Question

Which compound will have the larger R_f value on a SiO₂ TLC plate using 10% ethyl acetate/ hexane as eluent: 3-decanone or 3-decanol, toluene or benzoic acid, cyclooctane or cyclooctanone? Why?

Answer 1

Rf value depends on the polarity of the solvent used as the mobile phase. Rf value is defined as the distance travelled by the solute upon distance travelled by the solvent. Higher Rf means faster elution. The basic principle of chromatographic elution is like dissolves like. Thus, polar solvents will dissolve polar compounds and non-polar solvents will dissolve non-polar compounds. Hexane is non-polar solvent, whereas, ethylacetate is a polar solvent. In chromatography a mixture of hexane and ethylacetate is used in various combinations to separate organic compounds. In the present case, we have 10% ethylacetate in hexane as the eluent, so the solvent is slightly polar in nature. It still has more percentage of hexane (90 %) and thus has higher degree of non-polar character, will dissolve non-polar or less polar compounds preferably.

Now organic compounds with only hydrocarbon in their structure are non-polar in nature. Introduction of heteroatom such as an oxygen or nitrogen increases its polarity. These compounds then interacts with the mobile phase differently depending upon its polarity.

Among the given compositions, thus,    

3-decanone will have higher Rf value then 3-decanol since 3-decanone is less polar then 3-decanol.

Toluene will have higher Rf value then benzoic acid. Benzoic acid is more polar then toluene which is an aromatic hydrocarbon. Infact, acids tend to not move at all and stay at the base in TLC. It requires much higher polarity solvents for elution.

Cyclooctane again a hydrocarbon is less polar and will have higher Rf value then cyclooctanone a ketone.