Rf value

[Csv321]

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I'm finishing up some last minute ochem but was hoping someone could clarify what exactly the Rf value is...

I know it's distance compound travels/distance solvent travels but what does that really mean intuitively?? I think I had a problem like this on the April MCAT and was completely lost because I didn't have either of those numbers to solve..

I know everyone is sick of this stuff and is probably pretty much done studying but I'd really appreciate it if someone could reply...thanks!

 

[MEG@COOL]

I think rf is just ratio to front, front being the compound that has traveled the farthest. So something may have traveled 75% of the distance as the longest mover(solvent probably).... I might be wrong though since its been awhile.

 

[BenU29]

It tells you the affinity that the compound has to the solvent. The higher the Rf value, the more affinity the compound has to the solvent; the lower the Rf value, the lower affinity the compound has to the solvent.

 

[chopper]

Yep.

USUALLY - the solvent is hydrophobic (organic solvent like diethyl ether), and the solid support is very hydrophilic (silica gel)

So, if I compound is very hydrophobic, in interacts with the solvent and moves with the solvent as it is pulled up the TLC plate by capilarry action.

If a compound is hydrophilic, it tends to interact with the silica gel and NOT move with the solvent front.

Sooooo - how far up the plate it moves is a relative measure of how much hydrophobic/hydrophilic tendency it has, and Rf quantifies this.

 

[murphomatic]

yea, it's polarity

Polar molecules don't travel as far as non polar molecules, because they are more tighly bound to the TLC plate

High Rf= little or no polar groups

low Rf= more polar groups

 

[OHare]

Yep.

USUALLY - the solvent is hydrophobic (organic solvent like diethyl ether), and the solid support is very hydrophilic (silica gel)

I don't think ether is hydrophobic