Higher Rf Value? (TLC Chromatograpghy)

[negatifzeo]

Homework Statement

Suppose you carry out a TLC separation of acetaminophen and phenacetin on silica gel using a nonpolar developing solvent. Which should have the higher Rf value, and why?

Homework Equations

Im not sure. Is polarity a factor here? I don't think the polarity difference is that great here. Could molecular weight play a factor?

The Attempt at a Solution

 

[chemisttree]

It is mostly polarity. Look at the structures! Both structures contain the same aromatic acetamide functionality but Acetaminophen has a phenolic group whereas phenacetin has an aromatic ether on the aromatic ring. Can you estimate which will be most polar and least polar?

 

[Blueshift5]

In TLC chromatography, the Rf value (retention factor) is a measure of how far a compound travels relative to the solvent front. It is calculated by dividing the distance traveled by the compound by the distance traveled by the solvent front. The higher the Rf value, the further the compound has traveled and the less it has interacted with the stationary phase (silica gel in this case).

In this scenario, acetaminophen and phenacetin are both relatively nonpolar compounds and are unlikely to have a significant difference in polarity. Therefore, the Rf value will not be greatly affected by polarity.

However, molecular weight can play a factor in the Rf value. Generally, larger molecules have a lower Rf value as they have a higher affinity for the stationary phase and therefore travel a shorter distance. In this case, phenacetin has a higher molecular weight than acetaminophen and is likely to have a lower Rf value.

Therefore, in this TLC separation, phenacetin is likely to have a lower Rf value than acetaminophen due to its higher molecular weight and stronger interactions with the stationary phase.