Hydrogen rearrangements of ionized propanoic acid and isomeric ions

Ionized propanoic acid and its enol isomer lose H ˙ in both normal and metastable decompositions. These C 3 H 6 O 2 ions lose their distinctness prior to undergoing metastable decomposition in the mass spectrometer. Deuterium labeling indicated that this results from the transfer of a β‐hydrogen to oxygen in the propanoic acid ion followed by rapid exchange of hydrogen and deuteriums between the α‐ and β‐carbons and slower exchange between the carbons and the oxygens. 3‐ and 5‐Membered ring hydrogen rearrangements are preferred over 4‐membered ring transfers. It is concluded that 1,2‐hydrogen shifts following removal of hydrogen from alkyl chains may sometimes cause site‐specific hydrogen rearrangements to appear non‐site specific.