Chromatography Forum

Hello,

I have a problem with TLC. What is the order in the TLC lab of those substance: Salicylamide, Aspirin, Cafeine? I use the silica plate with solvent: methyl chloride.
Here is what I think:
Sal: H-bonding in OH, NH_2 groups + C=O+ smallest HC group= smallest Rf
Asp: H-bonding OH group+ C=O +smallest HC group = next smallest Rf
Caff: no H bonding + R3N+ C=O --> least polar = largest Rf

But when I develope the TLC, it turns out that caff has a smallest Rf while Asp and Sal have almost the same Rf. Why does Caff have a smallest Rf and between Sal/ Asp which one has a smaller Rf?

Thank you very much!

Caffeine would have the smallest Rf because it is the most basic. The basic nitrogens of caffeine have a very high affinity for Si-OH groups.

As for asprin and salicylamide, remember that the carboxylic acid -OH group of aspirin is very polar too.