Interpreting Infrared Spectra: Ether Linkages and Hydroxyl Groups

Infrared (IR) spectroscopy is one of the key techniques used to determine the structures of organic compounds.

It works by detecting vibrational absorption signals produced by different functional groups within specific wavenumber ranges, allowing us to infer which functional groups are present and gain insight into local molecular structures. Among the many common functional groups, ether linkages (C–O–C) and hydroxyl groups (–OH) are particularly important to identify because both contain oxygen and play significant chemical roles.

Hydroxyl groups contribute polarity and hydrogen bonding, while ether linkages are generally stable and relatively weakly polar. Although the vibrations of both groups involve oxygen atoms, their IR absorption bands differ considerably in position, shape, and sensitivity to the surrounding environment. This is largely because hydroxyl groups contain an O–H bond that can participate in hydrogen bonding, whereas ether groups feature a C–O–C linkage that may vibrate in symmetric or asymmetric modes.

These differences provide a useful basis for distinguishing and interpreting the two functional groups.

Ethanol (–OH)

Ethanol has a relatively simple molecular structure. In addition to the methyl and methylene groups discussed in earlier articles, it contains a hydroxyl group. In this instalment, let’s look at the characteristic features that this additional –OH group produces in an IR spectrum.

As shown in the spectrum above, adding an –OH group introduces vibrational absorption associated with the O–H bond, as well as absorption from the C–O bond. The broad, prominent band around 3332 cm⁻¹ is characteristic of the –OH group. Its distinctive shape and position make it a useful indicator of hydroxyl groups.

Absorption in the region of approximately 1050–1100 cm⁻¹ is generally associated with C–O single-bond vibrations. This can also be a characteristic feature, but because C–O bonds occur in many types of compounds, the band usually helps identify a broader class of structures rather than providing as specific an indication as the O–H band around 3300 cm⁻¹.

Polyethylene Glycol (C–O–C)

Looking at the structure of polyethylene glycol (PEG), aside from the methylene groups, the main features are the –OH groups and C–O bonds discussed above. More precisely, the C–O bonds in this structure form C–O–C ether linkages. These linkages are especially common in polymeric materials. What characteristic features do they produce in an IR spectrum?

As the spectrum shows, the most characteristic features of the C–O–C group are the strong band around 1100 cm⁻¹ and the band at approximately 1250 cm⁻¹. Together, these features can help indicate the presence of ether linkages.

Another feature worth noting is that the methylene band around 2869 cm⁻¹ does not split into two distinct peaks. This is related to the sample’s molecular structure. The sample contains many –OH groups, which form hydrogen bonds. These interactions can cause slight shifts in the vibrational frequencies of different molecular environments, broadening the absorption into a single band. A similar effect can be seen in the C–O–C absorption around 1110 cm⁻¹: in the ethanol spectrum above, the corresponding peak is sharp and well defined, whereas in this spectrum it is broader.

That concludes this instalment’s look at two fundamental structural features: hydroxyl groups (–OH) and C–O–C ether linkages, along with their characteristic IR spectral features.