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May 16th, 2018, 11:26 PM   #1
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Literally panicking - i'm not sure if paracetamol is more/less polar than aspirin

I did a laboratory experiment.

We used reverse phase liquid chromatography:
Mobile phase = water:methanol
Stationary phase: silica

This means that the mobile phase = polar and the stationary phase = non-polar.
The more polar a molecule, the more it will be attracted to the mobile phase, hence will travel through the column faster, hence shorter retention time.

My results from the HPLC was that paracetamol had the shorter retention time and caffeine had the longer retention time.
So based on this alone, Paracetamol is more polar than caffeine.

HOWEVER, i tried searching on the internet just to confirm. and to my horror, about 95% of websites say caffeine is more polar than paracetamol. i also compared the chemical structure of the 2 molecules and caffeine does have a lot more polar groups.
I'm not sure why my results say that paracetamol was more polar.
On the day of the experiment, my teacher looked at our chromatographs and said that they look correct.

please help me! i feel so stressed right now due to all these exams and now these experimental results are just confusing me so much.

Is paracetamol more/less polar than caffeine?
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May 16th, 2018, 11:37 PM   #2
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Update: I'm still panicking, but a bit relieved, but also still confused

I searched 'Paracetamol and caffeine HPLC' into google.

It came up with a bunch of chromatographs that were the SAME as mine. Yay! They also used a polar methanol mobile phase.

But i'm still confused why the structure of caffeine looks more polar than paracetamol. Shouldnt this mean that caffeine should have the shorter retention time? It doesnt make sense
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May 17th, 2018, 05:12 AM   #3
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Update 2: I scoured the internet and it seems this issue has been encountered by many people over the years. but i didn't find an answer why
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