Hiya. I have what I think should be one of those Simple questions that's making me tear my hair out because I'm not seeing something obvious. Here goes:
In the lab recently, we ran a Reversed Phase HPLC to find the concentration of an unknown solution containing Caffeine and paracetamol. Experiment went well, Worked out the concentration and such: now I'm just working on the theory that goes with it, and my problem is this. Under the conditions we used (Octadecylsilyl column, methanol/water mobile phase) Caffeine had the much larger retention time (almost double that of paracetamol). After drawing out the structures and trying to get my head around the polarity of both of them, I can only come to the conclusion that I think that's backwards.
Just looking at the amount of N's and O's in each of them would suggest that Paracetamol would be less polar, and therefore have the longer retention time in the column: but practically that's exactly what didn't happen. :S.
Strip away the confusion and my question is essentially, why is Paracetamol more polar than caffeine? What's the obvious thing I'm missing here. XD.
(And so you don't have to go hunting, Caffeine structure: http://a0.vox.com/6a00c2251d92a98fdb00cdf7e673d0094f-500pi
And Paracetamol Structure: http://www.chemdev.de/blog/wp-content/uploads/2007/10/paracetamol.png
Thanks in advance for anything you can offer