Massenspektrometrische Untersuchungen zur Genese von [M—Me] [M-Et] [C7H7]+ und [C7H8]+ aus n-Butyl- und n-Pentylbenzol: Zeitabhängige Nachbargruppeneffekte, Isomerisierungen und kompetitive Wasserstoff-Umlagerungen / Mass Spectrometric Investigations on the Formation of [M-Me] [M-Et] [C7H7]+ und [C7H8]+ from w-Butyl and w-Pentyl Benzenes: Time Dependent Neighbouring Group Participation, Isomerization and Competitive Hydrogen Rearrangements

Abstract

Abstract The details of the elimination of CH3· and C2H5· from the molecular ions of n-butyl and n-pentyl benzenes as well as the formation of C7H7+ and C7H7+ -have been established using a combination of different mass spectrometric techniques. Among these techniques are field ionization kinetics (FIK), collisional activation (CA), unimolecular decomposi-tion of metastable ions (MI), kinetic energy release determinations (Tkin)> appearance potential measurements (AP) as well as high resolution mass spectrometry. The appli-cation of these methods and the investigation of nine 13 carbon and eleven deuterium labelled n-butyl and n-pentyl benzenes clearly demonstrate that 1) the loss of methyl and ethyl from terminal positions of the alkyl chains are accompanied by an interaction with the phenyl ring and 2) the regioselectivity of these processes is time dependent. Contrary to previous conclusions it is shown that hydrogen transfer in the formation of C7H8+ -occurs via five-, six-and seven-membered transition states. The time dependence of the hydrogen exchange reaction preceding the alkene eliminations is discussed in detail.