How to be a Successful Organic Chemist

29 15 How to INTERPRET YOUR RESULTS Introduction Upon completion of the lab, you will move into a new phase of your lab experience. You now must now carefully evaluate your results with care and interpret them in a logical way. In this chapter, we will focus on the interpretation of the three most important types of results. 5.1 What does it all mean? We have already said that any experiment has some inherent goal. Many organic chemistry experiments are performed for the express purpose of obtaining a target com- pound. The obvious goal of these experiments is the target compound, but how can you satisfy yourself that the goal has been achieved? Of equal importance to the target are data and measurements that prove the identity and purity of the target. Some results that indicate the identity and purity of a compound are IR spectroscopy, melting point analysis, and TLC-analysis. As you will see, these results provide different kinds of information so you will normally collect more than one type of data. Let’s go through some typical scenarios, that illustrate the ways that data can be interpreted. 5.2 IR -spectroscopy: the workhorse IR-spectroscopy is perhaps the most frequently used technique in the organic chemistry labs at PSU. It is routinely used to identify products and to verify that an experiment has succeeded. One limitation of IR -spectroscopy that is worth remembering is that while the data can be used to distinguish clearly between many functional groups, the data is nearly useless for distinguishing between structural isomers, or charac- terizing carbon skeletons that contain the same functional groups An IR -spectrum routinely shows peaks from the range of 3600 to 500 cm -1 . IR -frequencies correspond to the frequencies of molecular vibrations. Molecules vibrate at many fre- quencies, because each vibration involves a particular group of atoms, and a particular motion such as stretches, bending, wagging etc.The same group of atoms will often vibrate at the same frequency, regardless of the surrounding molecular structure. This fact enables us to assign specific peaks to specific functional groups. There are several useful sources of IR-spectroscopy correlation tables, and a simplified version is supplied here. Although not very extensive, this table gives you information about the most important functional groups encountered in the organic chemistry labs at PSU. Something that is worth noting is that many functional groups undergo two types of vibrations and pro- duce peaks in two or more areas of the IR- spectrum. In these cases we should look for, and find, all of the