Infrared Spectra of Some Common Functional Groups

9 Infrared Spectra of Some Common Functional Croups 

As each functional group is discussed in future chapters, the spectroscopic behavior of that group will be described. For the present, though, we ll simply point out some distinguishing features of the more common functional groups. 

The O-H functional group of alcohols is easy to spot in the IR. Alcohols have a characteristic band in the range 3400-3650 cm 1 that is usually broad and intense. If present, it s hard to miss this band or to confuse it with anything else. 

The N-H functional group of amines is also easy to spot in the IR, with a characteristic absorption in the 3300-3500 cm 1 range. Although alcohols absorb in the same range, an N-H absorption is much sharper and less 

Carbonyl functional groups are the easiest to identify of all IR absorptions because of their sharp, intense peak in the range 1670—1780 cm-1. Most important, the exact position of absorption within the range can often be used to identify the exact kind of carbonyl functional group—aldehyde, ketone, ester, and so forth. 

As each functional group is discussed in future chapters, the spectroscopic properties of that group will be described. For the present, we ll point out some distinguishing features of the hydrocarbon functional groups already studied and briefly preview some other common functional groups. We should also point out, however, that in addition to interpreting absorptions that ore present in an IR spectrum, it s also possible to get structural information by noticing which absorptions are not present. If the spectrum of a compound has no absorptions at 3300 and 2150 cm-1, the compound is not a terminal alkyne if the spectrum has no absorption near 3400 cm -, the compound Is not an alcohol and so on. 

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-H bonds show a strong absorption from 2850 to 2960 cm-1, and saturated C—C bonds show a number of bands in the 800 to 1300 cm-1 range. 

Monosubstituted and disubstjtuted alkenes have characteristic =C—H out-ofplane bending absorptions in the 700 to 1000 cm-1 range( thereby allowing the substitution pattern on a double bond to be determined. Monosubstituted alkenes such as 1-hexene show strong characteristic bands at 910 and 990 cm-1, and 2,2-disubstituted alkenes (R2C=CH2) have an intense band at 890 cm-1. 

Aromatic compounds such as benzene have a weak C-H stretching absorption at 3030 cm-1, a series of weak absorptions in the 1660 to 2000 cm-1 range, and a second series of medium-intensity absorptions in the 1450 to 1600 cm 1 region. These latter absorptions are due to complex molecular motions of the 

Copyright 20l0Cengage Learning. All Rights Reserved. May not be coped, scanned, or diqilicated, in whole or in part. Due to electronic rights, sane third party content may be supiressed fron the eBook and/or eCh )ter( ). Editorial review has deemed tfiat any suppressed coitent does not materially affect the overall leamii experience. Cei age Leamii reserves the right to remove additional content at any time if subsequent rights restrictions require it. 

---

In this section, we discuss the identification process and the chemical structures of polymers as obtained from their infrared and Raman spectra. Many polymers have common features, and it is convenient to segregate polymers into groups, such that the characterization of the polymers in a group can be discussed together. A popular method for classifying polymers is by their modes of application. For instance, some polymers, such as polyvinyl acetate, polystyrene, and nylons, are classified as thermoplastics, while urea, melamine, and epoxide resins are classified as thermosets or thermosetting resins. In this chapter we will use a different approach to classify the polymers, based on their similarity of chemical structure. This enables us to utilize the correlation between the functional groups of polymers and their characteristic infrared and Raman frequencies. 

---