Intramolecular aromatic substitution reactions in substituted N,N‐dimethylthiobenzamide ions

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Abstract
The molecular ions of N,N‐dimethylthiobenzamide and its ortho substituted derivatives (substituents CH3, Cl, Br, I) lose a hydrogen atom and/or the ortho substituent. The mechanism of this process has been studied by measurements of the ionization energies, appearance energies of the product ions m/z 164 and the kinetic energy release during this process. The structure of the product ions m/z 164 and relevant reference ions have been investigated by mass analysed ion kinetic energy spectra, B/E linked scan spectra and collision induced decompositions. The results show clearly the formation of two different kinds of product ions m/z 164 depending on the substituent lost. Type a ions are formed by loss of a H atom or the CH3 substituent and correspond to protonated 3,4‐benzo‐N‐methylpyroline‐2‐thione. The formation of these ions occurs by a hydrogen rearrangement followed by an intramolecular substitution via a 5‐membered cyclic intermediate and is associated with a large release of kinetic energy. In contrast, the loss of the halogeno substituents to give type b ions probably occurs via a direct displacement reaction by the sulfur atom of the thioamide group giving rise to Gaussian shaped peaks mass analysed ion kinetic energy spectra.

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