alexleyenda
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Synthesis of acetylsalicylic acid - strange results
Hi !
I had a project, in class, where I decided to extract methyl salicylate from Gaultheria procumbens' leaves, transform it in salicylic acid, then
transform the salicylic acid into acetylsalicylic acid. ( Yeah, it's not that exciting, but the first project we proposed was refused because of
cancirogenic products :/ ) Something strange happened and even our teacher doesn't really understand...
The first two steps went very well, we ended up with fairly pure salicylic acid according to the melting point and the IR spectrum. However, the final
step didn't work. We heated, in a bath of boiling water, 4 g of salicylic acid with 8 mL of acetic anhydride and 1 mL of 98% H2SO4 for 20 min.
Obviously, the sulfuric did some damage as the solution turned brown, we might have used too much, though that's what was written in the protocol. It
looked like this after the 20 minutes :
We got rid of most of the crap through the filtration and a methanol/water recristallisation... However, here's where it gets strange... The reaction
did not work at all. The IR spectrum was a 96% match with salicylic acid, the melting point was the melting point of salicylic acid, and the point
where it gets really strange : the chromatography. We repeated it twice and got the same results again...
For the TLC, we used 50/50 cyclohexane/ethyl acetate and we tested 1- Lab grade salicylic acid 2- Lab grade acetylsalicylic 3- our product before
recristallisation 4- our product after recristallisation
Our product before recristallisation is a perfect match with lab grade salicylic acid : a large blue (under UV) dot with the same distance travelled
(around 45 mm). Our product after recristallisation matches perfectly the distance travelled and the size of the dot of the lab grade acetylsalicylic
acid : a small dot that travelled around 30 mm. However, the lab grade acetylsalicylic acid's dot is a brownish purple while our final product's dot
is the same blue as salicylic acid's dots (all the colours under UV)... and shall I remind you that the melting point and the IR spectrum of our final
product matches salicylic acid's.
In brief, we do not manage to understand why 1- The reaction did not work, even if the H2SO4 caused some damages, it should not have stopped the
reaction from working? 2-Our final product's dot matches perfectly the size and the distance of acetylsalicylic acid's dot, but it matches the color
of salicylic acid... while the IR spectrum and the melting point tells us it is salicylic acid. Even our teacher doesn't really know why, but he's
"only" a bachelor and doesn't know really much more than what he has to show us, so I thought someone here might understand what happened and could
help me understand ! Thank you for your help 
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DraconicAcid
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It was probably that you had too much sulphuric acid. When we do that reaction (on about half that scale), we use 4 drops of phosphoric acid as a
catalyst. When you add water, the solution is simply too acidic, and the product gets hydrolyzed.
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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alexleyenda
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It makes sense. However there's still the TLC's result that bugs me, why would the dot of our product, identified as salicylic acid by the other
tests, be exactly the same size and travel exactly the same distance as acetylsalicylic acid's one, but have salicylic acid's color?
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kavu
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How old is the lab grade ASA. If it has hydrolyzed and oxidized there might be some off coloring running at similar Rf values. Recrystallizing the
sample and adding acticated charcoal might help you get rid of any such compounds. You can check for leftover salisylic acid with iron(III)chloride
solution. It forms a blue/purple complex with many phenols.
[Edited on 26-4-2014 by kavu]
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alexleyenda
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I'll ask the lab technicians about that, thank you for sharing these possibilities
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Metacelsus
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Acetylsalicylic acid is blue under UV as well (I have personally observed it).
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alexleyenda
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Then the lab grade ASA we had is definitely too old and damaged, which makes sense considering there is only one classe of "organic chemistry 2" where
I go and I know that we are the only class that uses this compound as i passed all the other classes and we didn,t use it :p Thank you for pointing
that out !
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kmno4
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SA: 0,029 mole
Ac2O: 0,082 mole
H2SO4: 0,018 mole
These are very bad proportions.
Use proportions 1,05 mole of Ac2O per 1 mole of SA.
Below 10g amount of SA you can use 1 drop of H2SO4, but ~20 mg would also work.
ps. check quality of your Ac2O, just to be sure...
Слава Україні !
Героям слава !
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alexleyenda
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Well all the protocols I found suggest the use of a generous exces of anhydride. The amount of H2SO4 used however was far too much and is due to an
error I made. Sadly the whole project is over, and they closed the labs yesterday until the next session, so it's too late to try to change anything
or go back check the anhydride. Anyway, preparing this project from nothing and the remarks all of you made allowed me to learn a lot, it was great
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