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TLC and Polarity

Hi :smile:

Would somebody be able to please lend me a hand with answering this question?

In class we prepared diphenylmethanol and placed it on a TLC plate to measure the Rf value using a 9:1 petroleum ether : ethyl acetate solvent.
The question asks, if we were to use PURE ethyl acetate instead of the petroleum ether solvent would the Rf increase, decrease or have no change.

I guessed it would decrease since the polar diphenylmethanol would interact even more strongly with the ethyl acetate polar solvent (therefore less movement and a lower Rf value) however a friend suggests it would increase and now I am confused.

What is the answer and why??? My logic seems to make sense hence my confusion?

Thank you :smile:
Original post by Smoke_565
Hi :smile:

Would somebody be able to please lend me a hand with answering this question?

In class we prepared diphenylmethanol and placed it on a TLC plate to measure the Rf value using a 9:1 petroleum ether : ethyl acetate solvent.
The question asks, if we were to use PURE ethyl acetate instead of the petroleum ether solvent would the Rf increase, decrease or have no change.

I guessed it would decrease since the polar diphenylmethanol would interact even more strongly with the ethyl acetate polar solvent (therefore less movement and a lower Rf value) however a friend suggests it would increase and now I am confused.

What is the answer and why??? My logic seems to make sense hence my confusion?

Thank you :smile:


The distance a solute travels in chromatography is a function of the attraction that the solute has for both the stationary phase (the silica or whatever coats the plate) and the mobile phase.

In this case the mobile phase is the organic solvent. If the solute is more soluble in the solvent (mobile phase) it should travel further.

But, are you sure about your assertion that it is more soluble in pure ethyl ethanoate?
(edited 9 years ago)
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Smoke_565
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Hi charco,


Thanks for the reply!


Well I assumed it would be due to the hydrogen bonding with the polar ethyl acetate but I guess the two phenyl rings would also contribute to it being used in a petroleum ether non-polar solvent. Maybe I'm just confusing myself with what happens and terms here but I honestly did think it would travel less due to the OH group on the product.
Original post by Smoke_565
Hi charco,


Thanks for the reply!


Well I assumed it would be due to the hydrogen bonding with the polar ethyl acetate but I guess the two phenyl rings would also contribute to it being used in a petroleum ether non-polar solvent. Maybe I'm just confusing myself with what happens and terms here but I honestly did think it would travel less due to the OH group on the product.


I don't know the answer, like I said, it's a balance between attraction between the solute and the two phases.

If the stationary phase is silica then there should be considerable interaction between it and polar groups...

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