Question

Consider the two compounds of trans-stillbene or p-tolylmethanol,

a) Which compound has a lower Rf value when the mixture is developed in a 3:1 hexane/ether solvent system? Explain.

b) Which compound would you expect to come off first from the column when the eluting solvent is hexane first? Explain.

c) What happens if you use hexane as an only eluting solvent throughout the chromatography? Explain.

d) (Continue from c) Or if you use ethyl ether as an only eluting solvent? Explain.

Answer 1

OH trans-stillbene p-tolylmethanol a) For the TLC, the stationary phase is silica (silica is polar). The trans-stillbene does not contain any of the polar functional group i.e stillbene is non polar. The p-tolylmethanol contains a polar hydoxy group i.e., p-tolylmethanol is polar. Therefore the interaction between the silica and p-tolylmethanol will be more than that of the trans-stillbene therefore, stillbene moves faster on TLC than the p-tolylmethanol. Thus, the Rf value of p-tolylmethanol will be less according to the formula for Rf trans-stillbene Distance travelled by compound Rf Distance travelled by solvent Less interacction with stationary phase p-tolylmethanol More interacction with stationary phase b) Chromatography (both TLC and column)are based on the affinity of compounds towards the mobile phase and stationary phase. Because of less interaction with the stationry phase (stationary phase polar), the trans-stillbene will move faster in column when eluting with the hexane (a non polar solvent) c) Note that hexane is a nonpolar solvent. therefore, trans-stillbene will elute first due to more affinity for mobile phase. The o-tolylmethanol will not show movement or will move very slowly when eluting with the hexane due to less affinity towards the mobile phase and more affinity towards the stationary phase. d) When only ether ether will be used as a solvent. the separation between the Rf values of the both the compounds in ethyl ether will deside the their separation in the column. However, ethyl ether i.e, mobile phase polarity more than the hexane, hence both the compounds will be eluted from the column.