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There are two types of stationary phases used in thin layer chromatography. One stationary phase is made of aluminium oxide. Which of the following molecules would be expected to have the smallest $R_f$ using aluminium oxide as the stationary phase on the glass slide?

(a) $\ce{CH3CH2CH2CH2CH2COOH}$
(b) $\ce{CH3CH2CH2CH2CH2OH}$
(c) $\ce{CH3CH2CH2CH2NH2}$
(d) $\ce{CH3CH2CH2CH2CH3}$

$\ce{AL2O3}$ is a non-polar stationary phase, therefore if the molecule is also non-polar it would display high adsorption to the stationary phase, and therefore a smaller $R_f$ value.

The answer is (a).

Isn't (a) polar?

Wouldn't this increase the $R_f$ value, because it doesn't adsorb to the non-polar stationary phase? Can someone please explain, correct me?

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  • $\begingroup$ A is the most polar of the group, and D is non-polar. Since the answer is A, then your statement "Al2O3 is a non-polar stationary phase" is in question. "In extreme situations, very polar substances on Aluminum Oxide do not migrate very far from the starting point" from infohost.nmt.edu/~jaltig/TLC.pdf $\endgroup$
    – LDC3
    Apr 7 '14 at 3:42
  • $\begingroup$ so AL2O3 is polar then? $\endgroup$
    – confused
    Apr 7 '14 at 3:44
  • $\begingroup$ Go to Figure 7.5 here: courses.chem.psu.edu/chem36/Experiments/… $\endgroup$
    – LDC3
    Apr 7 '14 at 3:51
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Aluminum Oxide is more polar than Silica Gel and polar compounds do not migrate far from the starting point.

Thin Layer Chromatographic Analyses

7. Thin-Layer Chromatography

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"There are two types of stationary phases used in thin layer chromatography." Sigma-Aldrich sells silica gel (unmodified, modified/bonded, chiral and high purity), aluminum oxide, cellulose (fibrous, microcrystalline), and polyamide 6 stationary phase TLC plates. That barely scratches the surface.

Alumina is highly polar and highly basic unless modified. Yellowed tetramethylenediamine run down a Dispopipette of dry Activity 0 basic alumina has everything bothersome ripped out of it within an inch or so.

The carboxylic acid will ionize and salt bond, given an Rf of about zero.

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