Question on TLC w nonpolar eluent, p280 of AAMC guide to mcat

[luctoretemergo]

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Question: After the reaction w/acetaldehyde in Scheme II, the identity check of 2,3 dimethloxazolidine is performed by TLC w the secondary amine together on the same silica gel plate. If a 9:1 hexane:ethyl acetate mxture is used as the eluent, which of the following is mostly likely to be observed in terms of Rf?

Answer: B) 2,3 dimeth... has larger Rf value than does secondary amine.

I don't understand this, the explanation says that the eluent is nonpolar and like dissolves like so the less polar dimeth will have a larger Rf. But from eluents being mostly polar, and the polar molecules having a LOWER Rf since they 'stick' to the eluent, this seems backwards to me, and now I'm all confused. Opinions?

 

[Web]

It took me a while but I see your confusion. Rf is a measure of the movement of the chemical of interest as a fraction of the solvent front. So a polar chemical in a polar mobile phase will move further and paradoxically have a higher ******ation factor. Since the dimeth and the solvent are both fairly non-polar the dimeth spends more time in the solvent than the amine does so its Rf is higher.

Rf=chemical migration/solvant front migration.

 

[SN2ed]

Thread moved. These types of questions belong in the MCAT Study Question Q&A forum.

 

[luctoretemergo]

It took me a while but I see your confusion. Rf is a measure of the movement of the chemical of interest as a fraction of the solvent front. So a polar chemical in a polar mobile phase will move further and paradoxically have a higher ******ation factor. Since the dimeth and the solvent are both fairly non-polar the dimeth spends more time in the solvent than the amine does so its Rf is higher.

Rf=chemical migration/solvant front migration.

Still confused...
I thought Rf was higher if the chemical moved MORE, so a nonpolar solvent would cause the nonpolar dimeth to be attract to it and move less=lower Rf.

Like how a polar solvent gives a lower Rf to the more polar chemical on it since it travels less?

 

[Rabolisk]

The solvent is the mobile phase/eluent. Silica is the polar stationary phase. The solvent has an Rf of 1. Nonpolar molecules move along with the solvent, resulting in Rf close to 1.