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?
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.