Question

In your TLC analysis of Anacin it is quite likely that you were able to see another spot in addition to that for the acetylsalicylic acid (aspirin) (an idealized TLC of Anacin is shown below). This is because in addition to acetylsalicyclic acid, Anacin also contains some caffeine. Ignoring detailed intermolecular forces for now, state which would be described as more POLAR in the context of TLC, aspirin or caffeine? In answering this question you will realize that hydrogen-bonding is not always the most important I.M.F.

Answer 1

Caffeine is more polar than Aspirine because of pressence of more basic 4 nitrogen atoms, so these lone pairs can form the Lewis-acid base interaction with acidic SiO2 i.e silical gel and adsorbed very strongly than Aspirine.

whereas in case of Aspirine eventhough free hydroxy functinality is there , it can not adsorbed strongly because of acidic nature of silical gel