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.

Why shouldn't I applied too much of one compound on the TLC plate?

To correct the error in this case, the first step would be to choose a more appropriate solvent for the TLC experiment. Acetone, being a polar solvent, is not suitable for separating the nonpolar compound 1-octene and the slightly polar compound 1,4-dimethylbenzene. A more nonpolar solvent like hexane would be a better choice.

Next, the TLC plate should be prepared by applying separate spots of each compound or by preparing separate mixtures of each compound with a known ratio. This will ensure that individual spots are observed on the TLC plate.

After developing the TLC plate using the appropriate solvent (in this case, hexane), the plate should be visualized using appropriate visualization techniques such as UV light or staining with iodine. This will help in observing different spots on the plate representing the individual compounds.

Finally, the Rf value for each compound can be determined by measuring the distance traveled by the compound and dividing it by the total distance traveled by the solvent. This will give a numerical value that can be used for identification and comparison.

Overall, the key steps to correct the error would be to choose a more appropriate solvent, ensure separate spots or mixtures of each compound, properly visualize the TLC plate, and calculate the Rf values for each compound.

Based on the information provided, it seems that the error in running the TLC experiment was the choice of solvent. Acetone, being a polar solvent, was not appropriate for separating the non-polar compounds 1-octene and 1,4-dimethylbenzene. The compounds should have been dissolved in a non-polar solvent, such as hexane, in order to achieve better separation on the TLC plate.

The fact that only one spot with an Rf value of 0.95 was observed suggests that the two compounds have very similar properties, perhaps similar boiling points or molecular structures. Since they are both non-polar, they were able to dissolve in acetone to some extent and travel a considerable distance on the TLC plate. However, because of their similarity, they did not separate into distinct spots.

To correct this error and obtain better separation, one would need to choose a non-polar solvent like hexane to dissolve the compounds and run the TLC experiment again. This would allow for better differentiation in the movement of the compounds on the TLC plate based on their differing polarity.

To understand more about the principles and techniques of TLC, it would be useful to refer to reliable sources such as the textbook "Organic Chemistry" by Janice Gorzynski Smith, as mentioned in the question. Additionally, the link provided (http://orgchem.colorado.edu/hndbksupport/TLC/TLC.html) seems to provide helpful information on TLC, including solvent selection and understanding Rf values.