Question

Hi can someone please help with question #1. It deals with Rf values and the data collected is given below!

Results: Rf Values Of Compounds In Solvents Solvents 100% 50% hexanes 75% hexanes 25% ethyl 100% ethyl Hexane 25% hexanes 75% ethyl acetate 50% ethyl Compounds acetate acetate acetate trans-Stilbene 0.125 0.419 0.781 0.594 0.906 9-Fluorenone 0.156 0.677 0.906 0.375 0.843 Benzoic Acid 0.094 0.742 0.813 0.656 0.937

Answer 1

Rank	trans stilbene	benzoic acid	9-Fluorenone
1	100% ethyl acetate	100% ethyl acetate	50% ethyl acetate and 50% hexane
2	50% ethyl acetate and 50% hexane	50% ethyl acetate and 50% hexane	100% ethyl acetate
3	75% ethyl acetate and 25% hexane	25% ethyl acetate and 75% hexane	25% ethyl acetate and 75% hexane
4	25% ethyl acetate and 75% hexane	75% ethyl acetate and 25% hexane	75% ethyl acetate and 25% hexane
5	100% hexane	100% hexane	100% hexane

In TLC, we have a stationary phase which is fixed and a mobile phase which is moving. The stationary phase we usually see is made up of polar substance like silica gel or alumina . The mobile phase can vary from non-polar to very polar solvents.

The polar molecules in the compound will be attracted to the silica gel due greater adsorptivity, thus the interaction will be more and it will strongly bind with stationary phase and will move slowly while the more non-polar molecules will stay in the solvent (mobile phase). The solvent if is non , then does not have enough capacity to pull  off the polar compound off the silica or alumina and move it further up the plate. So in this case, we would have a lower R_f value since the molecules did not travel far.

We could then increase the polarity of the solvent which would help elute the molecules and move them further up the silica plate by breaking the adhesive force between the sample and stationary phase and making even stronger attraction between itself and the sample . This would result in larger R_f values.