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Lab Handouts and Lab Report Formats
CH342-02, Fall 2006
Dr. Hathaway
Look at these before you come to lab, to be sure you catch any changes in the procedure, or things you need to make sure you do during the lab.
Melting Point Determination
Melting point (although more accurately it should be called melting range) is a fundamental physical property of a substance. A pure substance should have a melting range of one degree of less. The melting range should also be close to the reported value in the chemical literature. Impure substances usually have wide melting ranges (over several degrees), and generally melt lower than the reported value.
There are several techniques and pieces of apparatus that can be used to take a melting point. We will be using the Meltempâ apparatus. Although rather simple-looking, this apparatus costs over $300 to replace now, so handle it carefully!
The melting point of a substance is determined in the following manner.
1. Introduce a small amount (about 1-2 mm) of dry, finely powdered crystals into a capillary tube. Be sure that the crystals pack tightly. This can be accomplished by tapping the closed end of the capillary tube on the benchtop.
2. Place the capillary tube (closed end down) in the slot of the Meltemp®. Be careful not to break the tube in the apparatus. Notice that the Meltemp® can hold three tubes at once.
3. Heat the sample and record the melting range. This range starts when the sample just begins to liquefy, and ends when the sample has totally liquefied. You will have to look in the eyepiece and watch the sample. When the sample just begins to liquefy, you will have to look up at the thermometer and record the temperature. You then look back down at the sample and watch it until it has totally liquefied, then look back up at the thermometer, and record this temperature.
The rate of heating is critical for obtaining a good melting point. If you heat the apparatus too fast, you may miss the end of the melting range, because the sample will have completely melted before you can look back down at it. Therefore, you should set a rate of heating of no more than about 5° per minute while your sample is melting. This is done by adjusting the voltage appropriately.
For example, let’s assume you have a sample that melts at 150°. Looking at the table of heating curves, you can see that some voltage settings would not be appropriate. Voltages of less than 40 would never reach 150°. Voltages of 80 or higher would have the temperature increasing so rapidly, you would miss the melting point. A voltage in the range of 50-60 would have a reasonable rate of temperature increase, and would probably work well. Notice that at 50 volts, it would take approximately 12 minutes to reach 150°.
So what do you do if you don’t know what the melting point is supposed to be? One approach is to start with a reasonably low voltage (such as 40), then gradually increase the voltage to maintain a level of heating of about 5° per minute. The disadvantage of this approach is that it will probably take a long time if the melting point is >150°. A second approach would be to take a melting point at a high voltage, such as 90, first. This melting point would be inaccurate, but would give you a rough idea of what range you are looking for. You would then go back and take a second melting point, using a more appropriate voltage setting. The disadvantage of this approach would be having to use two samples, and the need to let the Meltemp® cool back down to about 20° below the melting point before you could use it again. Of course, if another Meltemp® is available, you could just use it.
Precautions:
1. The heating block of the Meltemp® gets very hot (surprise!). Therefore, do not touch it with your fingers or nose (yes, your nose can get close to it) when taking a melting point.
2. Check the temperature on the thermometer before you insert your capillary. If the temperature is higher than the melting point of your sample, it will melt immediately, and you will have to prepare another sample.
3. Don’t break off the capillary tubes in the Meltemp®. If you do, or if you find broken tube pieces in a Meltemp®, please notify your instructor so it can be cleaned out. Broken tubes reduce the efficiency of the apparatus.
Melting Point Calibration
Your Name:______
Number of Melting Point Apparatus Used:Standard / Literature Melting Point (ºC) / Your Melting Point Range (ºC)
2,6-Dibenzylidine-4-methylcyclohexanone
/ 99
2,6-bis(4-Methoxybenzylidine)cyclohexanone
/ 159
2,5-bis(4-Methoxybenzylidine)cyclopentanone
/ 212
Using either graph paper or Microsoft Excel, for each standard plot the literature melting points on the Y axis, and the midpoint of your melting ranges on the X-axis. Draw the best straight line through the points (DO NOT CONNECT THE DOTS!) Determine the slope of the line and the Y-intercept. These values will allow you to correct melting points you take on this MelTemp apparatus, assuming that the melting ranges you take today are good. Include your graph with this report.
