TLC - thin layer chromatography

[dwc929]

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What is the effect of changing what the compounds are eluted with? Does that mean the solvent?

The question is compound x has a Rf of 0.7 in a 20% ethyl acetate + hexane elute. If elute is now 100% hexane, what will the new Rf be? Above 0.7, 0.7 or greater than 0.7?

The answer is lower. Is it b/c more nonpolar solvent won't travel as fast up the plate regardless of how polar/nonpolar the solute is?

 

[gogopogo]

What is the effect of changing what the compounds are eluted with? Does that mean the solvent?

The question is compound x has a Rf of 0.7 in a 20% ethyl acetate + hexane elute. If elute is now 100% hexane, what will the new Rf be? Above 0.7, 0.7 or greater than 0.7?

The answer is lower. Is it b/c more nonpolar solvent won't travel as fast up the plate regardless of how polar/nonpolar the solute is?

Hexane is less polar than ethyl acetate, so a 100% hexane solvent would be less polar than the 20/80% mix.
TLC pulls molecules based on their relative polarity to the solvent. So, if a less polar solvent is pulling, the sample molecules won't be "stuck" to them as strongly. This means the Retention Factor (Rf = the distance solute travels/distance solvent travels) will be a lower number than initial.

-Again, less polarity of the solvent means it can't "pull" the solute along as far, and Rf will be less than it was

Hope this helps
P.S. - Remember that the chromatography will be left to run until the solvent reaches the end of the strip/glass. Changing the solvent may change the time the TLC takes, but not the separation distances that the chromatographs will show for a particular combination of solute/solvent.