Question

In standard chromatography,

a). explain the relationship between polarity of compounds and Rf Value

b). predict the change of Rf (retention factor) value when more polar eluent is used

Answer 1

Chromatography is used to separate the pure individual compounds from a mixture.

Thin Layer Chromatography (TLC) is the one of the chromatographic technique which is mostly widely used for the monitoring the progress of a reaction.

In TLC there will be two phases one is a stationary phase and second will be the mobile phase. The stationary phases usually used are silica or alumina. A mobile phase is a mixture of different organic solvents which one has to choose based on the polarity of the compound under study.

Elution is the process in which the compound under study is placed on a TLC plate (small dot) and it has to dipped in a TLC chamber followed by addition of mobile phase (mixture of organic solvents).

Rf value (Retention factor) is known as distance traveled by the compound upon the distance traveled by the solvent. Rf value will be always less than one.

a) If the compound under study has polar groups they will have interactions with the stationary phase and they will run lesser height on the TLC plate during elution. Hence the Rf value is lesser for the polar compounds than the non-polar compounds (these compounds does not have any interaction with the stationary phase) given that the mobile phase is same.

b) If you use more polar eluent (that is mobile phase) than it will lessens the interactions of the polar groups with the stationary phase and the polar compounds will travel more distance on the TLC plate. Hence the Rf value also will be higher in this case.