Rf value---flashcard confusion?

[sotired]

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My kaplan flashcard says that a compound with a high Rf value would be near the top of the plate.

I would think it would be near the bottom.

Especially since I just saw an AAMC question that more polar molecules would have a lower Rf value. This made sense to me, because polar molecules wouldnt travel as far as nonpolar mlc so the ratio would be small..

someone please clarify if i'm wrong!!

 

[Brandon81081]

My kaplan flashcard says that a compound with a high Rf value would be near the top of the plate.

I would think it would be near the bottom.

Especially since I just saw an AAMC question that more polar molecules would have a lower Rf value. This made sense to me, because polar molecules wouldnt travel as far as nonpolar mlc so the ratio would be small..

someone please clarify if i'm wrong!!

No, kaplan is right. high Rf values are obtained when the molecules travels with the NONpolar solvent front. Polar molecules stick to the plate...not moving very far...hence a low Rf. Look at the equation again

Remember ..the Rf value can never be greater than 1

 

[WildTumor]

Low Rf = Polar ad sticks to plate. So it travels slower.
High Rf= Nompolar so it is free to move quickly up the plate.

 

[estairella]

My kaplan flashcard says that a compound with a high Rf value would be near the top of the plate.

The highest Rf value is 1, which, by definition, means at the very top of the plate.
The lowest Rf value is 0, which again, by definition, means at the very bottom of the plate.

Rf values have nothing to do with polar or nonpolar, it is just another way of measuring the distance from the bottom of the plate. You can predict Rf values BASED on polarity, but you can only measure actual Rf values by observing the actual plate.

 

[sotired]

haha. i was picturing the plate in the wrong orientation. i think i need to stop reviewing