Applying Spectroscopic Methods to Qualitative Organic Analysis

5 APPLYING SPECTROSCOPIC METHODS TO QUALITATIVE ORGANIC ANALYSIS 

The spectral data serve to complement or supplement the wet qualitative classification tests and in many instances substitute for these tests entirely. For example, a strong IR absorption in the 1690-1760 cm region is just as indicative of the presence of a carbonyl group as is formation of a 2,4-dinitrophenylhydrazone, and absorptions in the 8 6.0-8.5 region of the NMR spectrum are a more rehable indicator of the presence of an aromatic ring than a color test with CHCI3 and AICI3. 

We emphasize that spectroscopic analysis also requires that you carefully interpret the data. Although you may solve some problems quickly and uniquely by modern spectroscopy, you may find that others require the intelligent application of both the modern methods and the classic methods. The remainder of this section illustrates the use of IR and NMR spectroscopy in structure determination. 

The research chemist usually obtains IR and NMR spectra of an unknown substance even before obtaining an elemental analysis or performing solubility tests because these types of spectra are easily and quickly measured on a small amount of sample. Only after analyzing them does the researcher undertake other experimental approaches for determining the structure. This process can save many hours of unnecessary laboratory work. Other useful information about a compound may be obtained from mass spectrometry (Sec. 8.5), which provides the molar mass of the compound and, if high-resolution data are available, its elemental composition. 

The following example illustrates applying spectral analysis to identifying an unknown compound. 

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