Organic Chemistry I

Table 6.1
and
Figure 6.3b
provide very useful information
to identify the presence of certain functional group, that can be generally summarized as:
The polar
O-H bond
(in alcohol and carboxylic acid) usually shows strong and broad absorption bands that are easy to
be identified. 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
).
The polarity of
N-H bond
(in amine and amide) is weaker than OH bond, so the absorption band of N-H is not as
intense, nor that broad as O-H, and the position is in 3300-3500 cm
-1
region.
The
C-H bond
stretching of all hydrocarbons occur in the range of 2800-3300 cm
-1
, and the exact location can be
used to distinguish between alkane, alkene and alkyne. Specifically:
•
≡
C-H (sp C-H) bond of terminal alkyne give absorption at about 3300 cm
-1
• =C-H (sp
2
C-H) bond of alkene give absorption at about 3000-3100 cm
-1
• -C-H (sp
3
C-H) bond of alkane give absorption at about ~2900 cm
-1
(see the example of IR spectrum of 2-hexanone
in
Figure 6.3a,
the C-H absorption band at about 2900 cm
-1
)
A special note should be taken for the C-H bond stretching of an aldehyde group that shows two absorption bands, one
at ~2800 cm
-1
and the other at ~ 2700 cm
-1
. It is therefore relative easy to identify the aldehyde group (together with the
C=O stretching at about 1700 cm
-1
) since essentially no other absorptions occur at these wavenumbers (see the example
of IR spectrum of butanal in