Extensive rearrangement reactions observed for differently N-substituted 2,5-dimethylpyrroles under electron ionization
✍ Scribed by Karoliina Joutsiniemi; Pirjo Leppälä; Pirjo Vainiotalo
- Book ID
- 101236473
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 152 KB
- Volume
- 12
- Category
- Article
- ISSN
- 0951-4198
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✦ Synopsis
The 70 eV electron ionization mass spectra of fourteen differently N-substituted 2,5-dimethylpyrroles were studied. In accord with the aromatic character of the pyrrole ring, all the compounds studied gave rise to an intense molecular ion peak. Regardless of the functional group at the 1-substituent, for all the compounds the principal fragmentation products were practically the same, although the relative peak intensities varied depending on the structure of the substituent. In addition, an amino group substituent prompted intense radical site initiated cleavages which were minor or totally absent with other compounds. The data showed that extensive rearrangement reactions took place. Some of these were seen directly from the mass spectra, and some only from closer examination of the ion structures. The structures of the ions at m/z 108 and 94, formally representing the [MÀ1] ions of 1,2,5-trimethylpyrrole and 2,5-dimethylpyrrole, respectively, were carefully studied using the collision induced dissociation (CID) technique. Measurements showed that ring expansion reactions took place, protonated 2,5-lutidine and protonated 2-picoline being the most likely isomers formed for the m/z 108 and 94 ions, respectively. The results obtained from the CID experiments were supported by semi-empirical calculations of heats of formation.