Spectroscopic Features of Ketones

IR Spectroscopy

Load the IR Spectrum

Identifying ketones starts with observing a carbonyl stretch (1650-1800); normally this is the strongest peak in the spectrum. Electronics are important; electronegative R groups may give slightly higher frequencies, but conjugation can also (more commonly) lower the frequency. As you can see, cyclohexanone, a typical ketone, shows a peak at 1720 cm^-1. Aromatic ketones are typically below 1700 cm^-1. Ring strain can raise the frequency.

¹H NMR Spectroscopy

Load the ¹H NMR

Since ketones have no hydrogen directly attached to the functional group, evidence in the ¹H NMR is indirect.
There is a deshielded signal for any proton on the alpha carbon; between 2-2.5 ppm and will couple normally to its neighbors. The electronegativity of the carbonyl group provides progressive deshielding as you work your way down the chain.

¹³C NMR Spectroscopy

Load the ¹³C NMR spectrum

The carbonyl carbon is in 190 ppm or further downfield. Only ketones and aldehydes occur this far downfield. The carbon signal is usually weak because of nOe enhancement of carbons attached to protons in proton-decoupled spectra.

Mass spectrometry