Carbonyl Compounds  IR Spectra

The carbonyl stretching vibration band C=O of saturated aliphatic ketones appears:

C=O stretch

• aliphatic ketones 1715 cm^-1
• alpha, beta-unsaturated ketones 1685-1666 cm^-1

Figure 8. shows the spectrum of 2-butanone. This is a saturated ketone, and the C=O band appears at 1715.

Figure 8. Infrared Spectrum of 2-Butanone

If a compound is suspected to be an aldehyde, a peak always appears around 2720 cm^-1 which often appears as a shoulder-type peak just to the right of the alkyl C–H stretches.

H–C=O stretch 2830-2695 cm^-1

C=O stretch

• aliphatic aldehydes 1740-1720 cm^-1
• alpha, beta-unsaturated aldehydes 1710-1685 cm^-1

Figure 9. shows the spectrum of butyraldehyde.

Figure 9. Infrared Spectrum of Butyraldehyde