker acid than carbonic acid but aspirin is a stronger acid than carbonic acid. Prepare a flow diagram, like the one in this experiment, showing a procedure for separating a mixture of sucrose, aspirin, and acetaminophen.
(H 2 CO 3 ),
NaHCO 3 NaHCO 3
NaHCO 3
Waste Disposal: Unless your in- structor directs otherwise, pour the filtrate down the drain.
A Greener Way: You can recover the dichloromethane by evaporating it under vacuum using a cold trap (see OP-19) and placing it in an appropriate waste container.
If it is quite impure, the unknown may remain liquid after all of the solvent is removed. It should solidify upon cooling.
54 Part I Mastering the Operations
Observe and Note: Can you detect any evidence for a chemical reac- tion upon adding HCl? If so, de- scribe it in your lab notebook.
Experiment 2 Separating the Components of “Panacetin” 55
5. Write balanced reaction equations for the reactions involved (a) when aspirin dissolves in aqueous , and (b) when aspirin is pre- cipitated from a sodium acetylsalicylate solution by HCl. Assuming that both reactions are spontaneous under standard conditions, label the stronger acid, stronger base, weaker acid, and weaker base in each equation.
Other Things You Can Do
(Starred items require your instructor’s permission.)
*1. Carry out Minilab 2 in Part III to help you visualize what happens during an extraction.
2. Write a short research paper about the chemistry and physiological effects of analgesic–antipyretic drugs using such sources as the Kirk– Othmer Encyclopedia of Chemical Technology [Bibliography, A8] and titles from section L of the Bibliography.
NaHCO 3
