Synthesis of salicylic acid
This page contains syntheses of salicylic acid. Because these are original works, it is asked that the texts not be edited from their original form except for clarity's sake. Suggestions or improvements can be put in the Notes section below their respective synthesis.
No Tears Only Dreams Now's synthesis from aspirin tablets
This section details Sciencemadness member No Tears Only Dreams Now's synthesis of salicylic acid from store-bought asprin tablets containing acetylsalicylic acid.
Preparation of salicylic acid
Salicylic acid, a useful carboxylic acid, can be easily prepared from acetylsalicylic acid(aspirin) with a very high yield. The synthesis begins with extracting ASA from aspirin tablets; if you already have purified acetylsalicylic acid, proceed directly to step 6.
1. Begin with any brand or type of tablets containing acetylsalicylic acid, preferentially at a high concentration. Avoid large brand names to lower costs. A common concentration is 325mg per tablet. If your tablets have a colored coating on the outside, you may wish to remove this by placing them in warm water, allowing the coating to dissolve, and then drying them. When you are ready to proceed, crush these tablets using a mortar and pestle until you have a coarse powder.
2. Add the powdered aspirin tablets to your choice of solvent. Recommended solvents are acetone and isopropanol, as lower alcohols will partially esterify with carboxylic acids, resulting in impurities and losses. Add 3-4mL of acetone or 4-5mL of isopropanol per expected gram of acetylsalicylic acid to an appropriate container, and add in your powdered aspirin.
3. Begin to heat the solvent/aspirin tablet mix, dissolving the ASA within until you have reached the solvent's boiling point. At this point, remove from heat, and feel the edges of the container. Once it is cool enough to hold in one's bare hand, check to see if there are any clear, needle-like crystals floating around. If so, you need to begin heating again and use more solvent to ensure that all of the acetylsalicylic acid dissolves.
4. Filter the solution through one or two coffee filters, squeezing the filters into the filtrate afterward to recover some of the liquid.
5.Boil off the solvent(you can recover this in a distillation apparatus if you like, but be aware that some acetic acid may come over if you boil too aggresively); this will leave you with a white crystalline solid composed of partially decomposed aspirin.
6. Add the powdered acetylsalicylic acid obtained from steps 1-5 or otherwise to a clean beaker, and add enough 10% hydrochloric acid solution to cover them completely, plus another few centimeters. Place this beaker on top of a hot plate or other heating source, and place a round-bottom flask full of ice water on top of the beaker. Reflux for 30 minutes at 100 °C.
7. After you are done refluxing and the contents of your beaker have cooled to room temperature, fill the remainder of the beaker with very cold ice water. Stir the salicylic crystals around thoroughly to wash them, removing acetic and hydrochloric acids, and then carefully decant off the liquid and any remaining ice.
8. Assuming you haven't lost too much of your salicylic acid, add about 10 mL of water to the beaker still containing the crystals for every gram of salicylic acid you expect to obtain, as we are going to recrystallize it. Boil the water until you are sure that all of the salicylic acid has dissolved, then slowly cool the beaker, and finally chill it in a refrigerator or ice bath. A huge amount of thin, needle or hair-like crystals will form; this is nearly pure salicylic acid.
9. Finally, filter out your salicylic acid crystals, wash the crystals in the filter once more with a small amount of ice-cold water, and dry the crystals. You now have air-stable crystals of pure salicylic acid! Store them in a glass or plastic container prior to use.
alexleyenda's synthesis from methyl salicylate
Salicylic acid from wintergreen oil (methyl salicylate)
Written and tested with success by Sciencemadness member Alexleyenda.
Wintergreen oil (which is at least 98% methyl salicylate) can be found for a good price in many drugstores.
You need: Wintergreen oil, a 6M solution of NaOH, dilute H2SO4, boiling chips, pH test paper, a filtration set-up, cold distilled water and a reflux set-up.
1- For every 1 mL of methyl salicylate you wish to start with, prepare 10 mL of a 6M NaOH solution and at least the quantity of dilute sulfuric acid necessary to neutralize it.
2- In a reflux set-up, place the methyl salicylate, the NaOH solution and several boiling chips. A white solid should form immediately which is sodium salicylate.
3- Reflux until the solid completely dissolves. At that point, reflux for another 15 minutes, then let it cool to the room temperature.
4- Add some sulfuric acid to the reaction flask. A white solid should form. Add sulfuric acid until no more solid forms.
5- Test the pH of the solution. It needs to be very acidic (pH <1). If it is not, add more sulfuric acid.
6- Filter the crystals
Additional steps and notes:
-DO NOT cool the solution under room temperature. The large excess of NaOH neutralized by the sulfuric acid produces huge amounts of Na2SO4 (sodium sulfate). The salt if very soluble in water at room temperature (around 440 g/L) while salicylic acid is almost insoluble (2,5 g/L). If you cool the solution however, you'll end up with a huge amount of solid Na2SO4 and will have to redilute everything again.
- It is advised to do a recrystallization. Proceed this way: Add your salicylic acid to a flask with the minimum of water it takes to dissolve it at 100°C (its solubility at 100°C is around 77 g/L). Heat the water to its boiling point to dissolve all the salicylic acid, then let it cool to room temperature and filter the nice needle shaped crystals. The recrystallization is very efficient, as salicylic acid is nearly insoluble at room temperature (2,5g/L), barely soluble in hot water (15 g/l at 75°C) and it hits a peak of 77g/L at 100 °C. It really gets rid of all other more soluble compounds with a loss of only nearly 3% by solubility.
- It has been suggested that the reflux steps in the synthesis be conducted at or around the boiling point of water (100°C), which acts as the primary solvent in this reaction.