Analyzing Infrared Spectra

The identity of functional groups (groups of atoms that have specific properties) can be determined by analyzing an IR spectrum. The IR spectrum is a graphical representation of the absorbance and transmittance of infrared light over a range of frequencies. By comparing the IR spectrum of a compound to the IR spectra data table, which is a table of characteristic absorptions for functional groups, an identification of the functional groups present in a molecule is possible. For example, if an IR spectrum contains a broad ${\rm{O{-}H}}$ stretch between 3,550 and 3,200 cm^–1, the compound likely contains an alcohol. If an IR spectrum contains a ${\rm{C{=}O}}$ stretch between 1,780 and 1,630 cm^–1 but not the corresponding ${\rm{C{-}O}}$ stretch between 1,150 and 1,050 cm^–1, then the compound likely does not contain an ester but rather a different type of carbonyl.

When analyzing a spectrum, it is just as important to identify what is not present as what is present. For example, if a spectrum does not contain a stretch between 3,550 and 3,200 cm^–1, the compound does not contain any functional group from that stretch, such as an ${\rm{O{-}H}}$ or ${\rm{N{-}H}}$. Likewise, if a spectrum does not contain a stretch between 1,700 and 1,600 cm^–1, the compound does not contain a carbonyl, which is the functional group found in that region.

Also, when analyzing a spectrum, it is not only important to note the location of a stretch but also the shape of the stretch. If a spectrum contains a small stretch between 1,700 and 1,600 cm^–1, it does not mean that the stretch is because of the presence of a carbonyl. Carbonyls give very strong stretches, so a short stretch in that region is most likely not because of the presence of a carbonyl but rather a result of statistical noise or some other factor. The IR spectra of two compounds can also be analyzed and compared to track the progress of a reaction. For example, the oxidation of cyclohexanol (C₆H₁₁OH) to cyclohexanone (C₆H₁₀O) converts an alcohol to a ketone. The two key functional groups in this reaction are the alcohol (${\rm{O{-}H}}$) and the ketone (${\rm{C{=}O}}$).

Obtaining IR spectra of the starting material (pure cyclohexanol) and the reaction as it progresses allows the chemist to track the progress of the reaction over time. The IR spectrum of the starting material should show a distinct broad ${\rm{O{-}H}}$ stretch between 3,550 cm^–1 and 3,200 cm^–1. The ${\rm{O{-}H}}$ stretch between 3,550 cm^–1 and 3,200 cm^–1 is one of two key stretches used to follow the reaction of cyclohexanol to cyclohexanone. As the reaction continues, this stretch for the ${\rm{O{-}H}}$ (3,550 cm^–1 and 3,200 cm^–1) should disappear and a sharp ${\rm{C{=}O}}$ stretch between 1,750 cm^–1 and 1,680 cm^–1 will form.

The appearance of the sharp ${\rm{C{=}O}}$ stretch between 1,750 cm^–1 and 1,680 cm^–1 indicates the formation of the ketone of cyclohexanone and the disappearance of the alcohol of cyclohexanol. This stretch represents the ketone that is being generated by the oxidation reaction. The reaction is complete when the ${\rm{O{-}H}}$ stretch is completely gone.