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Sample IR Spectra :
    By looking at IR spectra that contain known functional groups and comparing and contrasting them with other IR spectra, one can develop the skills required to be able to "interpret" an "unknown" IR spectra.  Remember that for an organic chemist, the primary role of IR is to identify the functional groups that are present. A few examples reflecting some of the more important functional groups are provided below.
Compare them to try to appreciate the subtle differences, comparing frequency, intensity and shape.
 

In the first example, of the aromatic hydrocarbon, toluene, we can see both the aromatic and aliphatic CH stretches, and two absorptions for the aromatic C=C stretches.

IR spectrum of methylbenzene (toluene)



Acetone (2-propanone) is the "classic" carbonyl containing compound with the obvious C=O stretch in the middle of the spectra. Note that the peak is a very strong absorption. Compare it with the C=C in the previous case which are weaker and sharper.

IR spectrum of 2-propanone

 


The characteristic absorption of the alcohol, 2-propanol, is the broad band due to the hydrogen bonded -OH group.

IR spectra of isopropanol

 

Carboxylic acids contain both C=O and OH groups. Note the broadness of both absorptions due to the hydrogen bonding and that the C=O is typically at slightly lower frequency than that of a ketone.

IR spectrum of butanoic acid

 


An ester has the follwoing key absorptions, the C=O and typically two bands for the C-O (not always easy to identify) since there are sp3 C-O and sp2 C-O bonds.

IR spectrum of methyl ethanoate
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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary