Experiment 7
Synthesis of Aspirin
Aspirin is a common drug used to treat minor
pain, such as headaches and arthritis, and has been
around for over a hundred years. It works by blocking a
chemical known as prostaglandin, the build up of which
causes muscles to contract and blood vessels to dilate,
causing pain.*
Aspirin can be synthesized by two simple
components, acetic anhydride and salicylic acid. The
reaction proceeds as:
The main product of this reaction is aspirin,
also known as acetosalicylic acid. The other product is
acetic acid, which is the main component of vinegar. We
will be performing this reaction using an acid catalyst.
After the product is formed, we will recrystallize to get
higher purity and yield. After this, we will use a vacuum
filtration apparatus.
There are two major calculations for this lab,
theoretical yield and percent yield. To calculate
theoretical yield, we need to perform a dimensional
analysis. If our original salicylic acid (for abbreviation,
SA) weighed 1.12 grams, we need to convert this to
moles, and then convert to moles of aspirin (for
abbreviation, Asp), and finally to mass of aspirin.
(1.12 g SA)(1 mol SA/138.13 g SA)(1 mol Asp/1 mol SA)
*(180.17 g Asp/1 mol Asp)
= 1.46 g of Aspirin
This is the amount that we should have obtained if
everything went right.
*Information taken from http://www.aspirin.org/cons02.html
Now, if we only obtained 0.97 grams of aspirin,
how do we calculate percent yield? Percent yield is
similar to percent error, except we do not subtract by our
actual value. So, our percent yield would be:
(actual / theoretical)*100% = (0.97/1.46)*100%
= 66%
This means that we obtained two thirds of what we
should have.
To determine purity of our sample, we will be
determining the melting point. The actual melting
point of aspirin is between 132 oC and 135 oC. We will
also be performing the acid test. We will dissolve some
of our sample in a water/ethanol mixture, and add a few
drops of dilute ferric chloride solution. It should react
with the aspirin and turn it violet.