Infrared spectrum functional group region

Know the meaning of the functional group region and fingerprint region of an infrared spectrum, and tell what kind of information can be obtained from each. 

A broad peak at about 1100 cm" appears in the infrared spectrum of diethyl ether and is assigned to be the C-O stretch. Is this peak in the fingerprint or the functional group region ... 

Table 14.3 shows the major absorptions for several common functional groups. Most but not all of these absorptions appear in the functional group region of the infrared spectrum (4800-1400 cm i). A few will be seen in the fingerprint region (1400-400 cm i). 

Fingerprint region (Section 13 20) The region 1400-625 cm of an infrared spectrum This region is less character istic of functional groups than others but varies so much from one molecule to another that it can be used to deter mine whether two substances are identical or not Fischer esterification (Sections 15 8 and 19 14) Acid cat alyzed ester formation between an alcohol and a carboxylic acid... 

The next most useful is vibrational spectroscopy but identification of large molecules is still uncertain. In the laboratory, vibrational spectroscopy in the infrared (IR) is used routinely to identify the functional groups in organic molecules but although this is important information it is not sufficient to identify the molecule. Even in the fingerprint region where the low wavenumber floppy vibrational modes of big molecules are observed, this is hardly diagnostic of structure. On occasion, however, when the vibrational transition can be resolved rotationally then the analysis of the spectrum becomes more certain. 

Figure 14.4. Infrared spectra showing common absorption bands. The —OH of acids and alcohols are hydrogen-bonded, leading to broadening.The —OH of hexanoic acid, which is unusually sharp, absorbs in the 3500-2500 cm-1 region. Bottom spectrum shows absorptions due to functional groups in the compounds making up the mixture.

The infrared region of the electromagnetic spectrum is used for the study of molecular vibrations. For any molecule, the pattern of absorption bands is as unique as a fingerprint, and many individual bands can be related to specific groups or structures. This feature may be used for the identification of a molecule or functional group and for the quantitative analysis of simple mixtures. As infrared spectra are complex, mixtures may need separation before measurements can be made. 

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