How to be a Successful Organic Chemist

18 How to prepare a TLC plate: 1. A horizontal line is drawn at the bottom of the plate (5-8 mm from the bottom edge) and another horizontal line is drawn at the top of the plate (8 mm from the top edge). A gentle soft and a very soft pencil must be used because too much pressure or harder pencils will crack the very thin coating on the plate, leading to unaccept- able TLC results after development (ink pens should never be used because the ink will wander up the plate). 2. Vertical ticks are drawn on the bottom line, depending on how many samples are analyzed. These are usually labeled (A, B, C, for example) 3. Solutions are prepared of all the samples that must be analyzed. These must neither be too concentrated, nor too dilute and a concentration of about 5% is a good starting point. The samples are applied to the ticks with capillary tubes without damaging the coating on the plate. 4. The TLC plate is placed into the developing chamber, and care is taken to make sure that the bottom edge of the plate is perpendicular to the solvent. If not, parts of the plate will overdevelop, and we will get poor resolution and unclear results. 5. The chamber is sealed with the watch glass and the solvent front is allowed to climb up to the topmost line. We then remove the plate and allow it to dry before we observe the plate under UV-light. We will discuss two common ways to use a TLC- analysis. The first is to verify the identify of a com- pound. A quick TLC analysis can be used to identify whether or not an unknown compound is the same as another known compound. We typically make three applications: one of the unknown sample, one of a stock (known) compound, and a third with one spot of both (called a co-spot). If we find that the two spots have the same R f -values, and the third spot only shows one spot, the two compounds are identical. The second common way to use a TLC- plate, is to monitor a reaction.We can use TLC-analyses to see whether or not a reaction has gone to completion. We typically see the disappearance of the starting mate- rial(s) and the emergence of a new spot that corresponds to the product. We usually have to make controls as described above, with spots of the starting material(s) to learn their R f -values. A B U A+U or B+U Figure 2.8. A labeled TLC plate.