PUB - Publikationen an der Universität Bielefeld

The gas-phase ion-molecule reactions of the radical cations of 2-chloropropene (1(+.)), 2-bromopropene (2(+.)), and 2-iodopropene (3(+.)), as well as of the corresponding three 3,3,3-trifluoropropenes (4(+.)-6(+.)) with ammonia have been studied by FT-ICR mass spectrometry complemented by ab initio calculations of the reaction thermochemistry. In all cases a deprotonation of the 2-halopropene radical cations by ammonia is distinctly exothermic. In spite of this, the substitution of the halo substituent by NH3 is the main reaction pathway for 1(+.) and 2(+.) and is still competing for the slowly reacting iodo derivative 3(+.). In the latter case deprotonation generates not only NH4+, but also the proton-bridged dimer [H3N .. H+.. NH3]. These effects prove that the first addition step of the substitution by an addition-elimination mechanism of the haloalkene radical cations can compete effectively with exothermic deprotonation and occurs without noticeable activation energy. In fact it appears likely that the reprotonation of the unsaturated radical cations proceeds also by an addition-elimination process. The calculation of the reaction enthalpy shows that addition of NH3 to the ionized 3,3,3-trifluoro-2-halopropenes 4(+.)-6(+.) is especially exothermic. Experimentally this effect is not only reflected in the increased reaction efficiency of the substitution product, even in the case of the iodo derivative 6(+.), but also in competing fragmentations of the strongly excited distonic intermediates generated by the addition step. This corroborates the postulate that the variation of the rate constants with the substituents, which is observed for the reactions of ionized haloalkenes with ammonia, is caused by the excess energy released in the initial addition step.