Section 13.8 Characteristic Infrared Absorption Bands 501Wavelength (μm)Wavelength (μm)4000380036003400320030002800260024002200 2000 1800 1600 1400 1200 1000 800 600
Wavenumber (cm−^1 )Wavenumber (cm−^1 )0100 2.52.6 2.7 2.8 2.9 3 3.5^4 4.5^5 5.5^6789101112131415162.52.6 2.7 2.8 2.9 3 3.5 4 4.5 5 5.5 6 7 8 9 10 11 12 13 14 15 164000380036003400320030002800260024002200 2000 1800 1600 1400 1200 1000 800 600% Transmittance% Transmittance0100a.b.CH 3 CH 2 CHCH 2 CH 3OHCH 3 CHCH 2 CH 2 CH 3OHFigure 13.14
The IR spectra of (a) 2-pentanol and
(b) 3-pentanol. The functional
group regions are very similar
because the two compounds have
the same functional group, but the
fingerprint regions are unique for
each compound.An IR spectrum can be divided into two areas. The left-hand two-thirds of an IR
spectrum is where most of the functional groups show absorption
bands. This region is called the functional group region. The right-hand third
of the IR spectrum is called the fingerprint regionbecause it is
characteristic of the compound as a whole, just as a fingerprint is characteristic of an
individual. Even if two different molecules have the same functional groups, their IR
spectra will not be identical, since the functional groups are not in exactly the same en-
vironment; this difference is reflected in the pattern of absorption bands in the finger-
print regions. Each compound shows a unique pattern in this region. For example,
2-pentanol and 3-pentanol have the same functional groups, so they show similar ab-
sorption bands in the functional group region. Their fingerprint regions are different,
however, because the compounds are different (Figure 13.14). Thus a compound can
be positively identified by comparing its fingerprint region with the fingerprint region
of the spectrum of a known sample of the compound.
13.8 Characteristic Infrared Absorption Bands
The stretching and bending vibrations of each bond in a molecule can give rise to an
absorption band, so IR spectra can be quite complex. Organic chemists generally do
not try to identify all the absorption bands in an IR spectrum. In this chapter, we will
look at some characteristic bands so that you will be able to tell something about the
structure of a compound that gives a particular IR spectrum. However, there is a lot
more to infrared spectroscopy than we will be able to cover. You can find an extensive
table of characteristic group frequencies in Appendix VI. When identifying an un-
known compound, one often uses IR spectroscopy in conjunction with information ob-
tained from other spectroscopic techniques. Many of the problems in this chapter and
11400 – 600 cm-^12
14000 – 1400 cm-^12