Spectroscopic analysis of aldehydes and ketones:

The IR spectra of aldehydes and ketones are characterized via strong absorptionsdue to C=O stretching. These take place in the region 1740-1720 cm^-1 for aliphatic aldehydes and 1725-1705 cm^-1 for aliphatic ketones. Though, conjugation to aromatic rings or alkenes weakens the carbonyl bond resultant in absorptions at lower wavenumbers. For instance, the carbonyl absorptions for aromatic aldehydes and ketones are in the regions 1715-1695 cm^-1 and 1700-1680 cm^-1 correspondingly. For cyclic ketones, the absorption shifts to higher wavenumber along with increasing ring strain. For instance, the absorptions for cyclohexanone and cyclobutanone are 1715 and 1785 cm^-1 correspondingly.

In case of an aldehyde, 2 weak absorptions because of C-H stretching of the aldehyde proton might be spotted, one in the region 2900-2700 cm^-1 and another close to 2720 cm^-1. The aldehyde proton provides a characteristic signal in the ¹H nmr in the region 9.4-10.5 ppm. If the aldehyde group is related to a carbon bearing a hydrogen, coupling will occur, generally with a small coupling constant of about 3 Hz. Signals of an aldehyde or ketone can be acquired indirectly from the ¹H nmr through the chemical shifts of neighboring groups. For instance, the methyl signal of a methyl ketone takes place at 2.2 ppm like a singlet.

The carbonyl carbon can be spotted like a quaternary signal in the ¹³C nmr spectrum in the region 205-218 ppm for aliphatic ketones and 200-205 ppm for aliphatic aldehydes. The subsequent regions for aromatic aldehydes and ketones are 190-194 ppm and 196-199 correspondingly.

The mass spectra of aldehydes and ketones frequently depict fragmentation ions resultant from bond cleavage on either side of the carbonyl group (α-cleavage). Aromatic aldehydes and ketones usually fragment to provide a strong peak at m/e 105 because of the benzoyl fragmentation ion [PhC=O] ⁺.

 The carbonyl groups of saturated aldehydes and ketones provide a weak absorption band in their uv spectra among the 270 and 300 nm. This band is shifted to longer wavelengths (300-350 nm) while the carbonyl group is conjugated with a double bond. The fixed position of the uv absorption band can be helpful in the structure determination of conjugated aldehydes and ketones.