Thin Layer Chromatography & Stationary Phase 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?
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