Organic Chemistry I

-1
region indicate that a carbonyl group (C=O) is present
.
Within
that range, carboxylic acids, esters, ketones and aldehydes tend to absorb in the higher wavenumber/frequency end
(1700-1750 cm
-1
), while conjugated unsaturated ketones and amides tend to absorb on the lower wavenumber/frequency
end (1650-1700 cm
-1
).
Stretching Vibrations
Generally, stretching vibrations the stretching vibrations require more energy and show absorption bands in the
higher wavenumber/frequency region. The characteristics stretching vibration bands associated with the bonds in
some common functional groups are summarized in
Table 6.1
.
Formula
Bond
Characteristic IR Frequency range (cm
-1
)
alcohol
O-H stretching
3200 – 3600 (broad)
carbonyl
C=O stretching
1650 – 1750 (strong)
aldehyde
C-H stretching
~ 2800 and ~ 2700 (medium)
carboxylic
acid
C=O stretching
1700 – 1725 (strong)
O-H stretching
2500 – 3300 (broad)
alkene
C=C stretching
1620 – 1680 (weak)
vinyl =C-H stretching
3020 – 3080
benzene
C=C stretching
~ 1600 and 1500 – 1430
(strong to weak)
alkyne
C
≡
C stretching
2100 – 2250 (weak)
terminal 
≡
C-H stretching
3250 – 3350
alkane
C-H stretching
2850-2950
amine
N-H stretching
3300-3500 (medium)
Table 6.1 Characteristic IR Frequencies of Stretching Vibrations
198 | 6.3 IR Spectrum and Characteristic Absorption Bands

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The information in
Table 6.1
can be summarized in the diagram that is easier to be identified (
Figure 6.3b
), in which
the IR spectrum is divided in several regions, with the characteristic band of certain groups labelled.
Figure 6.3b Approximate IR Absorption Range
The absorption bands in IR spectra have different intensity, that can usually be referred to as strong (s), medium (m),
weak (w), broad and sharp. The intensity of a absorption band depends on the polarity of the bond, the bond with higher
polarity will show more intense absorption band. The intensity also depends on the number of bonds responsible for
the absorption, the absorption band with more bonds involved has higher intensity.
The characteristic IR frequencies of stretching vibrations in