posted by Scott .
Q: Each of the solvent given should effectively separate one of the following mixtures by thin-layer chromatography. Match the appropriate solvent with the mixture that you would expect to separate well with that solvent. Select your solvent from the following. Hexane, Methylene chloride, or Acetone.
a) 2-phenylethanol and acetophenone: methylene chloride but can't figure it out why...
b) bromobenzene and p-xylene: Hexane?
c) benzoic acid, 2, 4-dinitrobenzoic acid, and 2, 4, 6-trinitrobenzoic acid: acetone maybe?
I assume that this is TLC on silica.
The more polar solvent is needed to move the more polar molecule. b) are the least polar molecules and so you would try hexane to separate them.
a) are the next polar pair, so as you say methylene chloride would be a good choice.
c) the most polar set and acetone is a good choice.
In practice you would choose a solvent and modify it you don't get a good separation you would try something else. The reason for the TLC is also important, if you are monitoring a reaction then it does not matter too much how good the separation is, provided you can see the separation. If you are using TLC to prepare a sample then you would want to make sure there is a clear separation.
You will also find that the solvent or solvent mixture will also affect the shape of the spot, so sometimes the solvent choice is a compromise.