[XiLaii]

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 x

[moniczka]

hiya.
sorry I can't help you but I was just wondering if you could help me. the thing is, that my next big assignment is about separating paracetamol and coffeine using HPLC system. I've never used it before so I find it a little difficult. I was thinking about aspirin too so if you could give me some advice, information, maybe tips where can i find more information I could use in my report (of course in library but maybe somewhere else) I would be very gratefull. thanks

[Train]

I think it has to do with all those methyl groups on the one, as well as the respective willingness of each to engage in hydrogen bonding with your mobile phase.

Also I would guess the paracetamol is probably more susceptible to pH effects than caffeine.