Question about TLC and Rf values

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studentdoctor08

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Hey guys,

I'm a little confused about TLC and Rf values. I thought that a polar compound would travel less distance, and thus have a smaller Rf value in a polar solvent than a non-polar compound would. However, this Kaplan question seems to suggest the opposite and I am confused...

Four compounds, I, II, III, and IV are separated by TLC. Compound III is most polar, II the least polar, and I and IV have intermediate polarity. The solvent system is 85:15 ethanol: methylene chloride. Which Rf would belong to compound III?

A) Smallest Rf
B) Second largest Rf
C) Largest Rf

I believe the answer should be A, the smallest Rf since both the compound and solvent are polar, and thus would dissolve in each other better and prevent further migration up the sheet. However, according to Kaplan the correct answer is actually C? Am I missing something here?

Thanks for the help.
 
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It is my understanding that the "solvent" is what travels up the chromatography apparatus via capillary action. Therefore, in this case the more polar the compound, the more likely it would dissolve into the solvent and travel farther up the column. This would lead to the most polar compound having the largest Rf value.

I think generally we say polar compounds have small Rf values because it's most common to see the column itself made of a polar substance (i.e. alumina) and the solvent made out of a non polar substance. Thus, the solvent would be likely to adhere to the binding column and have a small Rf value. In this question however, it states that the solvent is polar, so the expected Rf value for a polar compound is large.

Just my thoughts....feel free to correct me if I'm wrong. It's been awhile since I've looked at this.
 
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Think of it as the solvent dragging the compound along with it because they're associating with each other.
 
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