Aspirin Synthesis Lab - What chemicals are present in each step?

Question

Aspirin is prepared from salicylic acid via an esterification reaction, which involves forming an ester (ASA) from an alcohol (salicylic acid) and carboxylic acid derivative (acetic anhydride), in the presence of a catalyst (H₂SO₄).

 

The process involves forming solid crystals, filtration, mixed solvent recrystallization and then filtration again to purify the sample.

 

I'm trying to determine which relevant chemicals are present in each step. This is what I think so far:

 

Solid crystal I: crude aspirin, salicylic acid

Filtrate I: salicylic acid

Solid crystal II: Aspirin, salicylic acid

Filtrate II: Ethanol, salicylic acid

 

I'm not sure if Ethanol would be present in the second filtrate, but it is used along with water to recrystallize the sample.

I'm also wondering if acetic acid would be present in any of the steps from a decomposition of aspirin?

 

Thanks for your help!

 

Answer 1

Hello Haley

 

Salicylic acid + Acetic anhydride + H₂SO₄ (catalyst)  -->  Acetylsalicylic acid (ASA, aspirin) + Acetic acid

 

Assuming that salicylic acid is the limiting reagent, and that it reacts completely with acetic anhydride in the first step, the solid formed will only be crude ASA.  The filtrate will contain the by-product acetic acid and catalytic amounts of sulfuric acid.  If excess acetic anhydride is used in Step 1, then it will decompose to 2 moles of acetic acid when water is added to aid in the first filtration.

 

When the aspirin is recrystallized with ethanol-water and filtered (assuming again that there is no leftover salicylic acid), the solid will be pure aspirin while the filtrate will contain the 2 solvents, ethanol and water.

 

Hope the above is helpful.