Chapter 18, Problem 4Q

Interpretation Introduction

Interpretation:

The compound with greater ${\text{R}}_{\text{f}}$ value when ethyl acetate and dichloromethane are used as developing phase solvent should be determined.

Concept introduction:

Thin-layer chromatography is an analytical technique used to identify whether two compounds are identical or not and also to find the number of compounds in a mixture.

Metal, glass or plates made of plastics are utilized in TLC that are coated by a thin layer of adsorbent and it acts as a stationary phase. The stationary phase usually used is polar silica gel. A mixture of solvents or single pure solvent is the mobile. It is also known as a developing agent. Mostly nonvolatile solid organic samples are analyzed by thin-layer chromatography. However, TLC cannot be used for volatile liquid compounds because it leads to loss of the sample by evaporation from the TLC plate.

In TLC analysis, mixture in a low quantity is taken for separation and dissolved in the suitable solvent and applied on the TLC plate. Then TLC plate is kept in the closed chamber immersed in mobile phase known as developing solvent. Through capillary action the solvent rises on the stationary phase. As the solvent rises on the plate, the sample is distributed between the mobile phase and the stationary phase on the basis of their retention values. The more the compound interacts with the stationary phase, the more slowly it moves on the TLC plate. When silica gel that is polar in nature behaves as the stationary phase then mobile solvent rises with the nonpolar substances up on the plate most rapidly. In the chromatogram, the polar substances rise up plate with lowest rate.

The ratio of distances covered by the compound to the distance covered by developing solvent or mobile front is known as retention factor $\left({\text{R}}_{\text{f}}\right)$ .

The ${\text{R}}_{\text{f}}$ value of a compound depends on its structure, the mobile phase and stationary selected. To calculate the ${\text{R}}_{\text{f}}$ value for a given compound, the distance covered by compound from where it was spotted and the distance covered by solvent front from where the compound was spotted.