Hi Everyone,
Im trying to figure out why there are such large difference in solubility within the given xanthines.
Solubility / pKa*
Caffeine – 21600mg/L water / 10.4 pKa
Theophylline – 7360 mg/L water – 8.81 pKa
Theobromine – 330mg/L water – 10.4 pKa
* Data provided by http://chem.sis.nlm.nih.gov at same temp.
Observations:
- Caffeine and theobromine are almost identical differing only in a H as opposed to a methyl in caffeine (both in position 1) - they also have identical pKas.
But in the case of theophylline the H (instead of methyl) is on the 5 carbon purine ring - so this may explain the difference in pKa (10.4 vs 8.81)
(http://dl.dropbox.com/u/52496091/caffeine-theophylline-theobromine.png)
Question - can anyone explain how the pKa or presence/lack thereof a methy group effects solubility in water?
- If the difference is based on pKa then it wouldnt explain why caffeine is ~65 more soluble than theobromine (they have the same pKa). For theophylline I can see how the lower pKa can influence solubility since most of it will be in its ionized form, but if this were the case it would also be much more soluble than caffeine (again, all in water at 25C).
Can anyone provide some additional input? Thanks