Paper Chromatography, AAMC 7, #6

[MedGrl@2022]

In paper chromatography is the paper always or usually polar? In addition, why would a alcohol be more polar than a ketone?

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[Postictal Raiden]

In paper chromatography is the paper always or usually polar? In addition, why would a alcohol be more polar than a ketone?

Paper is made of cellulose. Cellulose is a polymer of glucose. Glucose is polar.

 

[sdm33]

usually polar unless stated otherwise in the passage, also it depends on what you are trying to accomplish.

Alcohol is more polar than a ketone due to a couple things, first alcohol has H bonding , higher boiling point, mostly molecules with higher boiling points are more polar.

Principle: The greater the forces of attraction the higher the boiling point or the greater the polarity the higher the boiling point. second, The greater the electronegativity difference between atoms in a bond, the more polar the bond.
Also the more oxygens the more polar, hence acids are more polar then alcohol.

O is more polar then N , but if they appear both in a compound they can be more polar together. Eg Amide is more polar then amine, but alcohol is more polar then amine.

 

[MedGrl@2022]

usually polar unless stated otherwise in the passage, also it depends on what you are trying to accomplish.

Alcohol is more polar than a ketone due to a couple things, first alcohol has H bonding , higher boiling point, mostly molecules with higher boiling points are more polar.

Principle: The greater the forces of attraction the higher the boiling point or the greater the polarity the higher the boiling point. second, The greater the electronegativity difference between atoms in a bond, the more polar the bond.
Also the more oxygens the more polar, hence acids are more polar then alcohol.

O is more polar then N , but if they appear both in a compound they can be more polar together. Eg Amide is more polar then amine, but alcohol is more polar then amine.

I guess I was confused because the problem had two rings... one had a ketone in the middle and the other was the same structure but had an -OH group where the ketone was... I was thinking that the double bonded oxygen would be more electron withdrawing than the alcohol group (as it has a hydrogen) and I assumed it might be more happy because it was bonded to two different atoms and thus have less desire for electrons... But you are saying that the alcohols ability to hydrogen bond makes it more polar... because hydrogen bonds are the strongest intermolecular force?

Also, just to clarify, in paper chromatography the paper is usually polar and the solvent which dissolves the sample is non-polar... the non-polar parts of the sample will have the greatest Rf value (closest to 1) and the polar parts of the sample will have the least Rf value (closer to 0) because they tend to stick to the polar paper... Is this correct?

Thank you! 🙂

 

[Incontrol]

I just aamc7 today. And I had the same reasoning as yours.

But here is the thing. When we talk about polar, we just care about those partial charges, and not where they are.

In the case of ketone you have the partial charges between C=O due to the two covalent bonds that has difference in the electronegativity between the atoms.

Now lets look at the alcohol, we have one C-O bond. But the 2nd bond is O-H which has a greater electronegativity difference than O-C bond; meaning that overall there are stronger partial charges in O-H than in O-C.

So when we come and compare everything togather. Both C=O and C-O-H have one C-O bond, so we can leave that one alone. What we are left with is comparing The other C-O bond in the ketone, with the O-H in the alcohol. If you do that, you end up with having alcohol being more polar than ketones, because when you look at Oxygen it will have a bigger Partial negative (it had that weak sissy Hydrogen to steel more e- from compared to Carbon), and also you have Hydrogen having a bigger partial positive compared to the carbon in C-O bond.

Hopefully this helps