|Ph.D Student||Tamiri Tsippora|
|Subject||Specific Hydrogen Transfer in Difunctional Systems under|
Electron and Chemical Ionization
|Department||Department of Chemistry||Supervisors||Professor Emeritus Asher Mandelbaum|
|Dr. Shmuel Zitrin|
Isomeric mixed methyl ethyl citraconates, 2-substituted succinates and glutarates show different mass spectra under electron ionization (EI) and chemical ionization (CI). The mechanistic pathways of the specific fragmentation processes were studied in this work. Deuterium labeling and collision induced dissociation (CID) measurements were used for structural assignment of the fragment ions.
Isomeric mixed methyl ethyl 2-phenylsuccinates, 2-phenylglutarates and 2-benzylsuccinates showed specific loss of alcohol from the position adjacent to the phenyl group after the migration of a benzylic hydrogen. Isomeric mixed methyl ethyl 2-methylglutarates underwent two competing McLafferty rearrangements. The pathway involving the migration of a secondary hydrogen was three times higher than that involving the migration of a tertiary hydrogen.
1-Ethyl-4-methyl citraconate exhibited a highly specific unusual loss of the methyl group from position 2, while the other isomer, 1-methyl-4-ethyl citraconate, showed an efficient elimination of methanol. Mechanistic pathways were proposed for the two isomer-specific processes based on deuterium labeling and CID measurements and on theoretical calculations.
Under CI conditions, using methane and isobutane, the isomers with the ethyl group adjacent to the substituent underwent preferential elimination of ethanol. No specific alcohol elimination (or little in some cases) was found in isomers where the methyl group was adjacent to the substituent.
This different mass spectral behavior may be of analytical value as a diagnostic tool in the structural assignment of such close isomers, which show very similar infrared and NMR spectra.