Rearrangement Reactions following Electron Impact on Ethyl and Isopropyl Esters

The mass spectra of ethyl and isopropyl propionate and butyrate show rearrangement ions RlCOOH+ and RlC(OH)2+ formed by reactions [a] and [b]: R,COOR2+ --t RiCOOH+ + (R2 H), RlCOOR2+ --t R I C ( O H ) ~ + (R2 213). The mass spectra of ethyl-d2 propionate and butyrate show that both [a] and [b] occur with essentially complete scrambling of the alkoxy hydrogens. For the isopropyl esters the mass spectra of the isopropyl-d, molecules show that reaction [a] occurs by transfer of one of the methyl hydrogens to the acid moiety while reaction [b] occurs predominately by transfer of two methyl hydrogens.