Ketone infrared spectra

In addition to the obvious C=O stretch, ketones also have a C-CO-C bend:

Aromatic ketones have this stretch at the higher end, aliphatic ketones have this stretch at the lower end.

Methylene and methyl groups next to the carbonyl tend to absorb with greater intensity than usual.

The 1715 C=O stretch produces a very weak overtone band at 3430. Avoid confusing it with O-H absorptions, which are much more intense.

The ketene above can be considered an extreme example of a strained ring. S character of the double bond increases with ring strain, and this reaches a maximum when attached to a sp-hybridized carbon in a ketene.

1,2-diketones, also called α-ketones, have one strong absorption about 1716. As expected with normal carbonyls, this peak is lower when the diketone is conjugated. Diketones may also produce a narrowly-spaced doublet due to the asymmetric and symmetric absorptions (eg. the carbonyls might stretch together, or one might stretch while the other contracts).

1,3-diketones, also called β-ketones, show more complicated patterns due to tautomerization to the enol form:

The enol form is particularly stable for diketones due to the hydrogen bond, so spectra often show both C=O peaks.

The shifted C=O stretch in the enol can be explained by the hydrogen bond, and by resonance:

The relative intensities of the two types of C=O can provide a method for seeing which form is more prevalent.

Ketone examples:
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Cyclopentanone

Mesityl oxide

Acetophenone

Cyclopentanone