Organic functional groups spectral characteristics

Table 6 Spectral characteristics of some organic functional groups...

Tables 6.3-6.5 record data developed to undertake structural analysis in systems possessing chromophores that are conjugated or otherwise interact with each other. Chromophores within a molecule interact when linked directly to each other or when they are forced into proximity owing to structural constraints. Certain combinations of functional groups comprise chromophoric systems that exhibit characteristic absorption bands. In the era when UV-VIS was one of the principal spectral methods available to the organic chemist, sets of empirical rules were developed to extract as much information as possible from the spectra. The correlations referred to as Woodward s rules or the Woodward-Fieser rules, enable the absorption maxima of dienes (Table 6.3) and enones and dienones (Table 6.4) to be predicted. When this method is applied, wavelength increments correlated to structural features are added to the respective base values (absorption wavelength of parent compound). The data refer to spectra determined in methanol or ethanol. When other solvents are used, a numerical correction must be applied. These corrections are recorded in Table 6.5.

Spectroscopic techniques (IR, MS, and NMR) are covered in Chapters 12 and 13, so that they can be included in the first semester. This early coverage is needed to allow effective use of spectroscopy in the laboratory. Still, a large amount of organic chemistry has been covered before this digression into structure determination. The principles of spectroscopy are practiced and reinforced in later chapters, where the characteristic spectral features of each functional group are summarized and reinforced by practice problems. 

Infrared (IR) spectra of organic compounds are characteristic of various functional groups in the molecules. IR spectral information is somewhat complementary to mass spectral information. Therefore, the combination of GC with IR spectroscopy is, after GC/mass spectrometry, the second most important structural identification tool. Since conventional IR spectroscopy is less sensitive than most GC detectors, the necessary sensitivity enhancement is achieved through the use of Fourier transform techniques. With the advent of refined optical systems and fast computational techniques, the combination of GC with Fourier-transform IR spectrometry is becoming widely used, although its sensitivity is currently less than that of mass spectrometry. Special optical cells were designed for the purposes of this combination. 

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