Spectroscopy Tutorial: Aldehydes

IR Spectroscopy

Aldehydes show a the carbonyl stretch from 1740-1685. If the carbonyl is adjacent to saturated, aliphatic carbons, the C=O stretch is from 1740-1720. If the carbons alpha and beta to it are not saturated (for instance, if the carbonyl is on an aromatic ring), the C=O stretch shows up from 1710-1685.

Another useful diagnostic band for aldehydes is the C–H stretch of the aldehyde group in the region 2830-2695. If you suspect a compound to be an aldehyde, always look for a peak in this region; it often appears as a shoulder-type peak just to the right of the alkyl C–H stretches.

NMR Spectroscopy

The proton on the carbonyl carbon—the aldehydic proton—appears in the NMR from 9-10. It is usually a small singlet. As in ketones and esters, the proton on the carbon adjacent to the aldehyde (adjacent to the carbonyl) shows up from 2-2.7 ppm.


Two of the aldehyde examples have an aromatic ring and one an ether - if you are doing these problems in the order on the directory, you might need to jump ahead to the aromatic and ether discussions.

Example 6

Example 7

Example 8