thin layer chromatography explanation to save my life

[SuperSaiyan3]

Somebody saveeeeeee meeeeeeeeeeeeeeeeeeeeeeeee (smallville sound track playing in the background)


I don't understand TLC. What do they mean when it "depends on the relative attraction of components of mobile and stationary phases?"

And WHY would something that is little attracted to the stationary phase keep moving (mobile) until it makes a spot further UP? Does it ever make a spot further down?

I did TLC in organic chem II lab but I had no idea what I was doing and ended up getting a 60 on the lab.

So explanations that are dumbed down to the academic level of a goldfish would be appreciated (and only in the scope of MCAT please!)

Thank you,
from SS3.

Keep powering up.

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[capn jazz]

So you have the plate, coatedin adsorbent. Then you have the molecule to be run, dissolved in the solvent. Let's say your molecule is polar and your solvent is acetone. Since acetone is also polar, there will be an attraction between the solvent and the molecule, so the molecule wants to hang out with the solvent more and will travel further on the plate (thus getting a higher Rf value).

If you have a polar molecule in a non-polar solvent, it won't travel as far.

 

[loveoforganic]

One thing to note with your example capn, is that the adsorbent is normally a polar polymer. This means that even with a polar solvent, a nonpolar molecule might travel farther up the TLC plate than a polar molecule, due to attraction to the adsorbent.

 

[capn jazz]

One thing to note with your example capn, is that the adsorbent is normally a polar polymer. This means that even with a polar solvent, a nonpolar molecule might travel farther up the TLC plate than a polar molecule, due to attraction to the adsorbent.

Do you think it would identify the adsorbent as polar in the passage/question stem, or is that background knowledge?

 

[loveoforganic]

I'm not sure. TBR told us we should know that TLC adsorbents are typically polar, but it seems like that'd just be a miscellaneous factoid that a passage would tell us.

 

[SuperSaiyan3]

So you have the plate, coatedin adsorbent. Then you have the molecule to be run, dissolved in the solvent. Let's say your molecule is polar and your solvent is acetone. Since acetone is also polar, there will be an attraction between the solvent and the molecule, so the molecule wants to hang out with the solvent more and will travel further on the plate (thus getting a higher Rf value).

If you have a polar molecule in a non-polar solvent, it won't travel as far.

I'm confused.

So you have THREE things:
1. adsorbent
2. solvent
3. the sample molecule

So I think you've got some terms confused there, because it's not making any sense to me.

- You said that the plate is coated in adsorbent.
- molecule is dissolved IN solvent
- if your solvent is acetone and your molecule is polar, what is the basis on the molecule running further up the plate? Wouldn't it already be SURROUNDED by acetone since it IS the solvent?

Did you mean that the ADSORBENT is acetone? Because that would make more sense... but if the entire plate is coated in adsorbent, wouldn't the adsorbent just be already all over the plate? Or is the "plate" at the top when the molecule is trying to get to? That would make more sense...

So to conclude: FURTHER THE MOLECULE TRAVELS, HIGHER THE Rf?

Thank you capn.
Planet Vegeta commends you

 

[SuperSaiyan3]

One thing to note with your example capn, is that the adsorbent is normally a polar polymer. This means that even with a polar solvent, a nonpolar molecule might travel farther up the TLC plate than a polar molecule, due to attraction to the adsorbent.

This doesn't make sense to me.

So you are saying by point form:

1. adsorbent is arbitrarily polar.
2. even if solvent is polar, a NONPOLAR molecule will travel further up the TLC plate than a polar molecule because it's attracted to the POLAR adsorbent?

#2 doesn't make sense. Why would a nonpolar molecule be attracted to a POLAR molecule? Unless #1 was wrong to begin with. Did you mean that adsorbent is NONpolar? Because that would make your claim understandable.

Sorry I am very slow witted after battling majin buu.
Thank you for your patience.

SS3.

 

[capn jazz]

I'm confused.

So you have THREE things:
1. adsorbent
2. solvent
3. the sample molecule

So I think you've got some terms confused there, because it's not making any sense to me.

- You said that the plate is coated in adsorbent.
- molecule is dissolved IN solvent
- if your solvent is acetone and your molecule is polar, what is the basis on the molecule running further up the plate? Wouldn't it already be SURROUNDED by acetone since it IS the solvent?

Did you mean that the ADSORBENT is acetone? Because that would make more sense... but if the entire plate is coated in adsorbent, wouldn't the adsorbent just be already all over the plate? Or is the "plate" at the top when the molecule is trying to get to? That would make more sense...

So to conclude: FURTHER THE MOLECULE TRAVELS, HIGHER THE Rf?

Thank you capn.
Planet Vegeta commends you

So the plate is stood up in the solvent at the bottom. a dot of sample is placed on the plate the solvent slowly travels up the plate, dissolving the dot of sample into constituent parts, which travel up with the solvent, assuming there is sample-solvent attraction. the adsorbent is simply the material coating the plate.

