Homework Help: Chemistry/ OChem

Posted by Neets on Thursday, October 9, 2008 at 3:28pm.

Consider the following errors that could be made when running TLC. Indicate what should be done to correct the error.
a) a two-component mixture containing 1-octene and 1,4-dimethylbenzene gave only one spot with an Rf value of .95. The solvent uses was acetone.
b) a two-component mixture containing a dicarboxylic acid and tricarboxylic acid gave only one spot with an Rf value of .05. The solvent used was hexane.
c) when a TLC plate was developed, the solvent front ran off the top of the plate.

thanks in advance for all the help... i have been stuck on this question for a while!

Chemistry/ OChem - DrBob222, Thursday, October 9, 2008 at 5:19pm
http://siggy.chem.ucla.edu/VOH/136/TLC.pdf
The above site explains a little about why some analytes move fast and some slow.

Check my thinking on these.
a. Aren't the Hs at the double bond slightly acidic? But I would expect 1-4 dimethylbenzene to be less polar. The fact that the Rf factor was so high means that the solvent used was much more polar than either of the components. I would try a different mobile phase; i.e., one less polar.
b. The Rf value of only 0.05 means the components hardly moved at all; i.e., they were strongly attracted to the TLC plate surface. Hexane is a non-polar solvent; the solution is to use a more polar solvent.
c. When the solvent moves up past the top of the plate it means you let it develop too long.

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