TLC = That’s Lousy Chromatography?

This year I changed one of the solvents in the TLC lab that begins the first semester of Organic Chemistry lab. Instead of dichloromethane, we used ethyl acetate. Ethyl acetate is a less hazardous and more environmentally friendly solvent that dichloromethane. I thought the solvent strength on silica gel would be similar between the two. Ethyl acetate (eluotropic value on silica 0.38 – 0.48) is a little higher on the eluotropic scale than dichloromethane (0.32). The other two solvents we use are hexane (0.00 – 0.01) and methanol (0.70 – 0.73). The compounds we work with in this lab are all benzene derivatives: biphenyl, 2-naphthol, m-toluic acid, 4-chloroaniline, acetophenone, and 2-methoxynaphthalene. Surprisingly, the Rf values for these compounds developed by the students in ethyl acetate were consistently higher than in methanol. I don’t see an easy explanation why this would happen. The difference in solubility of the compounds in ethyl acetate and methanol might be a factor. A molecule less soluble in methanol is less likely to pushed along the TLC plate by that solvent. Another aspect to consider would be that Rf values over 0.8 are more-or-less meaningless. Dichloromethane gives Rf values between 0.38 (benzoic acid) and 0.73 (biphenyl) so it is easy to see why dichloromethane, and not ethyl acetate, would have been originally chosen for this experiment. It is also worth noting that TLCs are rarely (if ever) done in a single solvent. Evidently, the push-pull effect of combining two or more solvents adds an important dimension to TLC development.


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