The Rf value of compound A is 0.34 when a TLC plate is developed in pentane and 0.44 when the plate is developed in diethyl ether. Compound B has an Rf value of 0.42 in pentane and 0.60 in diethyl ether. Which solvent would be better for separating a mixture of A and B by TLC?

The retention factor is measure of how much interaction that exists between the solvent and the solutes. A high retention factor means that the solute travels too quickly through the mobile phase.

The value of Rf shows how good a solvent is in achieving separation of the components of a mixture by TLC.

If the Rf is moderate, then the solute does not travel too quickly through the column. If the solute travels too quickly through the column(too high Rf), minimum separation is achieved. If the Rf is too low, the solute has more affinity for the stationary phase than it does for the mobile phase hence separation is not feasible.

We can see that in both cases, diethyl ether has a moderate Rf hence it achieves a better separation of compound A and B than pentane.