Question

Explain how you used TLC analysis to identify each product obtained from the dihydroxylation reactions. Explain why 100% ethyl acetate was used to develop your TLC plate (and not 100% hexane or a mixture of hexane/ethyl acetate)?

Answer 1

Answers

TLC Analysis of Dihydroxylation reactions

Dihydroxylation reaction can be carried out in different ways like Epoxidation hydrolysis, Permanganate oxidation and Woodward dihydroxylation. In some reactions, the products formed are both -cis and -trans diastereoisomers. But it can be easily separated by TLC with different Rf values. Another method is to stain the sample spots with p-anisaldehyde stain. The -cis and -trans diastereomers can be identified with different colours. For –cis it shows red stain and for –trans it shows blue stain. There are all the methods to analysis the products of dihydroxylation reaction.

Why 100% ethyl acetate is used to develop TLC?

There are two phases in the Thin Layer Chromatography. (i) Stationary Phase and (ii) Mobile phase.

1. Stationary Phase – Not moving phase. The TLC plate is a silica coated aluminium plate. So the silica used in TLC is a stationary phase.
2. Mobile Phase – Moving phase. Solvents or a mixture of solvents is used to run the TLC is a mobile phase. The polarity of the solvents is used to identify/separate the organic samples present in the TLC plate based on its Rf values.

Important Note: If the sample is more polar, then the attraction between the sample and the silica is more and the sample will dwell with the silica rather than eluting/moving. In this case, we should use polar solvents to elute such type of compounds.

In dihydroxylation reaction, the product formed with two hydroxyl groups (-OH). As the hydroxyl groups present in the products are highly polar, it will attract by the silica and dwell to it. To move/separate the sample spots in TLC, more polar solvents like ethyl acetate are used as a mobile phase to elute the sample spot. The polarity index of ethyl acetate is 4.4. Since the hexane is a non-polar solvent (polarity index–0.1), the mixture of hexane and ethyl acetate or 100% hexane will decrease the polarity of the mobile phase. This is the reason why the 100% ethyl acetate is used as a mobile phase for TLC