Synthesis of Salicylic Acid from Methyl Salicylate

Reading and Other Preparation

Procedure: Loose-leaf Mohrig Lab Manual, microscale procedure starting on page 11. 

Background: Loose-leaf Mohrig Lab Manual, page 7 through the first paragraph on page 9. The reaction is the first one discussed in section 21.11, starting on page 1015. There is some information on salicylic acid here.

Techniques book: Background and Safety Information in Chapters 1-4, including the Essays starting on page 1 (Role of Lab) and page 41 (Carrying out Reactions); Measurements-Chapter 5; Heating-Chapter 6; Microscale reflux-Section 7.1, with apparatus in Fig 7.1b; Filtration-Chapter 9, and Fig. 9.7a (left),; Changing Solubility with Acid-Base Chemistry (section 10.2); Recrystallization-Chapter 15 with miniscale procedure in section 15.5. Melting points-Chapter 14; Carbon NMR-sections 23.1 and 23.2. (The Techniques book sections given in the  Loose-leaf Mohrig Lab Manual are for a previous edition of the Techniques book.) 

Techniques (online): The webpage associated with the Techniques book has videos on Handling of Waste, Greasing Ground Glass Joints, Refluxing, Vacuum Filtration, Packing a Melting Point Tube, Recrystallization, and How to Prepare an NMR Sample, listed in order of the Technique book chapters. The CU Bolder site has pictorial guides for filtration, recrystallization,  melting point determination,  equipment, and taking NMR spectra. There is a link to the Mel-Temp heating curves under OChem Lab Information to the left. There are videos in OChem.com (in the In the Lab section) on reflux, filtration, recrystallization and melting point determination, but it is impossible to link directly to the individual videos.

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: everything in the box on page 9 plus salicylic acid. Please don't copy the purpose on page 7; not only does this constitute plagiarism, but it's a fairly bad example of one. This lab is the one used for the synthesis Lab Notebook Sample.

Practice Problems: Procedure 1-5 on p 13; 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.

Lab Notes

  1. We will do the microscale version. We will analyze the product by NMR, not by IR, so you may leave out references to IR in your procedure. You need not write down any procedure for the NMR analysis in advance.
  2. 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..
  3. 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 ($$$).
  4. Don't rely on the volume of methyl salicylate to determine an accurate mass. You need to weigh it. Really. The syringe and pipete method for measuring liquids (figure 5.8, page 60) is reasonably accurate, but not ideal for a limiting reagent. Volumetric pipets are often worse. Thus, proceed as follows: weigh the reaction flask resting in the smallest beaker you have (flasks don't sit well on their own) so you have the mass of flask plus beaker, tare it if you want, remove it from the balance pan, add 0.20 mL of reagent (*), place it back on the balance pan, and determine the weight. Don't worry if it's not exactly 230 mg (0.230 g); within 10% is OK. (*)Measure the 0.20 mL using the syringe and pipete method with a 1-cc syringe or add a couple of drops to the tared flask, reweigh, and then add more as needed to bring it to the correct mass. It is critical to weigh the salicylic acid into the reaction flask, as opposed to in another container and then transferring it: the amount is tiny and much of it will be left behind in the first container.
  5. Heating options (in order of preference)
    1. Aluminum block on top of a heating mantle. The pear-shaped flask fits nicely into one of the openings and the round-bottomed flask fits OK. Don't use your glass thermometer to monitor the temperature of the block ask they tend to break off ($$$): 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.
    2. Ceramic heating mantles (small size only) with sand or just air heat (which seems like it won’t work, but does).
  6. 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.
  7. 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.
  8. 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).
  9. Recrystallization
    1. 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.
    2. Transfer the crude product directly to the 10-mL Erlenmeyer flask. We may not have weighing paper.
    3. 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.
    4. Use a boiling stick, rather than a stone, for any recrystallization as it's easier to remove.
    5. 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.
  10. 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.
  11. Cleanup - boiling sticks and chips go in solid waste container, filtrates go in inorganic waste, methyl salicylate and salicylic acid go in organic waste.
  12. 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.
  13. Link to carbon-13 NMR spectrum of methyl salicylate | Link to better-looking spectrum but without peak assignments. 
  14. 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.

Lab Report

Watch the plagiarism videos before preparing your lab report.

Typed Portion

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.

Experimental Data
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.

  1. Mass of methyl salicylate used:
  2. Theoretical yield of salicylic acid:
  3. Volume H2SO4 added, with units (drops or mL):
  4. Mass of crude salicylic acid obtained:
  5. Volume of water used as recrystallizing solvent:
  6. Mass of purified salicylic acid:
  7. Percent yield of purified salicylic acid from reaction:
  8. Melting point of purified product:
  9. Name of NMR solvent used and its chemical shifts:


Conclusion

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.

  1. 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%.
  2. 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.
  3. 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.

Questions
Don't retype the questions. Just number your answers. 

  1. Draw the structure of the form of methyl salicylate present when NaOH is first added to the methyl salicylate, but before heating has commenced. You may draw the structure electronically and paste it into your document or draw it out by hand, take a photo or scan it, and paste the resulting file into your document.
  2. Use the solubility table on page 13 of the loose-leaf pages to answer this question. Show work either by writing it out by hand, taking a photo, and pasting it electronically into this document or in your notebook with a page number reference in this document. Base your calculations on the amount of crude salicylic acid that you isolated. a. Suppose that you dissolved the crude salicylic acid in just enough water to dissolve it at 50°C and then allowed the resulting solution to cool down to 25°C. What is the maximum percent recovery you could obtain, assuming that the "crude" was actually pure salicylic acid? (This does raise the question of why  it was being purified, but that assumption needs to be made).  b. What is the maximum percent recovery possible if the same procedure was followed, but the higher temperature was 90°C and the lower one was 25°C? c. What is the maximum percent recovery possible if the higher temperature was 90°C and the lower one was 10°C?
  3. Why is it important to perform a recrystallization using a minimum amount of solvent at an elevated temperature? Be sure to address both the amount and temperature in your answer. You may use your calculations from question #2 to help to answer this question.
  4. Suppose a compound (mp 150°C) has the following solubility properties. In solvent A, 15 g of the compound dissolves in 100 mL of A at room temperature and 45 g of the compound dissolves in 100 mL of A just below the boiling point of A. In solvent B, 1.5 g of the compound dissovles in 100 mL of B at room temperature and 35 g of the compound dissolves in B just below the boiling point of solvent B. Which solvent would be better for the recrystallization and why?
  5. How many signals would be present in the NMR spectrum of acetylsalicylic acid (structure on first page of loose-leaf pages for experiment)? If you were given a carbon NMR spectrum that you knew was either methyl salicylate or acetylsalicylic acid, what is one way that you could quickly determine which compound it was by looking at the spectrum 

Notebook Portion

Write your name and lab day or section in the top right portion of the first page of your notebook pages if you turn in hard copies.

Calculations. Show calculations for the determination of theoretical and percent yield in a labeled section.

Chemical Shift Table and NMR Spectrum
The spectrum of the product will be distributed in class. Prepare a 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. Paste the spectrum itself into your notebook (you may need to reduce its size in order for it to fit in your notebook), along with a structure of the compound. 

The table should look something like this, but with more rows:

SA Table



Last updated 8.8/2014