Question:
Redraw 2,5-bis(4-methoxybenzylidine)cyclopentanone in another sheet of paper. Circle and label all of the functional groups in this molecule. See chapter 2 of your lecture text to help you.
Modifications to Panacetin Experiments 2 & 3
1. You will be given an unknown sample. Weigh it and use all of it.
2. For “Separation of Aspirin” on the middle of page 32, we will be using 5% sodium bicarbonate solution (NaHCO3), instead of 1M sodium hydroxide. Sodium hydroxide sometimes reacts with the ester group of aspirin.
3. For “Isolation of the Unknown Component” on the bottom of page 32, we will do the following:
a. Weigh your clean and dry 100 mL round-bottomed flask.
b. Transfer your dichloromethane layer form the separation of Aspirin to the round-bottomed flask.
c. Put a label on your cork ring with your name on it.
d. Take your round-bottomed flask and the cork ring to the rotary evaporators in the back of the room, and have your dichloromethane removed. If the rotary evaporators are in use, leave them there.
e. When your dichloromethane has been removed, reweigh the flask to determine the mass of the unknown.
4. After you have isolated the unknown in step 3 above, scrape it out into a 125 mL Erlenmeyer flask. Any remaining unknown can be removed from the flask by rinsing the flask with a few mL of boiling water.
5. Be sure to allow your aspirin and unknown to dry thoroughly before you weigh them and determine their melting point ranges.
Lab Report Format for Separating the Components of “Panacetin” (2) and Recrystallization and Melting Point Measurement: Identifying the Components of “Panacetin” (3).
1. Title Page
a. A descriptive title with between 15-25 words.
b. Dates the experiment was performed.
c. Course and section numbers.
d. Your name
e. Tape your TLC plate to the bottom of the page. Clearly label each column of spots on the page so I know what it is.
2. Body of the report (start a new page)
a. Panacetin sample number.
b. Weight of initial panacetin sample.
c. Weight of sucrose collected.
d. Weight of aspirin collected.
e. Melting point range of aspirin.
f. Weight of unknown compound isolated in Expt. 2.
g. Weight of recrystallized unknown compound in Expt. 3.
h. Melting point range of recrystallized unknown compound.
i. Mixture melting point ranges of the unknown compound with acetanilide and with phenacetin.
j. Show your calculations of the percent recoveries of sucrose, aspirin and your unknown (divide the weight of each solid by answer in b., ´ 100%).
k. Calculate Rf values for all of the spots on your TLC plate. Show your work.
l. What was the identity of your unknown? Explain in detail how you determined it, comparing the data you obtained with known values. Explain any discrepancies.
3. Questions
a. Where in the separation procedure could you lose one or more of the Panacetin components? Explain which component could be lost in which step. Ignore trace amounts left in containers, or blatant spills.
b. In the separation scheme, the dichloromethane solution is extracted with sodium hydroxide. What would happen if the dichloromethane solution was extracted with hydrochloric acid instead (which has actually happened)? Would the separation scheme still work? Explain your reasoning.
c. Acetaminophen (the active ingredient in Tylenol) is insoluble in water, soluble in sodium hydroxide solution, insoluble in hydrochloric acid, and insoluble in dichloromethane. If your Panacetin sample contained acetaminophen, where would it be separated in the separation scheme? Explain your reasoning.
Thin-Layer Chromatography (TLC) Of Recovered Panacetin Products
In this experiment, you will be assessing the purity of your crops of recrystallized Panacetin unknown, as well as your aspirin. Your unknown is either acetanilide or phenacetin. Since aspirin, acetanilide, and phenacetin show up under UV light, we can easily detect them with TLC.
