15.11: Mass Spectrometry: Aldehyde and Ketone Fragmentation

In mass spectrometry, the fragmentation of aliphatic aldehydes and ketones generally occurs through three key mechanisms: α-cleavage, inductive cleavage, and the McLafferty rearrangement.

1. α-Cleavage
   α-Cleavage is a common fragmentation in carbonyl compounds where the bond adjacent to the carbonyl group is broken. This process produces an alkyl radical and an acylium cation, which is often detected in the mass spectrum due to its stability.
2. Inductive Cleavage
   In inductive cleavage, fragmentation yields an acyl radical and an alkyl cation. This occurs when electrons are drawn toward the carbonyl, weakening adjacent bonds and resulting in a distinctive fragmentation pattern. For example, in the mass spectrum of 5-methyl-2-hexanone, inductive cleavage produces an alkyl cation at a mass-to-charge (m/z) ratio of 71.
3. McLafferty Rearrangement
   The McLafferty rearrangement is a specific fragmentation pattern for carbonyl compounds with γ-hydrogen (hydrogen on the third carbon from the carbonyl group). The molecule undergoes a six-membered cyclic transition in this rearrangement, yielding a radical cation and a neutral alkene. For 5-methyl-2-hexanone, this rearrangement results in a peak at m/z 58.

Example: Fragmentation of 5-methyl-2-hexanone
In the mass spectrum of 5-methyl-2-hexanone, the molecular ion undergoes these fragmentation patterns:

1. Inductive cleavage produces an alkyl cation at m/z 71.
2. α-Cleavage results in the base peak at m/z 43.
3. McLafferty rearrangement creates a distinct peak at m/z 58.

An additional diagnostic feature in aldehydes is the M−1 peak, which arises due to the α-cleavage of the aldehyde proton. In mass spectrometric analysis, this peak can help identify aldehydes, specifically among other carbonyl compounds.

Understanding these fragmentation pathways is crucial for interpreting mass spectra, particularly in identifying functional groups and distinguishing between similar compounds.