MCAT TLC

[deleted783484]

Which TLC solvent system (2:1 Hexanes/Ethyl acetate or 4:1 Hexanes/Ethylacetate) will have a higher Rf when analyzing isoamyl acetate? would it be 4:1 because isoamyl acetate is non polar so it would migrate more?
@aldol16

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[NextStepTutor_1]

Hi @ys2327,

Somewhat surprisingly, it's a general rule of thumb that a more polar solvent will result in a higher Rf value regardless of the polarity of the substance being analyzed. The core idea here is that interactions between the polar silica stationary phase and the analyte are what cause the analyte to 'get stuck' -- and this is always a polarity-driven reaction. By making the mobile phase more polar, you're effectively allowing it to compete with the polar stationary phase, which will delay the point at which the analyte 'gets stuck' and thereby increase the Rf value.

(A side note on isoamyl acetate -- it may not be "polar" in the grand scheme of things, but it does have a dipole due to the carbonyl carbon, and this is enough for it to interact with the silica plate and be affected by the polarity of the solvent. Moreover, the polarity of the analyte is actually a trap that the question-writers introduced here, to try to induce you into thinking 'like dissolves like' -- but in general, a more polar solvent will increase the Rf of any analyte for the reasons outlined above).

Hope this helps clarify things!

 

[aldol16]

While like-dissolves-like is a good principle to follow, what's happening here is a bit more complicated than that. Here, you basically have three components. Your sample, the TLC plate, and elutant. So basically, you load your sample onto the TLC plate, which basically means that the molecules in your sample associate with silica. How strongly they associate will depend on their properties, such as polarity and H-bonding. Now, what happens - and this is the principle of TLC and silica column separations - is that your elutant comes in and competes for binding to the silica gel. If your sample is only loosely held, it will come off and travel up the plate. If it's held tightly, it will stick on until the elutant becomes too strong and washes it out. So basically, no matter whether your sample is polar or not, a strongly polar solvent will always move it up more than a non-polar solvent like hexanes. This is because with a strong, polar solvent, all your sample come off immediately and travel up the TLC plate.

The difference with using different strength elutants is to maximize your resolution. If you start off with a strong, polar elutant, everything in the sample will come out and will travel more or less together. You won't have separation of spots.