15.11 : 质谱中醛和酮的碎裂

The major fragmentation patterns for aliphatic aldehydes and ketones include ⍺-cleavage, inductive cleavage, and  McLafferty rearrangement.

Fragmentation by ⍺-cleavage results in the formation of an alkyl radical and an acylium cation.

On the other hand, fragmentation by inductive cleavage yields an acyl radical and an alkyl cation.

If the carbonyl compound undergoes McLafferty rearrangement, it yields a radical-cation and a neutral alkene.

Consider the mass spectrum of 5-Methyl-2-hexanone, where the molecular ion fragments via inductive cleavage to form an alkyl cation at a mass-to-charge ratio of 71.

The base peak at a mass-to-charge ratio of 43 corresponds to ⍺-cleavage.

Another peak at the mass-to-charge ratio of 58 results from the McLafferty rearrangement of the molecular ion.

Notably, aldehydes can be identified by the presence of the M−1 peak resulting from the ⍺-cleavage of the aldehyde proton.