Rf value

[chiddler]

Suppose you have two compounds, one polar and one non polar, in an diethyl ether solvent. Run it on a TLC and you see a high Rf value for the polar and a low Rf value for non polar, as expected.

Now I want to increase Rf value of the polar. So I add some methanol.

Does adding polar methanol decrease the Rf value of the organic solvent?

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[MedPR]

Suppose you have two compounds, one polar and one non polar, in an diethyl ether solvent. Run it on a TLC and you see a high Rf value for the polar and a low Rf value for non polar, as expected.

Now I want to increase Rf value of the polar. So I add some methanol.

Does adding polar methanol decrease the Rf value of the organic solvent?

Are you asking will the new solvent travel less than the original solvent? Or are you asking if the methanol will make the non-polar compound have a lower Rf value?

Also, why would you expect a higher Rf for the polar? If you are doing TLC, the polar will stick to the stationary phase more readily than the nonpolar, especially if you have a relatively unpolar solvent. So the nonpolar should have the higher Rf.

 

[SaintJude]

I thought diethyl ether is a relatively nonpolar solvent and thus the nonpolar compound will have a higher Rf value.

And I haven't read of a TLC involving two separate solvents on the same plate. I imagine it would make more sense to create a new plate, where you now use replace the old solvent with a polar choice, like methanol. Now the polar substance will have a higher Rf value than the nonpolar substance.

 

[MedPR]

I thought diethyl ether is a relatively nonpolar solvent and thus the nonpolar compound will have a higher Rf value.

And I haven't read of a TLC involving two separate solvents on the same plate. I imagine it would make more sense to create a new plate, where you now use replace the old solvent with a polar choice, like methanol. Now the polar substance will have a higher Rf value than the nonpolar substance.

A polar solvent won't necessarily make the polar substance have a higher Rf value. If the polar stationary phase has higher affinity for the polar substance than it does for the polar solvent, and if the polar stationary phase has higher affinity than the polar solvent for the polar substance, the polar Rf could still be lower than the non-polar Rf.

 

[SaintJude]

Ah, yeah you're right. I just assumed that only thing that mattered was solvent. But it's still accurate the substance with the most similar polarity to the solvent will travel furthest (and have highest Rf values) right?

 

[chiddler]

Reading your latter comment, I think we disagree about something, so i'll address that first and then ask my specific question.

My understanding is that two things contribute to how far something will travel on a TLC plate. One, how much affinity it has for the solvent. Two, how polar solvent and sample is because a very polar sample will be difficult to move since it attaches to the silica strongly.

Adding methanol will allow the solvent to compete with the silica for dipole bonds with the polar sample. This means that the sample will attach less strongly to the silica due to the solvent and will therefore move further up the silica. A higher Rf value.

So. My question is after we add some methanol to the non-polarish diethyl ether, giving it more polar character and allowing the polar sample to move up further, what happens to the non polar sample? Does it move less?

I thought diethyl ether is a relatively nonpolar solvent and thus the nonpolar compound will have a higher Rf value.

And I haven't read of a TLC involving two separate solvents on the same plate. I imagine it would make more sense to create a new plate, where you now use replace the old solvent with a polar choice, like methanol. Now the polar substance will have a higher Rf value than the nonpolar substance.

I think you're right. But i'm asking what happens after adding methanol to the solvent? This question is posed by EK and, I may be remembering incorrectly, but I think I did this in ochem lab as well. So i'm fairly sure this is possible.

 

[milski]

Rf of the solvent is always 1, is not it? Also, like MedPR said, the polar will have a lower Rf, regardless of the solvent. Having a more polar solvent will increase the Rf for both compounds. It's effectively slowing down the solvent and scaling up the Rfs of the compounds.


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[milski]

You should run separate solvents on separate plates. The more polar solvent will move slower but you will wait longer in this case to reach about the same height on the plate. Since you're running the TLC longer, both compounds will move further up.


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[chiddler]

Rf of the solvent is always 1, is not it? Also, like MedPR said, the polar will have a lower Rf, regardless of the solvent. Having a more polar solvent will increase the Rf for both compounds. It's effectively slowing down the solvent and scaling up the Rfs of the compounds.


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Yes I understand about Rf solvent.

Why would a more polar solvent increase Rf of both compounds? What if the compounds are non polar?

