Ion/molecule reactions of 2-chloro- and 2-bromopropene radical cations with methanol and ethanol—FT-ICR spectrometry and DFT calculations

Abstract

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 β-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 α-CH 2 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 H/D exchange between 1 + and CD 3OH during their slow reaction.