Question

TLC analysis of an unknown sample using pure ethyl acetate as a developing solvent gives a single spot with an Rf value of 0.08.

A. Explain why this result doesn't guarantee that the sample is a pure compound.

B. Suggest two changes that could be made to the experiment to improve the analysis of the unkonwn sample.

Answer 1

A) A single spot does not necessarily indicate the purity of the compound. A single substance gives a single spot no matter which solvent is used to develop the plate. However, in mixtures, it may be possible to have a single spot if the appropriate solvent to separate the components of the mixture is not used. Moreover, the solvent used is Ethylacetate, and the Rf value is very less (0.08). Since Ethyl acetate is more polar than hexane because of its oxygens. Therefore, the developing solvent is a polar solvent, the more polar compounds will run up the TLC plate with the solvent.

B) When dealing with compounds of similar properties, such as isomers, multiple developments of the TLC plate with various solvents may be necessary to effectively separate compounds and so to determine purity.

i) Change the polarity of the solvent in a trail and error method and measure Rf value.

initially use Hexane (non polar solvent) then increase the % of polar solvent like Ethylacetate.

We can use the mixture of both like 80:20 (80% Hexane and 20% ethylacetate) or 50:50 and so on. In this way we can easily observe wether the spot is single or mixture of compounds.

A high Rf (i.e 0.92) would refer to a substance that is very non-polar. i.e that substance moved a 92% of the entire distance the solvent traveled. A low Rf value (0.10) would refer to a substance that is very polar.

ii) The other most important technique should be used is NMR.

Record a H1 (proton) NMR of unknown sample and observe the peaks. Pure compound gives a nice NMR spectrum where as mixture gives a complex spectrum.