Acetanilide Phenacetin Aspirin
Procedure:
Prepare two spotting capillaries out of open-ended capillary tubing, as shown in the pre-lab discussion. Do not obtain any solvents until everybody has finished with this part of the experiment. Obtain a TLC plate, and draw a light pencil line across the width of the plate, about 1 cm from the edge. Place 5 light “tic marks” on the line, approximately equally spaced.
In separate test-tubes, dissolve tiny amounts of your aspirin and your recrystallized unknown in about 1 mL of acetone. Using one of the spotting capillaries you made, spot a tiny drop of the acetone solution of your aspirin on the left “tic-mark” - you may want to practice spotting on a paper towel or tissue, to make small spots. Allow it to dry, then check the plate under a short-wave UV light, to see if you have a visible spot. If not, spot your solution again on the same “tic-mark”. Do not spot any more times than needed - too much material may give results that are hard to interpret! Using a different capillary, spot the solution of your recrystallized unknown on the second to the leftmost “tic-mark”. After you have your samples spotted, take your plate to the standards in the hood, and spot acetanilide on the middle “tic-mark”, the phenacetin on the second to the rightmost “tic-mark”, and the aspirin on the rightmost “tic-mark”. Do not get the spotting capillaries contaminated! Check the spots under the UV light, to see if they are all visible.
To prepare a developing chamber, obtain a piece of 11 cm filter paper, and fold it about 1 inch from the edge, to obtain a flat edge. Place the filter paper, flat edge down, in your 400 mL beaker. Obtain 15 mL of the TLC solvent, methyl t-butyl ether, and pour it into the beaker, swirling the beaker to wet the filter paper with the solvent.
Carefully place the TLC plate in the developing chamber, spotted side down, trying not to splash the solvent on the plate. The level of solvent must be below the pencil line. Cover the beaker with a watch glass. The solvent will rise up through the stationary phase on the plate. When the solvent has risen to 1-2 cm from the top of the plate, remove the plate, and draw a light pencil line across the plate, at the level to which the solvent rose. Allow the solvent to evaporate (waving the plate in the air will speed this up), then look at the plate under the UV light. Circle all of the spots.
TLC Plate TLC Developing Chamber
You will identify the components of your unknown tablet by comparing the amounts the components traveled up the plate with the amounts the standards traveled. These amounts are reported as Rf (retention factor) values.
X Distance the spot traveled
Rf = --- = ------
Y Distance the solvent traveled
Measure the distance a spot moves from the center of the spot.
A developed plate may look like this:
From the locations of the spots, it would appear that the Your Aspirin (YA) contains Aspirin (A) and acetanilide (Ac). Perhaps you didn’t do a good job of separation in the extraction. Your unknown (YU) only contains Ac. The spot for Ac in the standard looks larger than the spot for Ac in YU: this is probably due to different amounts of material being spotted on the plate initially. The Rf value for the A spot would be X/Y: measure X from the middle of the spot. Separate Rf values are calculated for each of the spots in YA. Since the spots for the materials in the samples and in the standards are different sizes and shapes, they may have slightly different Rf values.
Cleaning Up
When you are finished with the experiment, pour the TLC solvent in the “Recovered TLC Solvent” container. The filter paper may be thrown away in the trash can. Used spotting capillaries should be placed in the “Clean Broken Glass” container. Your acetone solutions you used to spot with may be flushed down the sink with lots of water.
Purification of an Unknown Compound
Solubility Properties Needed for a Recrystallization Solvent
The Material we are trying to purify must be:
Soluble in the hot recrystallization solvent, and
Insoluble in the cold recrystallization solvent.
The Impurities must be:
Soluble in both hot and cold recrystallization solvent, or
Insoluble in both hot and cold recrystallization solvent.
Choosing a Recrystallization Solvent
1. In a medium test-tube, add a small amount of solid, and 1-2 mL of solvent.
2. If it dissolves in cold solvent, it won’t work, so try another solvent.
3. If it doesn’t dissolve in the cold solvent, heat the test-tube up. If the solid dissolves, cool it, and see if crystals form. If they do, it’s a good solvent. If not, try another solvent.