How to be a Successful Organic Chemist

31 C-O peak). The spectrum from the stock sample of product, lacks the OH-peak, but a very characteristic C=O peak is readily observed. The IR-spectrum of the student’s product also indicates the presence of the desired ketone (strong C=O peak), but it also reveals the same O-H peak found in the starting material’s spectrum. One logical conclusion from this analysis is that the student’s oxidation has not gone to completion or that the purification has not been satisfactory. This is a microcosm of the entirety of IR -analysis: looking for the appearance of disappearance of spe- cific peaks to answer the question of whether a reaction has worked or not. 5.3 Melting point analysis: identity and purity The melting point of a compound is useful in two ways: it says something about the identity of a compound, and something about the purity of a compound. The presence of impurities will influence the melting point of a compound, leading to wider and depressed melting point ranges. Because even small deviations in melting point ranges is meaningful, it is worth mentioning that a control should be made when running the melting point of an isolated compound, against a stock sample of the same compound. In general, a de- viation of more than 1 oC in obtained melting point usually means the material is not of acceptable purity. Because the margin is relatively small, it is always worth running your sample with an authentic sample of the compound in the same melting point apparatus. If there are even small deviations in the apparatus, you will find that the authentic sample’s melting point is also deviating from the literature value. Melting points are usually not the only analytical data obtained for a sample; routinely IR-spectroscopy is also performed. Any abnormality in either data set should reflect in the other data source. For example, an IR spectrum corresponding to a product with a depressed melting point will often show peaks that are not expected for a pure sample. It is worth noting that this is not necessarily the case, as the impurities must also produce strong peaks in non-crowded areas of the IR-spectrum to be detect- able. Let us look at a specific example. A student recrystallized a sample of 2-naphthol from ethyl acetate. The published literature on 2-naphthol lists its melting point as 121-123 ° C, but the student’s sample melted from 115- 121 ° C, a range that was much broader and much lower than the literature one. Figure 5.3. Student’s sample of potential product. Figure 5.4. An IR spectrum obtained by a student.