Infrared spectra carbonyl functional groups

Different types of carbonyl groups give characteristic strong absorptions at different positions in the infrared spectrum. As a result, infrared spectroscopy is often the best method to detect and differentiate these carboxylic acid derivatives. Table 21-3 summarizes the characteristic IR absorptions of carbonyl functional groups. As in Chapter 12, we are using about 1710 cm-1 for simple ketones and acids as a standard for comparison. Appendix 2 gives a more complete table of characteristic IR frequencies. 

The infrared spectrum of the product was similar to that for the starting polymer with additional peaks due to carboxylic acid and ketone carbonyl functional groups. Figure 1 shows the infrared spectra of the starting poly (IB-PMS) copolymer... 

Compound A undergoes hydrolysis of its acetal function in dilute sulfuric acid to yield 1,2-ethanediol and compound B (CgHg02), mp 54°C. Compound B exhibits a carbonyl stretching band in the infrared at 1690 cm and has two singlets in its H NMR spectrum, at 8 2.9 and 6.7, in the ratio 2 1. On standing in water or ethanol, compound B is converted cleanly to an isomeric substance, compound C, mp 172—173°C. Compound C has no peaks attributable to carbonyl groups in its infrared spectrum. Identify compounds B and C. 

One of the main routine uses of infrared spectroscopy is identification of specific functional groups present in an unknown molecule and, as a result, further characterization of the unknown. By far the most common example involves the carbonyl group. Location of a strong band in the infrared in the vicinity of 1730cm is almost certain proof that carbonyl functionality is present. This confidence is based on the fact that the characteristic frequency (the CO stretch in this case) is isolated, that is to say, it is sufficiently far removed from the other bands in the infrared spectrum to not be confused with them. It also assumes that carbonyl groups in different chemical environments will exhibit similar characteristic... 

These color characteristics are similar to those of the model phthalocyanine (VII) as well as unsubstituted phthalocyanine. Furthermore, the infrared spectrum of a sample of I cured as a thin film with added hydroquinone is very similar to that of the model phthalocyanine including the distinctly weak but sharp N-H band at 3290 cm. Notably absent are bands corresponding to carbonyl or triazine functional groups. 

Some functional groups, for example, C=0 or C=C, can be seen in the NMR spectrum because they contain carbon atoms, while the presence of others like OH can be inferred from the chemical shifts of the carbon atoms they are joined to. Others cannot be seen at all. These might include NH2 and NO2, as well as variations around a carbonyl group such as COCl, CO2H, and CONH2. Infrared (IR)... 

The presence of bands in the infrared spectrum of hydroxy compound 27 in the region 1710-1760 cm has been taken to mean that the compound exists as the oxo compound 28. However, the isomeric compound 29 is reported to have bands in the infrared characteristic of hydroxyl groups and amide carbonyl functions. Accordingly it is suggested that this is evidence for the presence of the equilibrium 29 30. Similarly the mercapto derivative 31 shows evidence of the equilibrium 31 32. In contrast, the quinoxaline analogue 33 is completely in the thioamide form. ... 

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