# 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.

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?

• 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
– LDC3
Apr 7 '14 at 3:42
• so AL2O3 is polar then? Apr 7 '14 at 3:44
• Go to Figure 7.5 here: courses.chem.psu.edu/chem36/Experiments/…
– LDC3
Apr 7 '14 at 3:51