Distinguishing Aldehydes and Ketones using IR

The following two spectra are simple carbonyl compounds. Formaldehyde, the simplest aldehyde, and acetone, the simplest ketone. Students often come to me frustrated because they can not tell one carbonyl compound from the next, or the peak will be right between the two literature values. The aldehyde or ketone question is simple. In both you will see a very prominent C-O stretch around 1700cm-1 area.

Ketone

Aldehyde

But in the aldehyde you should also see see a peaks around 2820 and 2720cm-1. They often look like a doublet and are sometimes referred to as a Fermi doublet. These are the C-H stretches between the aldehydic proton and the carbonyl carbon. The presence of these peaks along with a carbonyl peak is a good indication that you have an aldehyde.