Question

The procedure asked you to take TLC of the starting material and the reaction mixture in 20:80 of ethyl acetate-hexane. If you had accidentally made 80:20 of ethyl acetate-hexane solution for your TLC mobile phase, what would your TLC look like? Sketch a diagram of the TLC and describe what change(s) you would observe, if any, and why? (starting mixture ferrocene and reaction mixture is ferrocene, acetic anhydride and 85% phosphoric acid)

Answer 1

Thin Layer chromatography (TLC) is the technique used to monitor the progress of an organic reaction.

To know the progress of the organic reaction on an aluminum plate which is pre-coated with silica gel on it, one has to make the three spots of the reactant and reaction mixture and a co-spot which is having both the reactant and reaction mixture spot at the bottom of the aluminum plate.

After putting the above said three spots on the TLC plate it has to kept in a TLC chamber with suitable solvent mixture (this solvent mixture is called as mobile phase).

If one use 20:80 of ethyl acetate-hexane as mobile phase (in this mixture, ethyl acetate portion (more polar than hexane) is less (20%), the spots will move in the following manner

The reactant ferrocene may move up to 3.0 cm upside on TLC plate of 4 cm length. (Rf value is 3/4 = 0.75)

The acetylated ferrocene may move up to 1.5 cm upside on TLC plate of 4 cm length. (Rf value is 1.5/4 = 0.375)

If one has accidently made 80:20 of ethyl acetate-hexane solution (in this 80 % will be the more polar ethyl acetate, for your TLC mobile phase, the Rf values of the both the compounds will increase, this is due to the increase in the polarity of the mobile phase the interactions of the compounds with the mobile phase will increase and the compound spots will travel far.