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The following table provides a collection of such data for the most common functional groups. Following the color scheme of the chart, stretching absorptions are listed in the blue-shaded section and bending absorptions in the green shaded part.
16 Δεκ 2021 · The broad shape of the absorption band results from the hydrogen bonding of the OH groups between molecules. The OH bond of alcohol group usually has absorption in the range of 3200-3600 cm-1, while the OH bond of carboxylic acid group occurs at about 2500-3300 cm-1 (Figure 6.4a and Figure 6.4c).
IR SPECTRUM OF A CARBOXYLIC ACID. A carboxylic acid functional group combines the features of alcohols and ketones because it has both the O-H bond and the C=O bond. Therefore carboxylic acids show a very strong and broad band covering a wide range between 2800 and 3500 cm-1 for the O-H stretch.
By learning where characteristic functional-group absorptions occur, it’s possible to get structural information from IR spectra. Table 12.1 lists the characteristic IR bands of some common functional groups.
The IR spectrum of an alkane is fairly uninformative because no functional groups are present and all absorptions are due to C–H and C–C bonds. Alkane C... Skip to Content Go to accessibility page Keyboard shortcuts menu
After completing this section, you should be able to use an infrared spectrum to determine the presence of functional groups, such as alcohols, amines and carbonyl groups, in an unknown compound, given a list of infrared absorption frequencies.
The N–H functional group of amines is also easy to spot in the IR, with a characteristic absorption in the 3300 to 3500 cm–1 range. Although alcohols absorb in the same range, an N–H absorption band is much sharper and less intense than an O–H band.
