Question

This isn't a homework assignment more of a search for advice. The lab experiment that my group is conducting is to separate two mixtures, a solid-solid and a liquid-liquid mixture. Each mixture has 2 separate components. The solid-solid mixture was easy enough, a quick solvent extraction completely separated the two and soon we will do IR, NMR and melting point determination to try to identify the solids.

The liquids however are another problem all in themselves. The mixture is tinted yellow and is translucent. The two liquids are entirely miscible and there seems to be no obvious phycial way to separate them. There is nothing known about the liquids currently except what we've measured;

The mixture is neutral pH

The mixture has at least one aromatic comound

We've been unable to observe a major polarity difference in the mixture, we had time to run 2 TLC plates, one with polar eluent and one with a mixed 75/25 polar/non-polar eluent. The Rf values have overlapped on both so we believe the compounds have similar polarities, we plan on running a few more with different eluents. This is also why we've been unable to determine whether both compounds are aromatic or only one is.

There is a good chance that the compounds are constitutional isomers of each other based on the hints that our TA has been dropping. We're extremely reluctant to attempt a distillation because as far as we can tell the polarities are extremely similar, as it is directly related to boiling point it is unlikely that the boiling points will be different enough to separate the compounds without a professional distillation set-up. We only have microscale organic kits to work with and previous distillations using the kit resulted in major loss of compound if there is any separation at all.

The rules state we cannot use more than 5mL of the unknown mixture total, for all of the tests we utilize. As a result we cannot afford to lose any to faulty processes like distillation.

The Characterizational techniques we are experienced in using are IR, Melting point, boiling point, refractive index, polarimetry. We are NOT allowed to use Mass Spec. and we can ONLY use NMR after we can prove that we are reasonably confident in the identity of our unknowns.

We're out of ideas so we're hoping for some solid advice and discussion. Thank you!

Tools we have access to are;

TLC

Staining Agents

UV lights

Sep. Funnel

Microscale Distillation (not recommended)

Essentially unlimited glassware

Hot plates and ice-baths

Reagents: HCl, NaOH, Sodium Anhydride, various solvents: toluene, hexanes, ethyl acetate, diethyl ether, water, ethanol, methanol.

I highlighted any relevant information regarding the liquids.

Answer 1

Since the polarities are very similar, distillation might not be an ideal process (as already deciphered). As a suggestion, I was wondering if the cooling crystallization or fractional crystallition method could work in your case or not. Cooling crystallization method can be applied to mixtures whose components have nearly the same boiling point (as in your case) but different melting points. In this method, by lowering the temperature of the mixture, the compound with the highest melting point will crystallize first as solid crystals, and can then be separated. Of course, one needs to attain supersaturation in this method. As the temperatured is gradually lowered, the solubility of the solute will decrease, and crystals would gradually form. Since you have both hot plates and ice baths available, this method could be tried. (For a reference, this method is also used to separate para-xylene from a mixture of other xylenes (ortho and meta forms)).