Design lab - Acetylsalicylic acid (ASA) synthesis

Aspirin causes many side effects, the most common is that it causes bleeding in the lining of the stomach. Exceeding the safe dosage can be fatal as the salicylic acid leads to acidosis due to a lowering of the hydrogen ion concentration in the blood.[1]

Aspirin’s structure is as follows:

The reaction that will occur is the following:

(CH3CO)2O + HOC6H4COOH        CH3CO2C6H4CO2H + CH3COOH

                          acetic anhydride         salicylic acid                    acetylsalicylic acid             acetic acid

Apparatus

• Salicylic acid
• Acetic anhydride
• Sulphuric acid
• Water
• Distilled water
• Ethanol
• Weighing boat
• Digital balance
• Two beakers
• Measuring cylinder
• Two filter papers
• Two funnels
• Two conical flasks
• Stop watch
• Safety mat
• Goggles
• Laboratory coat
• Gloves

Procedure

1.  Using a weighing boat and a digital balance measure three grams of salicylic acid and then    pour them in a beaker
2.  Add them to 6cm3 of acetic anhydride and to eight drops of sulphuric acid
3.  Swirl the beaker to mix properly the solution
4.  Then, position the beaker in a larger one full of boiling water for approximately fifteen minutes
5.  Add twenty drops of freezing water to the warm solution (this will destroy the excess acetic anhydride)
6.  Next, add 20cm3 of freezing water to the flask
7.  Place the beaker in a larger one full of freezing water (this will speed crystallisation)
8.  In the meanwhile, prepare filter paper in a funnel and position them on a conical flask
9.  When crystallisation seems complete add some distilled water and pour the solution in the filter paper
10.  Wait for it to filter all
11.  To purify the product, perform another crystallisation by transferring the crystals in a clean beaker, adding 10cm3 of ethanol
12.  Then, swirl vigorously the beaker and heat it up by placing it in another larger beaker full of boiling water for a few minutes
13.  After crystals have dissolved, add 25cm3 of boiling water to the solution
14.  Let the solution cool down, crystals will reform
15.  As soon as they reappear, place the beaker in another one containing freezing water (this will complete re-crystallisation)
16.  Then, pour the content of the beaker in filter paper which will be already sitting in a funnel and position the funnel on a conical flask
17.  After some hours you will have acetylsalicylic acid

Variables

• Controlled and independent variables

• - Mass of salicylic acid
• - Volume of acetic anhydride
• - Volume of phosphoric acid
• - Volume of ethanol

• Dependent variables

• - Freezing water

Results

Physical data

[1] Chemistry, Geoffrey Neuss, Oxford University Press, 2007