12.6 : アルデヒドとケトンの NMR 分光法と質量分析

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.