Thin-Layer Chromatography (CIE A Level Chemistry)

Dinitrobenzene has the molecular formula C₆H₄N₂O₄. 

Draw the isomers of dinitrobenzene and name the type of isomerism.

State the factors that determine the distance travelled by a spot in thin-layer chromatography.

Samples of 1,2-dinitrobenzene and 1,4-dinitrobenzene are spotted on a thin-layer chromatography plate coated with silica gel.

The solvent used to run the thin-layer chromatography experiment is hexane.

Suggest why each sample is applied to the thin-layer chromatography plate using a thin capillary tube rather than a dropping pipette.

Fig. 1.1 shows the chromatogram with the spot for 1,4-dinitrobenzene.

Fig. 1.1

Draw the expected position of the spot for 1,2-dinitrobenzene. Explain your answer.

Explain what the student could do to reverse the relative positions of the 1,2-dinitrobenzene and 1,4-dinitrobenzene spots.

A mixture is analysed using thin-layer chromatography. 

Draw the labelled apparatus that would be used to identify the number of compounds in the mixture. 

A student runs a thin-layer chromatography experiment and plans to determine the compounds from their R_f values.

Describe the steps that the student needs to perform to determine the identity of the compounds. 

The student’s results are shown in Fig. 1.1.

Fig. 1.1

For their measurements, the student locates the centre of each spot by estimating its rough position by eye.

Suggest an improved method to locate the centre of each spot. 

The student runs another TLC practical on a mixture of benzaldehyde and benzyl alcohol using 7:3 pentane / diethyl ether as a solvent. 

The student's chromatogram is shown in Fig. 1.2.

Fig. 1.2

Calculate the R_f values for both compounds in the chromatogram. 

Explain why the maximum R_f value of a compound cannot exceed 1.