Question

1. What does the Rf value represent and what is the unit of measure?

2. Which component elutes first during column chromatography and why?

3. What should be the order of elution during column chromatography; alcohol, ether, ester and ketone, assuming similar molecular weights?

These questions relate to a TLC and column chromatography organic chemistry lab

Answer 1

1. Retardation factor (Rf value) is the distance travelled by a given component (analyte) divided by the distance travelled by the solvent (mobile phase) front. For a given system at a known temperature, it is a characteristic of the component and can be used to identify components.
There is no any unit for the Rf value.

Rf = migration distance of substance / migration distance of solvent front

2. In a given solvent system, the polar compounds shows lower Rf value whereas comparatively less polar compounds shows higher Rf values. This is because, polar compounds have polar bonds such as O-H, N-H and these bonds form strong interactions with silica gel mounted on TLC plates. Hence, inorder to break this bond higher polarity solvent is required. This strong intercation is not possible in comounds which do not have polar functionalities.

3.       Elusion order:

First ether then ketone then ester and finally alcohol in the last as it has a polar O-H group