12.6: NMR Spectroscopy and Mass Spectrometry of Aldehydes and Ketones

In aldehydes, the hydrogen atom connected to the carbonyl carbon helps distinguish aldehydes from other carbonyl compounds using ¹H NMR spectroscopy. The closeness of aldehydic hydrogen to the electrophilic carbonyl carbon highly deshields the hydrogen atom causing its signal to appear around 10 ppm in the ¹H NMR spectra. α hydrogens split the aldehydic proton signal, which helps identify the number of α hydrogens in the molecule. For instance, one α hydrogen creates a doublet for an aldehydic signal. α hydrogens are also deshielded and appear downfield compared to β and γ hydrogens. The α hydrogens further split into multiple peaks depending on the number of hydrogens present on the surrounding carbons. Likewise, signal splitting occurs for β and γ hydrogens, indicating the number of hydrogens on each adjacent carbon atom.

The ¹³C NMR spectra of aldehydes and ketones show a distinct peak around 190–200 ppm.

Mass spectrometry gives information about the molecular mass of the compound and the different molecular fragments. Since aldehydes can readily lose hydrogen, the mass spectra of aldehydes have M⁺–1 peak, whereas ketones show the molecular ion peak (M⁺) peak. Apart from the more common α cleavage that generates the acylium ion, the aldehydes and ketones that contain γ hydrogen can undergo the McLafferty rearrangement. The rearrangement occurs via β cleavage to form the corresponding molecular fragments.

In the ¹H NMR spectrum, the highly deshielded aldehydic proton resonates far downfield, quite distinctively, around 10 ppm.

The peak splits in the presence of ɑ protons, and the splitting pattern indicates the number of such protons.

α proton signals appear at much lower chemical shifts, whereas β and γ proton signals appear far upfield. This is because they are farther away from the carbonyl group and hence less affected by its deshielding effects.

In ¹³C NMR, the carbonyl carbon appears around 190–220 ppm, while the ɑ carbon signal appears upfield.

The mass spectrum of a ketone consists of a molecular ion peak, whereas an aldehyde has an M–1 peak resulting from the loss of the aldehydic proton.

The fragmentation of the molecular ion happens mainly through alpha cleavage, forming an acylium ion.

In ketones, the cleavage occurs at both alpha carbons and the ion fragment that generates the stable radical forms the base peak with maximum abundance.