I am studying fragmentation patterns in mass spectrometry and I am confused about why alpha cleavage occurs. Why does the carbon-carbon bond adjacent to a functional group break instead of the bond between the alpha carbon and the fucntionalized moiety?

  • $\begingroup$ Because then you'd have two unstable fragments. There's simply not enough energy in the system to do that. $\endgroup$ – Karl Dec 12 '16 at 0:31
  • $\begingroup$ I edited this question in an attempt to clarify what you are asking. Please let me know if I distorted your meaning, and please feel free to revert the edit. $\endgroup$ – Curt F. Jan 2 '17 at 3:20
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    $\begingroup$ You need to look at the principle modes of decomposition of radicals. $\endgroup$ – Zhe Jan 2 '17 at 3:31
  • $\begingroup$ It's worth noting that alpha cleavage occurs primarily for radicals, such as the radical cations formed in electron ionization, and sometimes other ionization techniques (e.g. MALDI). The fragmentation patterns that result from "even-electron" ions that predominate from e.g. electrospray are quite different. $\endgroup$ – Curt F. Jan 3 '17 at 22:51

After ionization occurs, the carbonyl donates its free radical into its own double bond. This causes the bond adjacent to the carbonyl to split into two electron s, one with spin antiparallel to that of the carbonyl radical and one of parallel spin. The one of antiparallel spin combines with the free radical to form another pi bond. This creates a carbon oxygen triple bond and carbon atom with a free radical (alkyl radical). The carbon atom with the radical is the atom alpha to the carbonyl, hence alpha cleavage. The oxygen bears a positive formal charge. Alpha cleavage basically makes substituted carbon monoxides, and side chain radicals. You will see the "carbon monoxide derivative" on the spectrum.

To do this quickly, just take the original molecule, draw lines showing fragmentation on both sides of the carbonyl, and then draw the carbon oxygen triple bond structure.


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