THE CHEMISTRY OF IONIZED ETHYL GLYCOLATE, HOCH2CO2C2H5-CENTER-DOT-CENTER-DOT+ - FORMATION OF IONIZED AND NEUTRAL TRIHYDROXYETHYLENE(, AND ITS ENOL ISOMER, HOCH=C(OH)OC2H5)

Citation
D. Suh et al., THE CHEMISTRY OF IONIZED ETHYL GLYCOLATE, HOCH2CO2C2H5-CENTER-DOT-CENTER-DOT+ - FORMATION OF IONIZED AND NEUTRAL TRIHYDROXYETHYLENE(, AND ITS ENOL ISOMER, HOCH=C(OH)OC2H5), European mass spectrometry, 1(6), 1995, pp. 545-559
Citations number
50
Categorie Soggetti
Spectroscopy,"Physics, Atomic, Molecular & Chemical
Journal title
ISSN journal
13561049
Volume
1
Issue
6
Year of publication
1995
Pages
545 - 559
Database
ISI
SICI code
1356-1049(1995)1:6<545:TCOIEG>2.0.ZU;2-Q
Abstract
The unimolecular reactions of ionized ethyl glycolate, HOCH2CO2C2H5.+, and its enol isomer, HOCH=C(OH)OC2H5.+, have been studied by a variet y of techniques including H-2-, C-13- and O-18-labelling experiments, kinetic energy release information, analysis of collision-induced diss ociation and neutralization-reionization mass spectra and thermochemic al measurements, The metastable ionized enol eliminates C2H4 essential ly exclusively by beta-hydrogen transfer to give (HO)(2)C=CHOH.+, whic h itself expels H2O with very high selectivity (similar to 99%), The m etastable ionized keto isomer also eliminates C2H4, but minor amounts (similar to 5%) of competing fragmentations resulting in expulsion of H2O, HOCO. and HCO. are also observed, Moreover, in this case, loss of C2H4 no longer involves specific beta-hydrogen transfer, This behavio r is interpreted in terms of rearrangement of the ionized keto form vi a a 1,5-H shift to the distonic ion HOCH2C+(OH)OCH2CH2., which undergo es a further 1,5-hydrogen shift to form HOCH=C(OH)OC2H5.+, from which C2H4 is lost to give (HO)(2)C=CHOH.+. The chemistry of these C4H8O3.species, in which unidirectional tautomerism of HOCH2CO2C2H5.+ to HOCH =C(OH)OC2H5.+ via two 1,5-H shifts is important, contrasts sharply wit h the behavior of the lower homologues, HOCH2CO2CH3.+ and HOCH=C(OH)OC H3.+, for which the analogous tautomerism via 1,4-H shifts does not oc cur, The mechanism for loss of C2H4 from HOCH=C(OH)OC2H5.+ is essentia lly the same as that observed for ionized phenyl ethyl ether in that t he reaction proceeds via an exothermic proton transfer in an ethyl cat ion-radical complex, Neutralization-reionization experiments show that neutral trihydroxyethylene in the gas phase is a remarkably stable sp ecies which does not tautomerize to glycolic acid, HO-CH2-COOH. Using G2(MP2) theory, the enthalpy of formation, Delta H-f(298), of the neut ral molecule (most stable conformer) was found to be -449.4 kJ mol(-1) while that of the corresponding ion was calculated to be 299.6 kJ mol (-1).