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 discussed in section 16.11 of Bruice, starting on page 746. There is some information on salicylic acid here.

Techniques book: Background and Safety Information in Chapters 1-4; Measurements-Chapter 5; Heating-Chapter 6; Microscale reflux-Section 7.1, with apparatus in Fig 7.1b; Filtration-Chapter 10, and Fig. 10.7a,; Recrystallization-Chapter 15 with miniscale procedure in section 15.2. Melting points-Chapter 14; Carbon NMR-sections 22.1-22.2, 22.4. The Techniques book sections given in the  Loose-leaf Mohrig Lab Manual are for a previous edition of the Techniques book. 

Techniques (other sources): 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 lab). 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 fairly bad for this particular lab. This lab is the one used for the synthesis Lab Notebook Sample.

Practice Problems: Procedure 1-5 on p 13; Techniques: page 182 - 1, 2; page 197 - 1, 2, 3, 6. page 399 - 1a, 2. 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 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 4.7 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 ($$$).
  4. Don't rely on the volume of methyl salicylate to determine an accurate mass. You need to weigh it. Really. Volumetric pipets don't do a great job of measuring small amounts of organic liquids. Proceed as follows: weigh the reaction flask resting in the smallest beaker you have (flasks don't sit well on their own) so you have the mass of flask plus beaker, tare it if you want, remove it from the balance pan, add the 0.20 mL of reagent, place it back on the balance pan, and determine the weight. Don't worry if it's not exactly 230 mg (0.230 g); within 10% is OK. You can measure the 0.20 mL using the method shown on page 46 of the Techniques book (Figure 5.8, but using a 1-cc syringe) or add a couple of drops to the tared flask, reweigh, and then add more as needed to bring it to the correct mass. It is critical to weigh the salicylic acid into the reaction flask, as opposed to in another container and then transferring it: the amount is tiny and much of it will be left behind in the first container.
  5. Heating options (in order of preference)
    1. Aluminum block on top of a heating mantle. The pear-shaped flask fits nicely into one of the openings and the round-bottomed flask fits OK. Don't use your glass thermometer to monitor the temperature of the block ask they tend to break off ($$$): 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 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 those out from the stockroom. 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. 

Lab Report

Watch the plagiarm 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
You may use the numbers in front of each of the items below in your report, or the both the numbers and the descriptions. 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. 

  • Start with a statement that mirrors the original purpose and states the percent yield. 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%.
  • Justify how you know that the purpose of the experiment was met: how the melting point helps to identify the product and assess its purity and how the NMR does the same. 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 your yield seems unusually low or if you lost quite a bit 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. Why does salicylic acid exist as the dianion in base? 
  3. Use the solubility table on page 13 of the loose-leaf pages to answer this question. Show work--either type it out or write it out by hand and photograph or scan it, as done for the structure in the first question. 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 75°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?
  4. Suppose a compound (mp 150°C) has the following solubility properties in two solvents. 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?

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: Peak, δ, ppm, in Methyl Salicylate | Corresponding Peak, δ, ppm, in Salicylic Acid | Type of Carbon (aromatic, C=O, methyl). The "Peak, δ, ppm" values are read from the spectrum: they are the numbers under each peak that are printed vertically. Place the structure of the product compound next to 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. Round the chemical shift values to the nearest 0.1 ppm. 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. 

Last updated 1/15/2014 (clarify NMR table?)