Infrared spectra of C=O stretch

The carbonyl group absorbs strongly at 1850-1650. Its frequency is very sensitive to attached atoms. Some typical values for various types are below:

An electronegative element may tend to draw in electrons between the C=O bond, so that the C=O becomes somewhat stronger/higher frequency. This is what raises an ester high than a ketone:

This effect is even stronger for acid chlorides, since chloride strengthens the induction.

This leaves the question of why carboxylic acids have a lower typical absorption than ketones, even though the acid has an oxygen to create the inductive effect.

The reason for this lower frequency is that acids tend to dimerize by hydrogen bonding, which weakens the C=O bond:

In the monomeric form, the acid absorbs higher then ketones (at 1760), as expected by the induction effect.

For amides, resonance gives the carbonyl more single-bond character:

The inductive effect also works the other direction with alkyl groups, which are electron donating. This explains why ketones have a lower frequency than aldehydes.