I spotted a commercial TLC plate, coated with silica gel, with a pure benzoin solution. After running the TLC plate in methylene chloride, the benzoin separated into two different spots. Rf values are 0.6 and 0.9. What explains this? A pure material should have only one spot that travels, should it not?
Here is a link. Pay attention to the S and R enantiomers.
https://en.wikipedia.org/wiki/Benzoin
The separation of a pure benzoin solution into two different spots on a TLC plate can be explained by a phenomenon known as tautomerism. Tautomers are isomers of organic compounds that can rapidly interconvert by the migration of a proton. In this case, benzoin can exist in two tautomeric forms: keto and enol.
When a pure benzoin solution is spotted onto a TLC plate coated with silica gel (which is polar in nature), the compound undergoes partitioning between the mobile phase (methylene chloride) and the stationary phase (silica gel). Tautomeric compounds may exhibit different degrees of partitioning due to their structural differences, resulting in separation into distinct spots on the TLC plate.
The Rf (retention factor) values of 0.6 and 0.9 indicate the distance traveled by the compound relative to the distance traveled by the solvent front. The Rf value represents the polarity of the compound and how strongly it interacts with the stationary phase.
In this case, the difference in Rf values indicates that one of the tautomeric forms of benzoin has a higher affinity for the stationary phase (silica gel) and therefore travels less distance, resulting in a lower Rf value (0.6). The other tautomeric form has a lower affinity for the stationary phase and preferentially travels further, resulting in a higher Rf value (0.9).
It's important to note that although the presence of multiple spots on the TLC plate suggests the existence of different tautomeric forms, further characterization techniques such as spectroscopy (e.g., NMR) would be needed to confirm the identity of these compounds.