How to be a Successful Organic Chemist

32 These data are a clear indication that the product is not pure, but it always is preferable to be able to identify the source of the impurity as opposed to simply stating that there were impurities present. The key to this puzzle is found in the student’s IR -spectrum. You will notice a clear indication of an OH-bond (around 3200 cm -1 ) that is likely due to 2-naphthol, but you will also notice something else: a clear C=O peak. It is very likely that this is trace ethyl acetate left over from the recrystallization, which indicates that the sample was not allowed to air-dry sufficiently. A powerful observation is found when the two analyses are combined and viewed together. 5.4 TLC: identity and purity TLC can be a very useful resource to obtain quick and usefyul data about purity and identity. Unlike melting point analysis, where measurements can be compared to liter- ature values, control TLC – experiments are essential to support the presence of com- pounds of interest. TLC has an advantage over melting point analysis, however, in that it can be applied to both liquids and solids. Let us say that a student brominated acetophenone using bromine (scheme 1). Be- cause the product contains a C-Br bond, IR -spectroscopy is not very useful (the peak assosicated with a C-Br lies in a region of the spectrum that is hard to observe). Instead, the student performed a TLC analysis. In order to get conclusive data from the TLC analysis, the student made up a TLC – plate with four samples. Spot A is the starting material acetophenone, spot B is a stock solution of the product, spot C is the product obtained by the student, and spot D is a co-spot of two solutions, the stock solution of product (spot B) and the student’s product (spot C). The TLC -analysis gives us important information about the experiment. The first result is the student’s sample, C, which appears to be two compounds. One of those (the darker spot) agrees with authentic prod- uct (B), but the other (the lighter spot) agrees with the starting material (A).This suggests that the student’s product is contaminated with unreacted starting material. Spot D supports this interpretation: samples B and C produce a single dark spot in D. Scheme 1. Bromination of acetophenone. Figure 5.5. The TLC plate obtained by the student.