Question

ORGANIC CHEMISTRY (TLC lab)

if you used a solution of 90% ethyl acetate / 10% hexane instead of 10% ethyl acetate / 90% hexane to elute your TLC plates, what would you expect to see?

Would the Rf valaues increase of decrease for each compound? Explain in detail.

Rf values

Naphthalene = 0.44

9-fluorene 0.56

Benzophenone = 0.39

Fluorene = 0.88

4-nitroaniline = 0.28

4- methoxyacetophenone = 0.24

Answer 1

The principle of TLC is based on the diference in adsorption of different subasatances on TLC plate, mainly a silica coated one. The differnece in movement of different substance depends on the polarity of the of the substarte and on the polarity of the mobile phase for silica coated TLC plate.

[90% EA (Ethyl Acetate) + 10% Hex (Hexane)] is higly polar moblie phase comapred to [10% EA + 90% Hex] moblie phase.

So, the substance which are polar will move faster in polar mobile phase and non polar with the less polar mobile phase.

Rf values

Naphthalene = 0.44, this is a hydrocarbon and non polar compound will move faster with less polar system. So, Rf value will increase if the system changed from (90%EA+10%Hex) to (10%EA + 90%Hex).

9-fluorene = 0.56, This is also a hydrocarbon and Rf will increase with non polar system

Benzophenone = 0.39, This is a polar compound and its Rf is expected to be lower with less polar mombile phase.

Fluorene = 0.88, non polar will move with the solvent fron of non polar mobile phase

4-nitroaniline = 0.28, This is highly polar compound its Rf will reduce in a non-polar mobile phase.

4- methoxyacetophenone = 0.24, polar compound, Rf will be decreased