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A three-component mixture (X, Y, Z) was separated using normal phase TLC. Three eluents of varying polarity were used. Select the TLC plate that was developed using the least polar eluent (click on the corresponding TLC number 1, 2, or 3 on the bottom of the plate). On TLC plate 3, select the least polar compound (click on X, Y, or Z spot on plate 3).

TLC exercise

Does normal phase TLC mean silica gel TLC? If this is the case then plate 3 has the least polar eluent because the compounds are at the top. For the same reason compound Z would be the least polar.

The term eluent means solvent doesn't it.

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  • $\begingroup$ Hello and welcome to Chemistry.SE. If you have any questions about the site, I recommend taking the short tour. $\endgroup$
    – airhuff
    Feb 25, 2017 at 21:01
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    $\begingroup$ Normal phase as opposed to reversed phase where polar substances have higher $R_{f}$ values. Eluent is a fancy name for solvent in chromatography. $\endgroup$
    – Zhe
    Feb 25, 2017 at 22:31
  • $\begingroup$ @Zhe there was confusion in the answer given below. We're told each unknown eluent has different polarity, but it's normal phase TLC. Based on that we should be able to infer from the compound positions on the plate. In normal phase TLC the lowest Rf will always be the most polar correct? Compound polarity is now known. Then for eluent polarity, it's the opposite of compound polarity isn't it? All three compounds in plate 2 is polar, so the eluent should be less polar/nonpolar. $\endgroup$
    – Jason
    Feb 26, 2017 at 20:08
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    $\begingroup$ In normal phase TLC, the lowest $R_{f}$ value corresponds to the substance with the highest affinity for the silica substrate. Generally, these are the most polar compounds, but less polar alcohols might also have high affinity due to hydrogen bonding. Furthermore, in general, higher eluent polarity just increases the $R_{f}$ for all eluates. The scaling isn't perfect because eluent-eluate interactions can be complex. $\endgroup$
    – Zhe
    Feb 26, 2017 at 22:17
  • $\begingroup$ @Zhe Well with the reasoning used, if you check the discussion below, I'd have to say plate 2 has the least polar eluent given all the polar compounds packed together. And on plate 3, compound z is the least polar. $\endgroup$
    – Jason
    Feb 27, 2017 at 17:43

2 Answers 2

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Never in my years in the lab have I seen a more polar eluent that causes a lower $R_\mathrm{f}$ value than a less polar eluent. It also does not make sense theoretically.

TLC 2 is obviously the one with the least polar eluent and compound $z$ is the least polar compound.

I fully disagree with half of MaxW’s proposed answer.

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You must learn the terminology to follow along in chemistry. If in doubt Google the term...

  • Eluent - As Zhe noted "Eluent is a fancy name for solvent in chromatography." It can be a gas or a liquid. For TLC or HPLC a liquid is used. For GC a gas is used.

  • Normal phase TLC - For normal phase TLC the stationary phase is polar and the mobile phase is typically non-polar or only moderately polar. Polar molecules are attracted to the stationary phase while non-polar compounds are carried along by the mobile phase.

  • Reverse phase TLC - For reverse phase TLC the stationary phase is non-polar and the mobile phase is typically moderately polar to strongly polar. Non-polar molecules are attracted to the stationary phase while polar compounds are carried along by the mobile phase.

The normal designations is because the first HPLC columns used alumina and silica packing, which are both polar, in columns. With setup The least polar compounds were eluted first and the most polar compounds were eluted last.

Switching to non-polar packing was then noted as reverse phase since it reversed the order of elution. The most polar compounds were eluted first and the least polar compounds were eluted last.

ANSWER

This is normal phase TLC so the stationary phase is polar.

Assuming the compounds are "relatively" polar

Plate 2 has the LEAST polar eluent because the compounds have lower $\text{R}_{\text{f}}$ values than they do on the other two plates.

Plate 3 has the MOST polar eluent because the compounds have higher $\text{R}_{\text{f}}$ values than they do on the other two plates.

On plate 3 the least polar compound would be Z.

Assuming the compounds are "relatively" non-polar

Plate 2 has the MOST polar eluent because the compounds have lower $\text{R}_{\text{f}}$ values than they do on the other two plates.

Plate 3 has the LEAST polar eluent because the compounds have higher $\text{R}_{\text{f}}$ values than they do on the other two plates.

On plate 3 the least polar compound would be Z.

So the problem would seem to be ambiguous. We don't know that any of the solvents is "absolutely" non-polar like hexane, or that one might be "absolutely" polar like water.

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  • $\begingroup$ I'm not an expert in TLC, but I think the wording is wrong in the definitions. Shouldn't normal-phase TLC involve polar stationary phases and non-polar solvents? (An example: naked silica gel and solvents like hexanes / ethyl acetate.) And shouldn't reverse-phase TLC involve non-polar stationary phases and polar solvents? (Example: octadecylsilane-derivatized silica and alcoholic or aqueous solvents.) $\endgroup$
    – Curt F.
    Feb 26, 2017 at 4:32
  • $\begingroup$ @CurtF. - Thank you for the correction. You are absolutely right. To make this more embarrassing I chastised the OP about knowing terminology. <Blush> I think I've fixed it now. $\endgroup$
    – MaxW
    Feb 26, 2017 at 6:19
  • $\begingroup$ @MaxW So if compounds are less polar, like on plate 3 because they have high Rf values, the eluent is more polar? Since lower Rf, more polar compounds. So compound Z on plate 3 should be least polar, since it has the highest Rf. $\endgroup$
    – Jason
    Feb 26, 2017 at 16:35
  • $\begingroup$ Hmmm... The polarity of the compounds. I had just assumed that they were polar. $\endgroup$
    – MaxW
    Feb 26, 2017 at 17:11
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    $\begingroup$ @MaxW Answer: Plate 2 used the least polar eluent (hence more polar compounds) $\endgroup$
    – Jason
    Feb 27, 2017 at 18:03

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