What is the equation for calculating percentage yield with aspirin made with salicylic acid and pyridine?

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mvcdc eNotes educator| Certified Educator

Aspirin or acetylsalicylic acid is made by reacting salicylic acid with a base. There are various ways to synthesize aspirin (but the method of calculating the percent yield is the same for any chemical reaction). One way is to react salicylic acid with acetic anhydride, a base, in the presence of pyridine as a catalayst. In brief, pyridine hydrogen bonds with the phenolic hydrogen, thus activating it for acetylation.  The chemical equation for this is as follows:

`2C_7H_6O_3 + C_4H_6O_3 -> 2C_9H_8O_4 + H_2O ` , where the chemical species are salicylic acid, acetic anhydride, aspirin, and water, respectively. Note that pyridine is not in the balanced chemical equation since it only acts as a catalyst and is not consumed in the chemical reaction.

Percent yield is a parameter used to determine efficiency of chemical reactions. It is very important in industrial and pharmaceutical chemical industries - in calculating cost-benefit of a reaction and synthesis of a drug, for instance, or in organic chemistry labs in determining the efficiency of their synthesis route. Percent yield is calculated as follows:

%yield = (actual yield / theoretical yield) x 100.

Actual yield is the amount of product empirically produced after conducting the reaction. Meanwhile, theoretical yield is the yield obtained by stoichiometrically calculating the amount to be produced using the chemical equation using the amount of starting materials provided.

------ An example

For instance, if we start with 100 g of salysilic acid (SA for simplicity, and I'll use A for aspirin) (and enough acetic anhydride), we can calculate the expected amount of aspirin produced using the chemical equation above.

mol aspirin produced = 100g SA / 138.121g/mol SA * 2 mol A / 2 mol SA = 0.76 mol of aspirin produced.

This is equivalent to 0.76 mol * 180.157g/mol = 137 g of aspirin produced theoretically.

If you performed the same reaction in a lab and was able to retrieve 100 g of aspirin, your theoretical yield will be: 100/137 * 100 = 73%.