Electron ionization mass spectra and tautomerism of 2-phenacylpyridines

Eur. J. Mass Spectrom. 12, 25–30 (2006)
DOI: 10.1255/ejms.786

Electron ionization mass spectra and tautomerism of 2-phenacylpyridines

Olli Martiskainen,^a Ryszard Gawinecki,^b Borys Ošmiałowski^b and Kalevi Pihlaja^a,*
^aStructural Chemistry Group, Department of Chemistry, University of Turku, FI-20014 Turku Finland. E-mail: kpihlaja@utu.fi
^bDepartment of Chemistry, Technical and Agricultural University, Seminaryjna 3, PL-85-326 Bydgoszcz, Poland

ABSTRACT:

The electron ionization mass spectra of 2-phenacylpyridine (ketimine form) and its 13 derivatives substituted in the benzene ring (1a–n: a R = H, b 3-Me, c 4-Me, d 4-NH₂, e 3-F, f 4-F, g 4-OMe, h 4-Cl,i 4- N(CH₃)₂,j 4-NO₂, k 4-CF₃, l 4-N(CH₂)₄, m 4-Br, n 3-Br) were recorded at 70 eV to determine the fragmentation routes and to screen the presence of their enolimine tautomers, (Z-)-2-(2-hydroxy-2-phenylvinyl)pyridines in the gas phase. The total ion currents (TIC) of the ions [M–H]⁺, [M–HCO]⁺, 2-PyCH₂O⁺, and RC₆H₄CO⁺ (= ArCO⁺ ) showed a fair or good correlation with the Hammett s constants (R = 0.859, 0.876, 0.912, and –0.926, respectively). The relative abundances (RA) of both the [M–CO]^+· and the [M–HCO]⁺ ion increased with the decreasing electron donating ability of the substituents and also correlated relatively well with the Hammett constants (R = 0.834 and 0.907, respectively). These observations, in comparison to the NMR results, show that the relative contribution of the ketimine tautomer also increases in the gas phase with the increasing electron donating ability of the phenyl substituent, i.e. the TIC of the ArCO⁺ ion decreases whereas that of [M–H]⁺ ion increases.

Keywords: heterocycles, mass spectrometry, tautomerism, substituent effects

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