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September 22, 2014

Homework Help: TLC error correction

Posted by Sheryl on Saturday, October 14, 2006 at 7:32pm.

Question: 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 0.95. The solvent used was acetone.

In my researches, I found that 1-octene is very nonpolar and 1,4-dimethylbenzene also is "nearly insoluble in water." Acetone is polar so obviously the wrong solvent has been chosen. What I don't understand is why there is only one spot and why it has such a high Rf value. Seems like neither substance should dissolve and certainly they shouldn't travel very far. Any insights on this?

My lab book is pretty bad. It asks a lot of questions that it doesn't cover, that I can ascertain anyhow. My textbook is great though - "Organic Chemistry" by Janice Gorzynski Smith.

Thanks from Sheryl


I think the most commonly used adsorbent on TLC plates is SiO2. That is polar. Therefore, those compounds that are less polar (or non-polar) will move further up the plate during the capillary phase. I assume that is why the two compounds you had moved so far; i.e., they are non-polar. There is a discussion at the site below that is VERY good. It has an example of just what you are talking about in a separate page (click on Rf value page when you come to it in the discussion). It also has a list of solvents to use and the order in which eluents do their thing.
http://orgchem.colorado.edu/hndbksupport/TLC/TLC.html


That was helpful.
In the question it says, "a two-component mixture." I'm assuming this means that only one spot containing both compounds was applied to the plate. Do you agree?

The only reason I can think of now that there would be only one spot is that the two compounds possess very similar properties although they are not isomers. Does this sound reasonable?

The book is not very good about why you choose appropriate solvents. I do gather that hexane would have been a better choice but I don't know what the problem might be if you chose a polar solvent like acetone. I had assumed that the compounds wouldn't dissolve enough to go up the plate.


Sheryl


I think I found the information I needed about solvent choice although I'm still contemplating it.
c) Acetone is a more polar solvent than is hexanes. If it were used to elute the same three compounds, each of the compounds would travel faster because the more polar eluting solvent is more proficient at eluting the compounds from the polar adsorbent. Since each compound travels faster, each compound would have a larger R f value if acetone were used to elute than when hexanes is used to elute the TLC plate.


I would interpret the question as you have stated it. And from the article we read about Rf values, I suppose the SiO2 is more polar than acetone. I, too, think another solvent would have been better at separating two compounds.

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