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Recently had a lab experiment to synthesize aspirin. I was instructed to add 4 drops of concentrated sulfuric acid but misread the "4 drops" as "4 ml". Ended up with a yield of just 10 %. But the crystals that I obtained upon recrystallisaion weren't of aspirin but of salicylic acid (as confirmed by a melting point of 160 °C).

Did the addition of excessive sulfuric acid affect this in any way? What could have caused salicylic acid crystals to be produced instead of aspirin?

Some technical details of the steps I took:

  1. mixing 2 g of salicylic acid with 5ml of acetic anhydride

  2. 4 ml of sulfuric acid added as catalyst (should've been 4 drops)

  3. Heated the mixture in a water bath for 15 min.
  4. Added 1 ml of cold water slowly then added 20 ml of cold water again quickly and stirred for 5 min.
  5. Allowed solution to cool at room temp.
  6. Crude product was dissolved in 5 ml of ethanol. (I noticed that my crude product was able to dissolve immediately whereas my peers' had to heat theirs and took awhile to dissolve, any reason for this?)
  7. 30 ml of hot distilled water was added to the mixture and heated on a hotplate.

  8. Allowed solution to cool to room temperature then placed in an ice-bath for 15 min.

  9. Crystals collection by suction filtration.

  10. Crystals were dried in the oven at 100 °C for 15 min.

  11. Crystals placed in desiccator for 10 min.

  12. Melting point of crystals were found to be 159–161 °C

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marked as duplicate by Loong Nov 23 '16 at 12:43

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Did the addition of excessive sulfuric acid affect this in any way? What could have caused salicylic acid crystals to be produced instead of aspirin?

The intended reaction was the formation of an ester from a a phenol and an anhydride.

However, with an excess of sulfuric acid, acidic hydrolysis of the product according to an $\mathrm{A_{Ac}2}$ mechanism takes place.

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  • $\begingroup$ i remember doing the same synthesis but used glacial acetic acid not H2SO4 and still ended up with salycylic acid crystals ? @Klaus Warzecha $\endgroup$ – Technetium Nov 23 '16 at 10:02

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