Sometimes more complicated heteroatomic functional groups, containing bonds to more than one heteroatom, have slightly different spectra. Carboxylic acids feature a hydroxyl group bonded to a carbonyl. Hexanoic acid, a carboxylic acid in a six-atom chain, is one example.
If you look at the IR spectrum of hexanoic acid:
- there are CH2 bending modes at 1500 cm-1.
- there is a very strong C=O peak around 1700 cm-1.
- there is a medium C-O peak around 1250 cm-1.
- the sp3 C-H and O-H stretching modes are less clear.
Figure IR12. IR spectrum of hexanoic acid.
Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 14 July 2008)
At first, the O-H peak appears to be absent. The C-H stretch appears to be very broad. The wide peak between 3000 and 2600 cm-1 is really the usual C-H stretch with a broad O-H stretch superimposed on it. The low frequency vibration of this O-H bond is related to the partial dissociation of protons due to strong hydrogen bonding.