Reading and Other Preparation to Be Done Before Lab
Always read the background information and techniques book sections, watch videos of any starred techniques, and prepare your notebook before lab.
Techniques (online videos): The webpage associated with the Techniques book has videos on Handling of Waste (1.1), Greasing Ground Glass Joints(4.1*), Refluxing(7.1*), Vacuum Filtration(9.2*), Packing a Melting Point Tube(14.1*), Recrystallization(15.1*), and How to Prepare an NMR Sample. (22.1)
Notebook Preparation: Since this is a preparation, or synthesis, lab you must have a reaction equation with structures in your notebook for both the conversion of methyl salicylate to the dianion of salicylic acid and for the conversion of the dianion to salicylic acid (see the middle of page 8 of the loose-leaf pages). You need appropriate physical and hazard data for all of the substances used in the lab. Then you need to have the procedure written in the notebook. Please don't copy the purpose from the handout. This lab is the one used for the synthesis Lab Notebook Sample.
Practice Problems: Techniques: page 21 (Chapter 1) - 1, 3; pge 32 (Chapter 2) - 5; page 39 (Chapter 3) - 1, 3; page 51 (Chapter 4) - 1-5; page 72 (Chapter 5) - 1-3; page 86 (Chapter 6) - 1, 2, 6; page 106 (Chapter 7) - 1,2; page 142 (Chapter 9) - 1,4; page 220 (Chapter 14) - 1, 2; page 235 (Chapter 15)- 1, 2, 3, 6. These don't need to be done before lab, but are useful preparation for the lab test.
- You may use either the 5-mL pear-shaped flask, the large conical vial, or the 10-mL round-bottomed flask. Make sure that your reaction vessel and condenser fit together before you put anything in them. If there is a problem, use a different flask or vial (the fit issue is with some of the vials, not the condensers). The flask and condenser connect via a threaded cap and o-ring: see diagram in figure 4.8 (page 49) in the Techniques book. When this is done correctly, the two glassware pieces are screwed together and don't have to be clamped or secured individually..
- Use stopcock grease on the connection between the flask and condenser. It is necessary for this reaction because one of the reactants is a strong base that can etch the glass and fuse the two pieces together ($$$).
- Don't rely on the volume of methyl salicylate to determine an accurate mass. You must weigh it. Really. The syringe and pipete method for measuring liquids (figure 5.8, page 60) is reasonably accurate, but not ideal for a limiting reagent. Volumetric pipets are often worse. Thus, proceed as follows: weigh the reaction flask resting in the smallest beaker you have (flasks don't sit well on their own) so you have the mass of flask plus beaker, tare it if you want, remove it from the balance pan, add 0.20 mL of reagent (*), place it back on the balance pan, and determine the weight. Don't worry if it's not exactly 230 mg (0.230 g); within 10% is OK. (*)Measure the 0.20 mL using the syringe and pipete method with a 1-cc syringe or add a couple of drops to the tared flask, reweigh, and then add more as needed to bring it to the correct mass. It is critical to weigh the salicylic acid into the reaction flask, as opposed to in another container and then transferring it: the amount is tiny and much of it will be left behind in the first container. Always read all digits off the balance.
- Heating options (in order of preference)
- Aluminum block on top of a heating mantle. The pear-shaped flask fits nicely into one of the openings and the round-bottomed flask fits OK. Don't use your glass thermometer to monitor the temperature of the block ask they tend to break off ($$$): we have metal-dial thermometers that fit into a hole on the side of the block, but most of them don't work and serve largely as handles. Or, don't worry about monitoring the temperature--basically the reaction mixture has to boil, so just heat it to boiling and maintain.
- Ceramic heating mantles (small size only) with sand or just air heat (which seems like it won’t work, but does).
- Remove the boiling chips (boiling stick chopstick method, forceps or decanting the solution away from the chips) before you add the acid as it's easier to remove them from a liquid than a mixture of liquid and a lot of solid.
- Transfer the reaction mixture to a small beaker before you add the sulfuric acid (second paragraph on page 12) as it nice to have a larger container and makes it easier to remove the solid that's obtained. A volume of 0.5 mL is 8-10 drops from one of the Pasteur pipets if you want to estimate the volume that way.
- Weigh the crude product, even though it's probably wet with solvent. Save a tiny amount of it (enough for a melting point). Why? If your recrystallization goes awry and you end up with no purified product, at least you have a crude weight to use to calculate your percent yield and some sample for a melting point. Better than nothing. You're not likely to be penalized grade-wise if your crystallization doesn't work (possibly bad luck), but you are if you forgot to save some of the crude product (definitely bad planning).
- We have 10-mL Erlenmeyer flasks for the recrystallization! Check one out from the stockroom if one is not in your locker. The product tends to get lost in larger flasks. Never use a beaker for a recrystallization.
- Transfer the crude product directly to the 10-mL Erlenmeyer flask.
- We can do the recrystallization by heating directly on a hot plate (or Al block) since water, the recrystallizing solvent here, is not particularly volatile. That will not be the case with all solvents.
- Use a boiling stick, rather than a stone, for any recrystallization as it's easier to remove.
