PUB - Publikationen an der Universität Bielefeld

Continuing the studies of ion/molecule reactions of haloalkene radical cations with nucleophiles, the reactions of the radical cations of 2-chloropropene, 1(.+), and 2-bromopropene. 2(.+) with methanol and ethanol, respectively, have been investigated by FT-ICR spectrometry and by computational analysis using DFT calculation (BHLYP/6-311 + G(2d,p)//BHLYP/6-31 + G(d) level). Only slow reactions (reaction efficiency <1%) are observed for 1(.+)/methanol and 2(.+)/methanol. Slow proton transfer is the main process for 1(.+)-methanol besides minor addition of methanol to 1(.+) followed by loss of HCl or Cl-.. Addition of methanol accompanied by loss of Br-. is the exclusive process observed for 2(.+)/methanol. In contrast, both 1(.+) and 2(.+) react efficiently with ethanol yielding protonated acetaldehyde as the exclusive (1(.+)) or by far dominant (2(.+)) primary reaction product. The computational analysis of these ion/molecule reactions shows that in the case of 1(.+)/methanol and 2(.+)/methanol all processes are either endothermic or blocked by large activation energies. Nonetheless, addition of methanol to the ionized C-C double bond of 1(.+) or 2(.+) is exothermic, yielding in each case a pair of isomeric beta-distonic methoxonium ions. A new reaction mechanism has been found for the HX (X = Cl, Br) elimination from the less stable isomer of the distonic intermediates. Further, an energetically favorable transition state has been detected for hydrogen atom transfer from the alpha-CH2, group of alcohol to the halogenoalkene radical cations. These findings lead to a revised mechanism of the oxidation process and provide a plausible explanation for the excessive HID exchange between 1(.+) and CD(3)0H during their slow reaction. (C) 2004 Elsevier B.V. All rights reserved.