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 IR8.1. IR spectrum of hexanoic acid.¹

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.

Problem IR8.1.

Isopentyl butanoate has a C-O stretch at 1200 cm^-1. We saw earlier that an ether had a C-O stretch around 1000 cm^-1. Explain the differences in these bond stretches.

Figure IR8.2. IR spectrum of isopentyl butanoate.¹