- Initially add just enough water to just cover the crude crystals (not necessarily 2 mL). If you add too much water relative to the amount of crude crystals, you may never see your product again. Heat up that mixture and then add more until the solid just dissolves. It's a good technique to have a container of heating solvent on the hot plate so you don't have to wait for the recrystallizing mixture to heat up after each solvent addition.
- We will weigh the purified product and determine its melting point the next week so you don't have to dry it in an oven. It's OK to leave a solid product in your locker for next time. Get it off the wet filter paper, put it in a (preferably weighed) beaker, and label it. Don't cover it--you want the solvent to evaporate. If you're nervous about it escaping or getting attacked, use a KimWipe secured with a rubber band on the top of the beaker.
- Cleanup - boiling sticks and chips go in solid waste container, filtrates go in inorganic waste, methyl salicylate and salicylic acid go in organic waste.
- Melting points, weighing and NMR analysis will be done the following week. You don’t need a procedure for the NMR. If you haven’t taken a melting point before you should have a brief (a few lines) procedure for that. We use the Mel-Temp apparatus. We won't take an IR.
- Link to carbon-13 NMR spectrum of methyl salicylate | Link to better-looking spectrum but without peak assignments.
- Cleanup. Used melting point capillaries go either in the dedicated mp capillary waste container in Sci 303 or in the broken glass container. Your instructor will collect your final product in a communal beaker.
Watch the plagiarism videos before preparing your lab report.
Write your name and lab day or section in the top portion of the first page of your notebook pages. The information below can go on the same page as the data collected during the second lab period—the mass and melting point of the purified salicylic acid.
Calculations. Show calculations for the determination of theoretical and percent yield in a labeled section.
NMR Spectrum and Chemical Shift Table
The spectrum of the product will be distributed in class.
Draw a representation of the carbon NMR spectrum in your notebook. It does not need to be super-detailed, beautiful or perfect.
Prepare a Chemical Shift Table in your notebook with the following three headings: Chemical Shift, δ, ppm, in Methyl Salicylate | Chemical Shift of Corresponding Peak, δ, ppm, in Salicylic Acid | Type of Carbon The "Peak, δ, ppm" values are read from the spectrum: they are the numbers that are printed vertically under each peak. Round the chemical shift values to the nearest 0.1 ppm and put them in order of decreasing chemical shift (largest ppm value at the top of the table). Do not include peaks attributed to solvent in the table. Assume that the chemical shift of a given carbon doesn’t change too much between methyl salicylate and salicylic acid when determining which signal in the product corresponds to a given signal in the reactant. Possibilities for the Type of Carbon column are: carbonyl, methyl, or aromatic. Draw the structure of the product, salicylic acid, next to the table. Do not follow the instructions in the Presenting Spectral Data in Lab Reports handout.
The table should look something like this, but with more rows:
This portion of the report will be typed and turned in via Turnitin. Please see Dr. Kline's Turnitin handout for additional information about Turnitin. Each boldfaced section name should be included in the report.
The first two lines of the report are your name and the title of the lab.
Use just the numbers in front of each of the items below in your report. Adding the descriptions will increase your percent originality score on Turnitin.
The data should be presented as an ordered (numbered) list.
- Mass of methyl salicylate used:
- Theoretical yield of salicylic acid:
- Volume H2SO4 added, with units (drops or mL):
- Mass of crude salicylic acid obtained:
- Volume of water used as recrystallizing solvent:
- Mass of purified salicylic acid:
- Percent yield of purified salicylic acid from reaction:
- Melting point of purified product:
- Name of NMR solvent used and its chemical shifts:
Write in paragraph form, not with bullet points or numbers. Conclusions should be written in the past passive tense and avoid the use of personal pronouns.
- Start with a statement that mirrors the original purpose and includes the percent yield obtained. You just want to state the percent yield, not the actual yield in grams and definitely not the amount of reactant used. The expected yield indicated in the Instructor's Guide for this lab is 65-70%.
- Discuss how it is known that the purpose of the experiment (to make salicylic acid) was met: how the melting point helps to identify the product and assess its purity, how the NMR does the same and any other evidence for product formation. For the sake of the reader’s sanity, it is best to discuss the melting point and NMR data separately rather than jumbling them together. When writing a conclusion treat any handed-out spectrum as if it was taken of the material that you isolated from the experiment.
- If the yield seems unusually low or quite a bit was lost during the recrystallization you should attempt to justify how this happened. Keep in mind that there is no necessary correlation between yield and purity: a high yield doesn’t infer a pure compound. In fact a yield that seems too good to be true may indicate a product that contains impurities that contribute to its apparent mass.
Don't retype the questions. Just number your answers. Do not restate the question as part of your answer.
- How many signals would be present in the NMR spectrum of acetylsalicylic acid (structure link)? If you were given a carbon NMR spectrum that you knew was of either methyl salicylate or acetylsalicylic acid, what is one way that you could quickly determine which compound it was by looking at the spectrum?
- Why is it important to use the largest possible temperature range (highest when dissolving, lowest at end when recovering crystals) when doing a recyrstallization. One of the practice problems in the online lab may be of use here.
- Why is it important to perform a recrystallization using a minimum amount of solvent?
- Which solvent, A, B, or C, would be best to use in a recrystallization of the compound whose solubility properties are graphed below? Why?
Last Revised 8/4/2015