 

[MedPR]

Reading your latter comment, I think we disagree about something, so i'll address that first and then ask my specific question.

My understanding is that two things contribute to how far something will travel on a TLC plate. One, how much affinity it has for the solvent. Two, how polar solvent and sample is because a very polar sample will be difficult to move since it attaches to the silica strongly.

Adding methanol will allow the solvent to compete with the silica for dipole bonds with the polar sample. This means that the sample will attach less strongly to the silica due to the solvent and will therefore move further up the silica. A higher Rf value.

So. My question is after we add some methanol to the non-polarish diethyl ether, giving it more polar character and allowing the polar sample to move up further, what happens to the non polar sample? Does it move less?



I think you're right. But i'm asking what happens after adding methanol to the solvent? This question is posed by EK and, I may be remembering incorrectly, but I think I did this in ochem lab as well. So i'm fairly sure this is possible.

I assume the nonpolar substance would move just as much in methanol as in diethyl ether. The diethyl ether isn't making the nonpolar move any faster, the nonpolar having no attraction to the stationary phase is why the nonpolar has a high Rf value. There's basically 2 ways of looking at it and both end up with the same qualitative result.

Way 1: In the nonpolar solvent, the nonpolar substance is binding to the nonpolar solvent, making it move more and giving it a higher Rf value. Probably true.

Way 2: In the polar solvent, the nonpolar substance is not bound very well to the mobile phase, however, there are also going to be way less available binding sites on the stationary phase because now the polar substance AND the polar solvent are going to be trying to bind to the stationary. So, you might think that the absence of the nonpolar solvent is going to hurt the nonpolar Rf value, but in reality it should probably stay about the same.

And yea, you don't add solvent to TLC, you would only add different solvents to column chromatography to get things to continue to elute.

 

[MedPR]

Yes I understand about Rf solvent.

Why would a more polar solvent increase Rf of both compounds? What if the compounds are non polar?

More polar solvent will block binding sites on the stationary phase. The substances will have no way to vacate the mobile phase.

 

[chiddler]

I assume the nonpolar substance would move just as much in methanol as in diethyl ether. The diethyl ether isn't making the nonpolar move any faster, the nonpolar having no attraction to the stationary phase is why the nonpolar has a high Rf value. There's basically 2 ways of looking at it and both end up with the same qualitative result.

Way 1: In the nonpolar solvent, the nonpolar substance is binding to the nonpolar solvent, making it move more and giving it a higher Rf value. Probably true.

Way 2: In the polar solvent, the nonpolar substance is not bound very well to the mobile phase, however, there are also going to be way less available binding sites on the stationary phase because now the polar substance AND the polar solvent are going to be trying to bind to the stationary. So, you might think that the absence of the nonpolar solvent is going to hurt the nonpolar Rf value, but in reality it should probably stay about the same.

And yea, you don't add solvent to TLC, you would only add different solvents to column chromatography to get things to continue to elute.

Well put. Thanks I understand.

 

[MedPR]

Ok, after re-reading the TLC stuff in TBR, it seems like a higher Rf value in polar (vs non polar) might indicate a polar compound.

Because a solute dissolves best into a solvent of "like" nature, the higher the value of Rf, the more that the solute is like the solvent.

Which makes sense considering a polar compound will travel more in polar solvent and the polar solvent will compete for the stationary phase as well.

 

[hellocubed]

Ok, after re-reading the TLC stuff in TBR, it seems like a higher Rf value in polar (vs non polar) might indicate a polar compound.

Wait, was it not established that nonpolar will also have a higher Rf value?
So you mean the solute that shows the Most improvement from a polar compound is most likely polar.


The TLC seems to me to be very unpredictable for every separate case. Both solutes could very well be nonpolar (Both would also improve Rf)

 

[MedPR]

Wait, was it not established that nonpolar will also have a higher Rf value?
So you mean the solute that shows the Most improvement from a polar compound is most likely polar.


The TLC seems to me to be very unpredictable for every separate case. Both solutes could very well be nonpolar (Both would also improve Rf)

I think TBR said something like the more similar something is to the solvent the higher Rf it will have. TLC does seem to be case-by-case, but that's probably why if it is tested it will be a specific case or very simple generalization.

Basically you should know that polar will stick to the stationary phase more since the stationary phase is polar.