2-Acetoxybenzoic Acid (C9H8O4)

2-acetoxybenzoic acid has many names including acetylsalicylic acid, acetol, O-acetylsalicylic acid and entrophen. This page shows how I extracted 2-acetoxybenzoic acid from common aspirin tablets. I'm currently using 2-acetoxybenzoic acid in the synthesis of another compound (details coming soon).

A standard tablet is usually made up of 2 parts. The first part of the tablet is the chemicals required to perform the specific task (e.g. pain killers). The second part of the tablet is the binder which simply binds the chemicals from the first part together. The binder also gives the tablet its shape.

In the case of aspirin the first part of the tablet is 2-acetoxybenzoic acid, which is what we are after. The binder in an aspirin tablet is dependent on the manufacturer, but a common aspirin binder consists of potato starch, lactose (C12H22O11) and talc (H2Mg3(SiO3)4). The aspirin tablets I'm using each have 300mg of 2-acetoxybenzoic acid. The weights of the chemicals in the binder are unknown, but the wieght of each tablet is around 340mg, so the total binder weighs approximately 40mg.

The soluability of 2-acetoxybenzoic acid is shown in the following table.

Alcohol5 - 7
Chloroform  17

The solubility table shows that 2-acetoxybenzoic acid is fairly soluable in alcohol but quite insoluable in water. These 2 facts can be used to our advantage when trying to seperate the 2-acetoxybenzoic acid from the binder. The following table shows the solubility of the chemicals in the binder.

Potato StarchInsoluableInsoluable

So to extract the 2-acetoxybenzoic acid we can mix the aspirin tablets with an alcohol and then filter out the insoluable binder.

There are lots of different ways to do this type of experiment, each with advantages and disadvantages. The following sections show some of the different approaches that are available.

In the following experiments large amounts of alcohol are evaporated and heated to boiling point. This is extremely dangerous and should NOT be attempted by anyone. The fumes are poisonous, flamable and even explosive under the right conditions. The dangers of the fumes rolling across surfaces and causing flashbacks are very real. When working with alcohol adequate ventilation MUST be used. NEVER work with alcohol indoors. The correct fire fighting equipment should always be at your side and you should always try and remain a safe distance from the setup. NEVER use an open flame to heat the alcohol (I really can't emphesize that enough). I wouldn't give this warning unless it was absolutely necessary.

Experiment 1 - Promoting Crystal Growth

In the first experiment crystal growth is promoted by adding ice cold water is added to the filtered alcohol to promote 2-acetoxybenzoic acid crystal growth. The crystals are then filtered out. Some losses are expected with this method as adding the cold water to the filterate won't promote 100 percent crystal growth. If a bigger final yield was desired then after filtering the binder out the filterate would have the alcohol evaporated (i.e. no ice cold water would be used).

100ml of isopropyl alcohol is heated to boiling point.
10.7g of finely powdered 300mg aspirin is added. The solution is then vigorusly stired for a couple of minutes.
Once the aspirin looks to have stopped disolving the solution is filtered. The binder should be left behind in the filter paper whereas the 2-acetoxybenzoic acid passes through with the alcohol.
During the filtering process 2-acetoxybenzoic acid crystal can be seen growing on the funnel.
The filterate then has 200ml of ice cold water added to it. The complete solution is then put in an ice bath. 2-acetoxybenzoic acid crystals can be seen form almost instantly.
A think layer of crystals form on top of the water and smaller columns appear at the bottom.
The crystals are then filtered out of the water and allowed to dry. You could dry them in an oven but you would need to pay attention to the temperature. They dry quite quickly in the air.
After drying you are left with the 2-acetoxybenzoic acid crystals. On this occasion the final product weighed 4.7g which is around half of the original aspirin's 2-acetoxybenzoic acid. Such a low return was expected due to using the ice cold water to promote crystal growth. Also quite a lot of 2-acetoxybenzoic acid was left on the beakers and filter paper.

Experiment 2 - Evaporation

In the second experiment I boil the filtered alcohol down to leave the 2-acetoxybenzoic acid. This method should result in a much greater yield than the first experiment.

This experiment uses twice as much aspirin as the first one, so 200ml of isopropyl alcohol is heated to boiling point.
Once the alcohol is boiling 21.9g (64 tablets) of 300mg powdered aspirin is added. The mixture is stirred for several minutes.
Once the aspirin looks to have stopped disolving it is filtered to remove the binder.
Quite a lot of residue is left in the original beaker. This can be collected with some extra alcohol and added to the liquid in the filter paper. If you do this though it's best to always use warm alcohol, otherwise you'll cool the liquid in the filter paper which could promote early crystal growth.
After filtering the binder is left behind.
The filtered alcohol could be left to evaporate, or if you want quicker results you can heat it up. During the heating process crystals can be seen to grow around the outside of the dish. I find it's best to use the hot alcohol to wash the crystals back into the center of the dish. This makes it easier to collect them later.
As the alcohol evaporates you should notice more crystals forming. You must make sure you don't expose the 2-acetoxybenzoic acid to excesive heat, so you shouldn't boil the alcohol alway down. Instead let the last bit of alcohol evaporate at room temperature.
The final dry product weighed 15.4g. The original aspirin tablets contained a total of 19.2g of 2-acetoxybenzoic acid. A return of 15.4g is pretty good as some 2-acetoxybenzoic acid was behind in the glassware.