Likewise the CH stretch still appears at about 3000 cm -1 . It also has a characteristic carbonyl band at 1700 cm -1 . You can see the strong carbonyl CO stretch at approximately 1700 cm -1 . You can also see that this spectrum is different from the spectrum for octanal. At this point in your study of IR spectroscopy, you can't tell which compound is an aldehyde and which is a ketone.
Carboxylic acids have spectra that are even more involved.
They typically have three bands caused by bonds in the COOH functional group. The band near 1700 cm -1 is due to the CO double bond. The spectrum for the carboxylic acid, diphenylacetic acid, appears below. It overlaps the CH stretch which appears near 3000 cm -1 . A strong carbonyl CO stretch band exists near 1700 cm -1 .