How to be a Successful Organic Chemist

17 One common problem with extractions is one that does not arise from improper technique, but from the annoying behavior of certain compounds: they form emulsions. These usually appear between the two lay- ers, and are chemically composed of the two solvents, and other compounds found in the original solution. The emulsion can resolve itself over a few minutes, or be very persistent. In the latter case, it can sometimes be a good strategy to drain the emulsion out in the phase that does not contain the product, and then re-ex- tract it several times. Let us say that we are extracting an aqueous solution with the organic solvent diethyl ether. Let us also say that the desired compound is organic, and in this example will be dissolved in the organic phase. A very persistent emulsion is formed. If we drain the aqueous phase and as little of the organic phase as we can (while still draining the emulsion out), we can obtain some organic phase without emulsion that is set aside. We then add the phase back to the separatory funnel and re-extract the aqueous phase that still contains some organic phase, as well as emulsion again. This usually disperse the emulsion. 2.4 TLC -analysis Thin-layer chromatography (or TLC) is a quick, cheap, and reliable way to evaluate the composition of a sample, and the identity of a given compound. The TLC plate works by chromatographic principles: a mobile phase (solvent or solvent mixtures) will climb up the plate material (stationary phase). Compounds that are less polar will wander farther on the plate, because they experience les1s attraction to the stationary material. Compounds that are more polar will wander lower on the plate, because they experience more at- traction to the stationary material. This process is separating the different compounds present in the crude sample Most compounds that we handle in the o-chem labs at PSU should give a single spot on the TLC plate. Ideally, each compound in a mixture will produce a distinct spot so a sample with two compounds will give two different spots, and so on. An important property of any compound, is its R f -value (retention factor). In simple terms, this value is an indication of how far up a TLC-plate a compound has wandered. A high R f -value indicates that the compound has travelled far up the plate and is less polar, while a lower R f -value indicates that the com- pound has not travelled far, and is more polar. This value is easily calculated by measuring the distance the spot has wandered, and dividing this by the distance the solvent has traveled. The R f -value is dependent on both the compound and the solvent used for development. If we analyze two compounds and they give the same R f -value with the same solvent system, the two compounds are most likely identical. Figure 2.7. Calculation of the R f -values of two spots. The apparatus for doing a TLC-experiment is very simple. We need a developing chamber where the mobile phase is kept.The chamber should be sealed, and although there are special TLC-chambers one can use, a beaker covered by a watch glass to create a seal often suffices.