We can learn a great deal of information from an IR spectrum; for the MCAT, all of the information
comes from the frequencies between 1400 and 4000 cm–1. Everything lower (in the fingerprint
region) is out of scope. IR spectra are plotted as percent transmittance, the amount of light that
passes through the sample and reaches the detector, vs. wavenumber.
Figure 11.2. IR Spectrum of an Aliphatic Alcohol
Broad peak at 3300 cm–1: –OHKEY CONCEPT
In an IR spectrum, percent transmittance is plotted vs. frequency. The equation relating
absorbance, A, and percent transmittance, % T, is A = 2 – log %T; this means that maximum
absorptions appear as the bottoms of valleys on the spectrum.Figure 11.2 shows the IR spectrum for an aliphatic alcohol: the large broad peak at 3300 cm–1 is due
to the presence of a hydroxyl group, whereas the sharper peak at 3000 cm–1 is due to the carbon–
hydrogen bonds in the alkane portion of the molecule, as listed in Table 11.1.
MCAT Concept Check 11.1:
Before you move on, assess your understanding of the material with these questions.