Mass spectrometry of

tert-butylnaphthalenesa comparison with the unimolecular fragmentation of tert-butylbenzene

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Eur. J. Mass Spectrom. 3, 271 - 280 (1997)

Mass spectrometry of tert-butylnaphthalenesa comparison with the unimolecular fragmentation of tert-butylbenzene

K. Weniger, J. Kleimann and H.-Fr. Grützmacher^*
Fakultät für Chemie, Universität Bielefeld, Universitätstraße 25, D-33615 Bielefeld, .

ABSTRACT:

The unimolecular fragmentation of ionized 1- and 2-tert-butylnaphthalenes 1 and 2 have been investigated using mass analyzed ion kinetic energy (MIKE) spectrometry and specifically deuterated and ¹³C labeled derivatives. Generally, the mass spectrometric fragmentations of 1 and 2 follow closely the reactions known for ionized tert-butylbenzene 3. Metastable molecular ions of 1, 2 and 3 lose exclusively a methyl radical yielding 2-naphthyl-2-propyl cations 1a and 2a or 2-phenyl-2-propyl cations 3a. Further fragmentations of 1a and 2a include, as the main processes, the elimination of C₂H₄ and of CH₃^". The loss of C₂H₄ is associated with a large kinetic energy release giving rise to a broad flat-topped peak in the MIKE spectra (as in the case of 3a) while loss of CH₃ gives rise to a Gaussian shaped signal. The latter reaction is not observed for metastable 2-phenyl-2-propyl cations 3a. The fragmentations of labeled derivatives of 1 and 2 show that H/D exchange in 1a and 2a, between the methyl groups of the side chain and the naphthalene ring, preceding the losses of C₂H₄ and of CH₃^", is much more extensive than in 3a. This H/D exchange decreases for metastable 1a and 2a decomposing in the second field-free region, and the interchange of the C atoms within the C₃H₆is less than for 3a. These observations are explained by H migrations between the side chain and the naphthalene rings of 1a and 2a, before the rearrangements within the side chain by the series of reversible 1,2-H shifts, of a 1,2-aryl shift, and of a 1,2-methyl shift known for the reactions of 3a. The differences observed for the reactions of 1a and 2a and of 3a are explained by the more basic aryl group of 1a and 2a. These fragmentation mechanisms are corroborated by an AM1 calculation of the minimum energy reaction pathways (MERP).

Keywords: tert-butylarene, fragmentation mechanism,rearrangement, metastable ions, kinetic energy release, deuterium labelling.

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