Oil of Wintergreen

This post is the video companion to this video.

Today's experiment will be making a minty-scented oil for the winter time: methyl salicylate, or oil of wintergreen. This can be made simply from two very common and easy to obtain chemicals, and with a very simple procedure. This is also one of my first forays into organic chemistry!

Materials

• 13.0 g aspirin, C9H8O4 (the actual tablets will weigh more!)
• 60 mL methanol, CH3OH
• 8 mL concentrated sulfuric acid, H2SO4
• Distilled water 

Reaction

1 mol aspirin + 2 mol methanol --> 1 mol methyl acetate + 1 mol methyl salicylate

The two reagents we will need are ordinary aspirin and methanol. My source of methanol is HEET antifreeze, which is almost entirely the alcohol. Make sure it is not the red version, Iso-HEET, which is isopropanol.

The first step is to crush the aspirin tablets into a powder. I'm using 40 tablets, and each has 325mg of aspirin. This equates to 13 grams of aspirin. The weight of the tablets themselves will be greater than this, because each pill contains binders and other inactive ingredients. These will not dissolve in any of our solutions, but they do not interfere with the reaction and can be ignored. Filtration is not necessary.

The crushed aspirin tablets are combined with 60mL of methanol in a conical flask, and swirled to dissolve as much as possible. To this mixture, 8mL of concentrated sulfuric acid is added. This only serves as a catalyst for the reaction, so this amount might actually be a bit overkill.

This final mixture is then placed on a hot water bath at 60 C for at least one hour. I've done this before and the yield is pretty small, so this time I let mine run for 4 hours.

I actually had to take the reaction off heat after two hours to go have dinner, and then resumed heating the next day for the remaining two hours. After this time, the temperature is raised to about 80 C to vaporize most of the methanol. I heated mine until I reduced the volume by about half. At this point, brown blobs of oil began to come out of solution.

I then took it off heating to cool. In the meantime, I prepared the washing solutions for workup of the product. This consists of two portions of 50 mL of distilled water, and a saturated solution of baking soda (3.9 g in 50 mL water). These solutions were placed in the fridge to cool down to near ice cold.

For the workup, we need to separate the oil from the rest of the mixture. This is easily done because while methyl salicylate is very soluble in methanol, it is nearly insoluble in water. Combining this solution with water causes all of the oil to come out of solution, and washes away any water soluble impurities.

When cool, I transferred my reaction mixture to a 250 mL separatory funnel. To this I added one of the 50 mL portions of water, and shook the funnel vigorously for about 1 minute, venting often. The oil is heavier than water, and so will sit on the bottom. I drained this oil layer off, emptied the funnel of the aqueous wash water, and returned the oil to the funnel. I repeated this process a second time with the other portion of water, and then finally a third time with the baking soda solution. It is especially important to vent often when shaking this last mix - as it neutralizes any leftover acid, it produces carbon dioxide that can blow the stopper off!

After allowing to sit and separate for a few hours, I drained my washed product oil off and into a small vial for storage. I obtained 7.3 g of oil. Theoretical yield was 10.98 g, so this equates to a 66.5% yield. This batch of oil was very brown, but previous batches I've made were more of a tan color. This might be due to running the reaction for too long, or too hot, which could decompose some of the oil. It's also possible that interrupting the reaction and letting it sit overnight wasn't good for it either. You can see the comparison below - the oil from this run is on the left and a sample from a previous batch run for a shorter duration is on the right. Much less oil, but it looks more pure.

Finally, the waste rinse water can be neutralized with baking soda and is safe to pour down the drain with plenty of water.

The oil looks a little gross, but smells great! It's perfect for oil burners (the kind with a little dish heated by a candle or something similar), but it should NOT be used for topical or food application. This crude oil likely has unreacted aspirin dissolved in it, and skin contact might lead to a concentrated dose of aspirin well beyond the recommended level.