 

[SuperSaiyan3]

So the plate is stood up in the solvent at the bottom. a dot of sample is placed on the plate the solvent slowly travels up the plate, dissolving the dot of sample into constituent parts, which travel up with the solvent, assuming there is sample-solvent attraction. the adsorbent is simply the material coating the plate.

ohhhhhhhhhhh i get it.

so you have the sample IN the solvent (regardless if they match in polarity or not). Then you run the TLC and then the solvent starts climbing up the plate... correct me if I'm wrong... like water climbing up a paper towel as you hold it over the water spilled on the table! This is there reason why the adsorbent is there right? To make the solvent crawl up the plate?? I remember in my lab now...

And so the sample would tend to stick WITH the solvent together if they are alike in polarity (they'll hold each other together) so the molecule will travel up further.

Whereas samples without the similiar polarity will fall behind as the solvent continues up the paper towel like water's cohesive properties.

Is this right?

 

[SuperSaiyan3]

One thing to note with your example capn, is that the adsorbent is normally a polar polymer. This means that even with a polar solvent, a nonpolar molecule might travel farther up the TLC plate than a polar molecule, due to attraction to the adsorbent.

hey, but I still don't get this.

Why would the nonpolar molecule dissolved in a polar solvent travel up FARTHER than a polar molecule? Shoulnd't the nonpolar molecule fall behind faster??

 

[Tutmos]

I'm not sure. TBR told us we should know that TLC adsorbents are typically polar, but it seems like that'd just be a miscellaneous factoid that a passage would tell us.

I'd tend to agree that the norm is plates are coated in polar substance meaning polar solutes tend to spot lower on the slide due to attraction between solute and slide. The higher it travels the less polar. This was also the norm for slides we used in organic lab class.

 

[SuperSaiyan3]

I'd tend to agree that the norm is plates are coated in polar substance meaning polar solutes tend to spot lower on the slide due to attraction between solute and slide. The higher it travels the less polar. This was also the norm for slides we used in organic lab class.

you mean that the adsorbent (the polar thingy) is at the BOTTOM? that would make sense !

 

[loveoforganic]

Ok, I'll break the whole thing down the best I can.

TLC uses these things called plates. What a plate is is a small rectangular piece of glass that's coated with a very thin layer of a substance. This substance is called adsorbent. Its purpose is to provide a surface onto which a solution will stick. Now what you do to the plate is take a drop of the solution you want to test (a very small drop). You place this drop at a point some small distance above the edge of one end (the bottom end) of the plate. You mark the point where you put the drop (call it point A). You now allow all of the solvent to evaporate, leaving a solid (but usually pretty much invisible) residue on the plate in the adsorbent.

You now find a beaker that's small enough that the plate will fit in it standing up, laying against the edge. You fill this beaker with a small enough amount of solvent such that when the plate is inserted in the solvent, the solvent does not reach point A (this is because the solvent would dissolve your compound into solution). The solvent will travel up the adsorbent and carry your compound with it. Compounds of different polarities will travel different distances.

You then take the plate out of the solvent before it reaches the other end of the plate. You mark the point the solvent reaches as point B. You can then identify how far up the plate your compound traveled by any number of ways. You take the ratio of the distance traveled by your compound over the distance traveled by the solvent.

That's the basis of TLC.

Applications:

1) You can check the progress of a reaction by taking a sample of your reaction solution and running TLC on it. You run the reaction solution along with your pure reactants and look for the appearance of new peaks.

2) You can check the relative polarity of two compounds in a certain solvent to determine which solvent would be good for a chromatography separation.

3) You can check the purity of a compound (by looking for multiple peaks).


Now on to my special case:

The adsorbent is often made of a polar polymer. For this reason, polar molecules have a certain tendency to stay bound to the polymer. If the force of attraction between the adsorbent and the compound is very significant compared to the force of attraction between the solvent and the compound, a polar compound could theoretically travel less up the plate (smaller Rf value) than a nonpolar compound (which experiences little to no attraction to the adsorbent).

 

[capn jazz]

Here is an explanation from wikipedia which helped me:

"Separation of compounds is based on the competition of the solute and the mobile phase for binding places on the stationary phase. For instance, if normal phase silica gel is used as the stationary phase it can be considered polar. Given two compounds which differ in polarity, the most polar compound has a stronger interaction with the silica and is therefore more capable to dispel the mobile phase from the binding places. Consequently, the less polar compound moves higher up the plate (resulting in a higher Rf value). If the mobile phase is changed to a more polar solvent or mixture of solvents, it is more capable of dispelling solutes from the silica binding places and all compounds on the TLC plate will move higher up the plate. Practically this means that if you use a mixture of ethyl acetate and heptane as the mobile phase, adding more ethyl acetate results in higher Rf values for all compounds on the TLC plate."

So silica = polar --> nonpolar samples will run further (higher Rf). But if the solvent is SUPER polar, everything will run further. So in nonpolar solvent, the Rfs for 3 cmpds may be.... 1 cm, 2 cm, 3 cm. In a polar solvent, the order is still the same, but the distances increase: 1.5 cm, 2.5 cm, 3.5 cm.

 

[SuperSaiyan3]

In a polar solvent, the order is still the same, but the distances increase: 1.5 cm, 2.5 cm, 3.5 cm.

this sounds like a key fundamental idea of TLC that everybody should be aware of. It also sounds like where the MCAT might make a question.

Thanks a lot guys.

You guys have been a lot of help.

-SS3