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I want to make aspirin. Normal method is to react salicylic acid with acetic anhydride, but it is extremely hard to find acetic anhydride. How may I synthesize aspirin with acetic acid and salicylic acid only?

I have watched a video where someone makes it with sulphuric acid, salicylic acid and acetic acid. Can I use phosphoric acid insted of sulphuric acid as a catalyst?

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  • $\begingroup$ This seems like the kind of question that could only be answered by searching in the literature. Without trying it out, it's pretty hard to say. However, it does seem plausible and worth investigating or giving it a try. $\endgroup$
    – Raphaël
    Sep 29, 2019 at 9:31
  • $\begingroup$ @Raphaël Is it theoratically possibble that I can use phosphoric acid insted of sulphuric acid. I will try it If it is theoratically plausible. Check this out youtube.com/watch?v=M5dQMvPsTEk $\endgroup$
    – cccccccaaaaccc
    Sep 29, 2019 at 9:35
  • $\begingroup$ Is it not possible to use salicylic acid and acetyl chloride ? $\endgroup$
    – Maurice
    Sep 11, 2022 at 10:13

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I just looked in the literature, and it has been done even without catalyst in a "reactive distillation column" at high temperatures (patent US201113194873). If you can remove the water formed, you can technically drive the reaction forward, even if it is slow. A catalyst will just make that process easier, requiring less heat and time.

Phosphoric acid is a much weaker acid than sulfuric acid, and it does not have the same dehydrating properties. However, if you have a proper setup, it should do the trick. A big problem with phosphoric acid is that the most concentrated you can normally get is 85%, which means you still have to remove 15% of water somehow. If you don't, you'll likely hydrolyse any aspirin formed and get your starting material back.

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  • $\begingroup$ Polyphopshoric acid would be a better option $\endgroup$
    – Waylander
    Sep 29, 2019 at 9:53
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    $\begingroup$ @Raphaël Can I use 99 percent sulphuric acid + salicylic acid + glacial acetic acid because I dont think I can find reactive distillation column $\endgroup$
    – cccccccaaaaccc
    Sep 29, 2019 at 10:33
  • $\begingroup$ @cccccccaaaaccc Yes, that would be ideal. That patent was mostly to make very large quantities. $\endgroup$
    – Raphaël
    Sep 29, 2019 at 10:47
  • $\begingroup$ @Raphaël OK thank you very much. Another question do I need to heat the mixture of sulphuyric acid, salicylic acid and acetic acid maybe to boil the water off. $\endgroup$
    – cccccccaaaaccc
    Sep 29, 2019 at 10:52
  • $\begingroup$ The water will co-distill with acetic acid, you may need to add more anhydrous acetic acid as the reaction proceeds $\endgroup$
    – Waylander
    Sep 29, 2019 at 11:19

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